LTC3638_15 [Linear]
High Efficiency, 140V 250mA Step-Down Regulator;
| 型号： | LTC3638_15 |
| 厂家： | 
Linear |
| 描述： | High Efficiency, 140V 250mA Step-Down Regulator |
| 文件： | 总26页 (文件大小：614K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3638
High Efficiency, 140V
250mA Step-Down
Regulator
FEATURES
DESCRIPTION
The LTC®3638 is a high efficiency step-down DC/DC
regulator with internal power switch that draws only 12μA
typical DC supply current while maintaining a regulated
output voltage at no load.
n
Wide Operating Input Voltage Range: 4V to 140V
n
Internal Low Resistance Power MOSFET
n
No Compensation Required
n
Adjustable 20mA to 250mA Maximum Output
Current
The LTC3638 can supply up to 250mA load current and
features a programmable peak current limit that provides
a simple method for optimizing efficiency and for reduc-
ing output ripple and component size. The LTC3638’s
combination of Burst Mode® operation, integrated power
switch, low quiescent current, and programmable peak
current limit provides high efficiency over a broad range
of load currents.
n
Low Dropout Operation: 100% Duty Cycle
n
Low Quiescent Current: 12µA
Wide Output Range: 0.8V to V
n
IN
n
n
n
n
n
n
n
0.8V 1% Feedback Voltage Reference
Precise RUN Pin Threshold
Internal or External Soft-Start
Programmable 1.8V, 3.3V, 5V or Adjustable Output
Few External Components Required
Programmable Input Overvoltage Lockout
Thermally Enhanced High Voltage MSOP Package
Withitswideinputrangeof4Vto140Vandprogrammable
overvoltage lockout, the LTC3638 is a robust regulator
suitedforregulatingfromawidevarietyofpowersources.
Additionally,theLTC3638includesapreciserunthreshold
and soft-start feature to guarantee that the power system
start-up is well-controlled in any environment. A feedback
comparator output enables multiple LTC3638s to be con-
nected in parallel for higher current applications.
APPLICATIONS
n
Industrial Control Supplies
n
Medical Devices
n
Distributed Power Systems
n
Portable Instruments
The LTC3638 is available in a thermally enhanced high
voltage-capable16-leadMSEpackagewithfourmissingpins.
L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
n
Battery-Operated Devices
n
Avionics
Automotive
n
TYPICAL APPLICATION
Efficiency and Power Loss vs Load Current
100
EFFICIENCY
90
5V to 140V Input to 5V Output, 250mA Step-Down Regulator
80
L1
220µH
70
60
50
40
30
20
10
0
V
V
V
= 12V
= 48V
= 140V
IN
IN
IN
V
V
IN
OUT
V
SW
IN
5V TO 140V
5V
1000
100
10
LTC3638
250mA
V
FB
RUN
C
IN
1µF
OVLO
V
250V
SS
V
PRG1
C
OUT
PRG2
22µF
GND
POWER LOSS
100
3638 TA01a
1
0.1
1
10
1000
LOAD CURRENT (mA)
3638 TA01b
3638fa
1
For more information www.linear.com/LTC3638
LTC3638
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V Supply Voltage................................... –0.3V to 140V
IN
1
3
SW
16 GND
14 RUN
12 OVLO
RUN Voltage............................................. –0.3V to 140V
V
IN
17
SS, FBO, OVLO, I
FB PRG1 PRG2
Voltages...................... –0.3V to 6V
SET
5
6
7
8
FBO
PRG2
PRG1
GND
GND
V
V
11
I
SET
V , V
, V
Voltages ......................... –0.3V to 6V
10 SS
Operating Junction Temperature Range (Notes 2, 3, 4)
LTC3638E, LTC3638I......................... –40°C to 125°C
LTC3638H.......................................... –40°C to 150°C
LTC3638MP....................................... –55°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)...................300°C
9
V
FB
MSE PACKAGE
VARIATION: MSE16 (12)
16-LEAD PLASTIC MSOP
= 150°C, θ = 40°C/W, θ = 10°C/W
T
JMAX
JA JC
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3638EMSE#PBF
LTC3638IMSE#PBF
LTC3638HMSE#PBF
LTC3638MPMSE#PBF
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3638EMSE#TRPBF
LTC3638IMSE#TRPBF
LTC3638HMSE#TRPBF
3638
3638
3638
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
16-Lead Plastic MSOP
–40°C to 125°C
–40°C to 125°C
–40°C to 150°C
–55°C to 150°C
LTC3638MPMSE#TRPBF 3638
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping
container.Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Supply (V )
IN
V
V
Input Voltage Operating Range
Output Voltage Operating Range
4
140
V
V
IN
0.8
V
IN
OUT
l
l
UVLO
V
Undervoltage Lockout
V
V
Rising
Falling
3.5
3.3
3.75
3.5
250
4.0
3.8
V
V
mV
IN
IN
IN
Hysteresis
I
DC Supply Current (Note 5)
Active Mode
Q
150
12
1.4
350
22
6
µA
µA
µA
Sleep Mode
No Load
RUN
Shutdown Mode
V
= 0V
V
RUN Pin Threshold
RUN Rising
RUN Falling
Hysteresis
1.17
1.06
1.21
1.10
110
1.25
1.14
V
V
mV
RUN
I
RUN Pin Leakage Current
OVLO Pin Threshold
RUN = 1.3V
–10
0
10
nA
RUN
V
OVLO Rising
OVLO Falling
Hysteresis
1.17
1.06
1.21
1.10
110
1.25
1.14
V
V
mV
OVLO
3638fa
2
For more information www.linear.com/LTC3638
LTC3638
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Output Supply (V
)
FB
V
V
Feedback Comparator Threshold
(Adjustable Output)
V
Rising, V
= V
PRG2
= 0V
= 0V
FB(ADJ)
FBH
FB
PRG1
l
l
LTC3638E, LTC3638I
LTC3638H, LTC3638MP
0.792
0.788
0.800
0.800
0.808
0.812
V
V
l
Feedback Comparator Hysteresis
(Adjustable Output)
V
Falling, V
= V
PRG2
3
5
9
mV
FB
PRG1
I
Feedback Pin Current
V
= 1V, V
= V = 0V
PRG2
–10
0
10
nA
FB
FB
PRG1
l
l
V
Feedback Comparator Thresholds
(Fixed Output)
V
V
Rising, V
Falling, V
= SS, V
= SS, V
= 0V
= 0V
4.94
4.91
5.015
4.985
5.09
5.06
V
V
FB(FIXED)
FB
FB
PRG1
PRG1
PRG2
PRG2
l
l
V
V
Rising, V
Falling, V
= 0V, V
= 0V, V
= SS
= SS
3.25
3.23
3.31
3.29
3.37
3.35
V
V
FB
FB
PRG1
PRG1
PRG2
PRG2
l
l
V
V
Rising, V
Falling, V
= V
= V
= SS
= SS
1.78
1.77
1.81
1.80
1.84
1.83
V
V
FB
FB
PRG1
PRG1
PRG2
PRG2
Operation
l
l
l
I
Peak Current Comparator Threshold
I
Floating
500
250
40
575
300
60
650
350
80
mA
mA
mA
PEAK
SET
100k Resistor from I to GND
SET
I
Shorted to GND
SET
R
Power Switch On-Resistance
Switch Pin Leakage Current
Soft-Start Pin Pull-Up Current
Internal Soft-Start Time
I
= –100mA
1.8
0.1
5
Ω
μA
μA
ms
ON
SW
I
I
t
V
V
= 140V, SW = 0V
< 2.5V
1
6
LSW
SS
IN
4
SS
SS Pin Floating
1
INT(SS)
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 3: The junction temperature (T , in °C) is calculated from the ambient
J
temperature (T , in °C) and power dissipation (P , in Watts) according to
A
D
the formula:
T = T + (P • θ )
JA
J
A
D
Note 2: The LTC3638 is tested under pulsed load conditions such that
where θ is 40°C/W for the MSOP package.
JA
T ≈ T . The LTC3638E is guaranteed to meet performance specifications
J
A
Note that the maximum ambient temperature consistent with these
specifications is determined by specific operating conditions in
conjunction with board layout, the rated package thermal impedance and
other environmental factors.
Note 4: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. The maximum
rated junction temperature will be exceeded when this protection is active.
Continuous operation above the specified absolute maximum operating
junction temperature may impair device reliability or permanently damage
the device. The overtemperature protection level is not production tested.
from 0°C to 85°C. Specifications over the –40°C to 125°C operating
junction temperature range are assured by design, characterization and
correlation with statistical process controls. The LTC3638I is guaranteed
over the –40°C to 125°C operating junction temperature range, the
LTC3638H is guaranteed over the –40°C to 150°C operating junction
temperature range and the LTC3638MP is tested and guaranteed over the
–55°C to 150°C operating junction temperature range.
High junction temperatures degrade operating lifetimes; operating lifetime
is derated for junction temperatures greater than 125°C. Note that the
maximum ambient temperature consistent with these specifications is
determined by specific operating conditions in conjunction with board
layout, the rated package thermal impedance and other environmental
factors.
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
3638fa
3
For more information www.linear.com/LTC3638
LTC3638
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current,
VOUT = 5V
Efficiency vs Load Current,
VOUT = 3.3V
Efficiency vs Load Current,
VOUT = 1.8V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
FIGURE 14 CIRCUIT
FIGURE 14 CIRCUIT
FIGURE 14 CIRCUIT
V
IN
V
IN
V
IN
= 12V
= 48V
= 140V
V
V
V
= 12V
= 48V
= 140V
V
V
V
= 12V
= 48V
= 140V
IN
IN
IN
IN
IN
IN
0.1
1
10
100
1000
0.1
1
10
100
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
3638 G01
3638 G02
3638 G03
Efficiency vs Input Voltage,
Feedback Comparator Trip
Threshold vs Temperature
RUN and OVLO Thresholds vs
Temperature
V
OUT = 5V
100
90
80
70
60
50
40
30
20
10
0
802
801
800
799
FIGURE 14 CIRCUIT
1.24
1.22
1.20
1.18
1.16
1.14
1.12
1.10
1.08
1.06
RISING
FALLING
I
I
I
= 250mA
= 10mA
= 1mA
LOAD
LOAD
LOAD
798
–55 –25
5
35
65
95 125 155
65
TEMPERATURE (°C)
125 155
–55 –25
5
35
95
0
25
50
75
100
125
150
TEMPERATURE (°C)
INPUT VOLTAGE (V)
3638 G05
3638 G06
3638 G04
Peak Current Trip Threshold
vs RISET
Peak Current Trip Threshold
vs Temperature and ISET
Peak Current Trip Threshold
vs Input Voltage
700
600
500
400
300
200
100
0
700
600
500
400
300
200
100
0
600
500
400
300
200
100
0
I
OPEN
SET
I
OPEN
SET
R
= 100kΩ
ISET
R
ISET
= 100kΩ
I
= GND
65
I
= GND
90
SET
SET
60
–55 –25
5
35
95 125 155
0
30
120
150
25 50 75 100 125 150
200
175
0
TEMPERATURE (°C)
V
VOLTAGE (V)
IN
R
ISET
(kΩ)
3638 G08
3638 G09
3638 G07
3638fa
4
For more information www.linear.com/LTC3638
LTC3638
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Supply Current
vs Input Voltage
Quiescent Supply Current
vs Temperature
Switch Pin Current
vs Temperature
15
10
5
35
30
25
20
15
10
5
15
10
5
V
= 140V
V
= 140V
IN
IN
SLEEP MODE
SLEEP
SW = 0.8V
CURRENT INTO SW
0
SLEEP
SW = 0V
CURRENT OUT OF SW
–5
–10
–15
SHUTDOWN
SHUTDOWN
0
0
–25
5
65
95 125 155
–55
35
0
30
60
90
120
150
–55 –25
5
35
65
95 125 155
V
VOLTAGE (V)
TEMPERATURE (°C)
TEMPERATURE (°C)
IN
3638 G12
3638 G10
3638 G11
Switch On-Resistance
vs Input Voltage
Switch On-Resistance
vs Temperature
Load Step Transient Response
3.0
2.5
2.0
1.5
1.0
4
3
2
1
0
OUTPUT
VOLTAGE
50mV/DIV
I
= 250mA
SW
LOAD
CURRENT
100mA/DIV
3638 G15
V
V
= 48V
200µs/DIV
IN
OUT
= 3.3V
1mA TO 250mA LOAD STEP
FIGURE 15 CIRCUIT
0
30
60
90
120
150
–25
5
65
95 125 155
–55
35
V
VOLTAGE (V)
IN
TEMPERATURE (°C)
3638 G13
3638 G14
Operating Waveforms, VIN = 48V
Operating Waveforms, VIN = 140V
Short-Circuit and Recovery
OUTPUT
VOLTAGE
50mV/DIV
OUTPUT
VOLTAGE
50mV/DIV
OUTPUT
VOLTAGE
1V/DIV
SWITCH
VOLTAGE
20V/DIV
SWITCH
VOLTAGE
50V/DIV
INDUCTOR
CURRENT
500mA/DIV
INDUCTOR
CURRENT
500mA/DIV
INDUCTOR
CURRENT
500mA/DIV
3638 G18
3638 G16
3638 G17
500µs/DIV
V
V
= 48V
10µs/DIV
V
V
= 140V
= 3.3V
10µs/DIV
IN
IN
OUT
OUT
= 3.3V
FIGURE 15 CIRCUIT
OUT
OUT
I
= 250mA
I
= 250mA
FIGURE 15 CIRCUIT
FIGURE 15 CIRCUIT
3638fa
5
For more information www.linear.com/LTC3638
LTC3638
PIN FUNCTIONS
SW (Pin 1): Switch Node Connection to Inductor and
Catch Diode Cathode. This pin connects to the drain of
the internal power MOSFET switch.
I
(Pin 11): Peak Current Set Input. A resistor from this
SET
pin to ground sets the peak current comparator threshold.
Leave floating for the maximum peak current (575mA
typical) or short to ground for minimum peak current
(60mA typical). The maximum output current is one-half
the peak current. The 5µA current that is sourced out of
this pin when switching is reduced to 1µA in sleep. Op-
tionally, a capacitor can be placed from this pin to GND
to trade off efficiency for light load output voltage ripple.
See Applications Information.
V (Pin 3): Main Supply Pin. A ceramic bypass capacitor
IN
should be tied between this pin and GND.
FBO(Pin5):FeedbackComparatorOutput. Connecttothe
V
pins of additional LTC3638s to combine the output
FB
current.Thetypicalpull-upcurrentis20µA.Thetypicalpull-
down impedance is 70Ω. See Applications Information.
V
, V
(Pins 6, 7): Output Voltage Selection. Short
OVLO (Pin 12): Overvoltage Lockout Input. Connect to
the input supply through a resistor divider to set the over-
voltage lockout level. A voltage on this pin above 1.21V
disables the internal MOSFET switch. Normal operation
resumes when the voltage on this pin decreases below
1.10V. Exceeding the OVLO lockout threshold triggers a
soft-start reset, resulting in a graceful recovery from an
input supply transient. Tie this pin to ground if the over-
voltage is not used.
PRG2 PRG1
both pins to ground for a resistive divider programmable
output voltage. Short V to SS and short V to
PRG1
PRG2
to ground
ground for a 5V output voltage. Short V
PRG1
and short V
to SS for a 3.3V output voltage. Short
PRG2
both pins to SS for a 1.8V output voltage.
GND (Pin 8, 16, Exposed Pad Pin 17): Ground. The ex-
posed pad must be soldered to the PCB ground plane for
rated thermal performance.
RUN (Pin 14): Run Control Input. A voltage on this pin
above 1.21V enables normal operation. Forcing this pin
below 0.7V shuts down the LTC3638, reducing quiescent
current to approximately 1.4µA. Optionally, connect to the
input supply through a resistor divider to set the under-
voltage lockout.
V
(Pin 9): Output Voltage Feedback. When configured
FB
for an adjustable output voltage, connect to an external
resistive divider to divide the output voltage down for
comparison to the 0.8V reference. For the fixed output
configuration, directly connect this pin to the output.
SS (Pin 10): Soft-Start Control Input. A capacitor to
ground at this pin sets the output voltage ramp time. A
50µA current initially charges the soft-start capacitor until
switching begins, at which time the current is reduced to
its nominal value of 5µA. The output voltage ramp time
from zero to its regulated value is 1ms for every 6.25nF
of capacitance from SS to GND. If left floating, the ramp
time defaults to an internal 1ms soft-start.
3638fa
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For more information www.linear.com/LTC3638
LTC3638
BLOCK DIAGRAM
1.3V
V
IN
ACTIVE: 5µA
SLEEP: 1µA
3
V
IN
+
I
SET
11
C
IN
PEAK CURRENT
COMPARATOR
+
–
RUN
14
12
+
–
1.21V
OVLO
L1
SW
LOGIC
1
V
OUT
–
+
D1 C
OUT
GND
16
1.21V
+
–
5V
SWITCH NODE
COMPARATOR
20µA
FEEDBACK
COMPARATOR
VOLTAGE
REFERENCE
5V
FBO
START-UP: 50µA
NORMAL: 5µA
0.800V
+
5
SS
70Ω
+
–
10
R1
V
FB
9
7
6
V
V
PRG1
PRG2
R2
GND
GND
8
V
V
V
R1
R2
PRG2
PRG1
OUT
17
GND GND ADJUSTABLE 1.0M
GND
SS
∞
IMPLEMENT DIVIDER
EXTERNALLY FOR
ADJUSTABLE VERSION
SS
GND
SS
5V FIXED 4.2M 800k
3.3V FIXED 2.5M 800k
1.8V FIXED 1.0M 800k
SS
3638 BD
3638fa
7
For more information www.linear.com/LTC3638
LTC3638
(Refer to Block Diagram)
OPERATION
TheLTC3638isastep-downDC/DCregulatorwithinternal
power switch that uses Burst Mode control, combining
low quiescent current with high switching frequency,
which results in high efficiency across a wide range of
load currents. Burst Mode operation functions by us-
ing short “burst” cycles to switch the inductor current
through the internal power MOSFET, followed by a sleep
cycle where the power switch is off and the load current is
supplied by the output capacitor. During the sleep cycle,
the LTC3638 draws only 12µA of supply current. At light
loads, the burst cycles are a small percentage of the total
cycle time which minimizes the average supply current,
greatly improving efficiency. Figure 1 shows an example
of Burst Mode operation. The switching frequency is de-
pendent on the inductor value, peak current, input voltage
and output voltage.
Externalfeedbackresistors(adjustablemode)canbeused
by connecting both V and V to ground.
PRG1
PRG2
In adjustable mode the feedback comparator monitors
the voltage on the V pin and compares it to an internal
FB
800mV reference. If this voltage is greater than the refer-
ence, the comparator activates a sleep mode in which
the power switch and current comparators are disabled,
reducing the V pin supply current to only 12µA. As the
IN
load current discharges the output capacitor, the voltage
on the V pin decreases. When this voltage falls 5mV
FB
below the 800mV reference, the feedback comparator
trips and enables burst cycles.
At the beginning of the burst cycle, the internal high side
power switch (P-channel MOSFET) is turned on and the
inductor current begins to ramp up. The inductor current
increases until either the current exceeds the peak cur-
SLEEP
CYCLE
rent comparator threshold or the voltage on the V pin
FB
SWITCHING
FREQUENCY
exceeds 800mV, at which time the switch is turned off
and the inductor current is carried by the external catch
diode. The inductor current ramps down until the switch
node rises, indicating that the current in the catch diode
BURST
CYCLE
INDUCTOR
CURRENT
is zero. If the voltage on the V pin is still less than the
FB
BURST
FREQUENCY
800mV reference, the power switch is turned on again and
another cycle commences. The average current during a
burst cycle will normally be greater than the average load
current.Forthisarchitecture,themaximumaverageoutput
current is equal to half of the peak current.
OUTPUT
VOLTAGE
∆V
3638 F01
OUT
The hysteretic nature of this control architecture results
in a switching frequency that is a function of the input
voltage, output voltage, and inductor value. This behavior
provides inherent short-circuit protection. If the output is
shorted to ground, the inductor current will decay very
slowly during a single switching cycle. Since the high side
switch turns on only when the inductor current is near
zero,theLTC3638inherentlyswitchesatalowerfrequency
during start-up or short-circuit conditions.
Figure 1. Burst Mode Operation
Main Control Loop
The LTC3638 uses the V
and V
control pins to
PRG2
PRG1
connect internal feedback resistors to the V pin. This
FB
enables fixed outputs of 1.8V, 3.3V or 5V without increas-
ing component count, input supply current or exposure to
noise on the sensitive input to the feedback comparator.
3638fa
8
For more information www.linear.com/LTC3638
LTC3638
(Refer to Block Diagram)
OPERATION
Start-Up and Shutdown
By connecting the FBO pin of a master LTC3638 to the V
FB
pin of one or more slave LTC3638s, the output currents
can be combined to source 250mA times the number of
LTC3638s.
IfthevoltageontheRUNpinislessthan0.7V,theLTC3638
enters a shutdown mode in which all internal circuitry is
disabled,reducingtheDCsupplycurrentto1.4µA.Whenthe
voltageontheRUNpinexceeds1.21V,normaloperationof
the main control loop is enabled. The RUN pin comparator
has 110mV of internal hysteresis, and therefore must fall
below 1.1V to disable the main control loop.
Dropout Operation
When the input supply decreases toward the output sup-
ply, the duty cycle increases to maintain regulation. The
P-channel MOSFET switch in the LTC3638 allows the duty
cycle to increase all the way to 100%. At 100% duty cycle,
the P-channel MOSFET stays on continuously, providing
output current equal to the peak current, which is twice
the maximum load current when not in dropout.
An internal 1ms soft-start function limits the ramp rate of
the output voltage on start-up to prevent excessive input
supply droop. If a longer ramp time and consequently less
supply droop is desired, a capacitor can be placed from
the SS pin to ground. The 5µA current that is sourced
out of this pin will create a smooth voltage ramp on the
capacitor. If this ramp rate is slower than the internal 1ms
soft-start, then the output voltage will be limited by the
ramp rate on the SS pin. The internal and external soft-
start functions are reset on start-up, after an undervoltage
or overvoltage event on the input supply, and after an
overtemperature shutdown.
Input Voltage and Overtemperature Protection
When using the LTC3638, care must be taken not to
exceed any of the ratings specified in the Absolute Maxi-
mum Ratings section. As an added safeguard, however,
the LTC3638 incorporates an overtemperature shutdown
feature.Ifthejunctiontemperaturereachesapproximately
180°C, the LTC3638 will enter thermal shutdown mode.
The power switch will be turned off and the SW node will
become high impedance. After the part has cooled below
160°C, it will restart. The overtemperature level is not
production tested.
Peak Inductor Current Programming
The peak current comparator nominally limits the peak
inductor current to 575mA. This peak inductor current
can be adjusted by placing a resistor from the I pin to
SET
The LTC3638 additionally implements protection features
whichinhibitswitchingwhentheinputvoltageisnotwithin
a programmable operating range. By use of a resistive
divider from the input supply to ground, the RUN and
OVLOpinsserveasapreciseinputsupplyvoltagemonitor.
Switching is disabled when either the RUN pin falls below
1.1V or the OVLO pin rises above 1.21V, which can be
configured to limit switching to a specific range of input
supplyvoltage.Furthermore,iftheinputvoltagefallsbelow
3.5V typical (3.8V maximum), an internal undervoltage
detector disables switching.
ground. The 5µA current sourced out of this pin through
the resistor generates a voltage that adjusts the peak cur-
rent comparator threshold.
During sleep mode, the current sourced out of the I pin
SET
isreducedto1µA.TheI currentisincreasedbackto5µA
SET
on the first switching cycle after exiting sleep mode. The
I
current reduction in sleep mode, along with adding
SET
a filtering network, R
and C , from the I
pin to
ISET
ISET
SET
ground, provides a method of reducing light load output
voltagerippleattheexpenseoflowerefficiencyandslightly
degraded load step transient response.
Whenswitchingisdisabled,theLTC3638cansafelysustain
inputvoltagesuptotheabsolutemaximumratingof140V.
Input supply undervoltage or overvoltage events trigger a
soft-start reset, which results in a graceful recovery from
an input supply transient.
For applications requiring higher output current, the
LTC3638providesafeedbackcomparatoroutputpin(FBO)
for combining the output current of multiple LTC3638s.
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LTC3638
APPLICATIONS INFORMATION
The basic LTC3638 application circuit is shown on the
front page of this data sheet. External component selec-
tion is determined by the maximum load current require-
ment and begins with the selection of the peak current
The internal 5μA current source is reduced to 1μA in sleep
mode to maximize efficiency and to facilitate a tradeoff
between efficiency and light load output voltage ripple, as
describedintheOptimizingOutputVoltageRipplesection.
programming resistor, R . The inductor value L can
ISET
The peakcurrent is internally limited to bewithin the range
then be determined, followed by capacitors C and C
.
IN
OUT
of 40mA to 500mA. Shorting the I
pin to ground pro-
SET
gramsthecurrentlimitto40mA,andleavingitfloatingsets
the current limit to the maximum value of 500mA. When
selecting this resistor value, be aware that the maximum
average output current for this architecture is limited to
halfofthepeakcurrent.Therefore,besuretoselectavalue
that sets the peak current with enough margin to provide
adequate load current under all conditions. Selecting the
peak current to be 2.2 times greater than the maximum
load current is a good starting point for most applications.
Peak Current Resistor Selection
The peak current comparator has a maximum current
limit of at least 500mA, which guarantees a maximum
average current of 250mA. For applications that demand
less current, the peak current threshold can be reduced
to as little as 40mA. This lower peak current allows the
efficiency and component selection to be optimized for
lower current applications.
The peak current threshold is linearly proportional to the
Inductor Selection
voltageontheI pin, with100mVand1Vcorresponding
SET
to 40mA and 500mA peak current respectively. This pin
may be driven by an external voltage source to modulate
thepeakcurrent, whichmaybebeneficialinsomeapplica-
tions. Usually, the peak current is programmed with an
Theinductor,inputvoltage,outputvoltage,andpeakcurrent
determine the switching frequency during a burst cycle of
the LTC3638. For a given input voltage, output voltage,
and peak current, the inductor value sets the switching
frequency during a burst cycle when the output is in regu-
lation. Generally, switching at a frequency between 50kHz
and 200kHz yields high efficiency, and 100kHz is a good
first choice for many applications. The inductor value can
be determined by the following equation:
appropriately chosen resistor (R ) between the I pin
ISET
SET
andground. ThevoltagegeneratedontheI pinbyR
SET
ISET
and the internal 5µA current source sets the peak current.
The value of resistor for a particular peak current can be
computed by using Figure 2 or the following equation:
R
= I
• 400k
PEAK
ISET
VOUT
f•I
VOUT
L =
• 1–
PEAK
where 40mA < I
< 500mA.
V
PEAK
IN
600
500
400
300
200
100
The variation in switching frequency during a burst cycle
withinputvoltageandinductanceisshowninFigure3. For
lower values of I
, multiply the frequency in Figure 3
PEAK
.
TYPICAL PEAK
INDUCTOR
CURRENT
by 575mA/I
PEAK
An additional constraint on the inductor value is the
LTC3638’s 150ns minimum on-time of the switch.
Therefore, in order to keep the current in the inductor
well-controlled, the inductor value must be chosen so that
MAXIMUM
LOAD
CURRENT
0
0
25 50 75 100 125 150 175 200
(kΩ)
R
ISET
3638 F02
Figure 2. RISET Selection
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LTC3638
APPLICATIONS INFORMATION
160
10000
1000
100
I
OPEN
SET
140
120
100
80
L = 47µH
L = 100µH
L = 220µH
60
40
20
0
10
10
30
60
90
0
120
150
100
1000
V
INPUT VOLTAGE (V)
PEAK INDUCTOR CURRENT (mA)
IN
3638 F03
3638 F04
Figure 4. Recommended Inductor Values for Maximum Efficiency
Figure 3. Switching Frequency for VOUT = 3.3V
it is larger than a minimum value which can be computed
as follows:
For applications where board area is not a limiting factor,
inductors with larger cores can be used, which extends
the recommended range of Figure 4 to larger values.
V
IN(MAX) •t
L >
ON(MIN) •1.2
IPEAK
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency regulators generally cannot
affordthecorelossfoundinlowcostpowderedironcores,
forcing the use of the more expensive ferrite cores. Actual
core loss is independent of core size for a fixed inductor
value but is very dependent of the inductance selected.
As the inductance increases, core losses decrease. Un-
fortunately, increased inductance requires more turns of
wire and therefore copper losses will increase.
whereV
isthemaximuminputsupplyvoltagewhen
IN(MAX)
switching is enabled, t
is 150ns, I
is the peak
ON(MIN)
PEAK
current, and the factor of 1.2 accounts for typical inductor
tolerance and variation over temperature.
For applications that have large input supply transients,
the OVLO pin can be used to disable switching above the
maximumoperatingvoltageV
inductor value is not artificially limited by a transient
condition. Inductor values that violate the above equation
will cause the peak current to overshoot and permanent
damage to the part may occur.
sothattheminimum
IN(MAX)
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequently output voltage
ripple. Do not allow the core to saturate!
Although the previous equation provides the minimum
inductorvalue, higherefficiencyisgenerallyachievedwith
a larger inductor value, which produces a lower switching
frequency. For a given inductor type, however, as induc-
tance is increased DC resistance (DCR) also increases.
HigherDCRtranslatesintohighercopperlossesandlower
current rating, both of which place an upper limit on the
inductance. The recommended range of inductor values
for small surface mount inductors as a function of peak
current is shown in Figure 4. The values in this range are a
goodcompromisebetweenthetrade-offsdiscussedabove.
Different core materials and shapes will change the size/
currentandprice/currentrelationshipofaninductor.Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate energy but generally cost more
than powdered iron core inductors with similar charac-
teristics. The choice of which style inductor to use mainly
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LTC3638
APPLICATIONS INFORMATION
depends on the price versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
TDK, Toko, and Sumida.
C and C
Selection
IN
OUT
The input capacitor, C , is needed to filter the trapezoidal
IN
current at the source of the high side MOSFET. C should
IN
be sized to provide the energy required to magnetize the
inductor without causing a large decrease in input voltage
Catch Diode Selection
(∆V ). The relationship between C and ∆V is given by:
IN
IN
IN
Thecatchdiode(D1fromBlockDiagram)conductscurrent
only during the switch off time. Average forward current
in normal operation can be calculated from:
2
L•IPEAK
CIN >
2•V •∆V
IN
IN
V – V
It is recommended to use a larger value for C than
IN
OUT
IN
I
D(AVG) =IOUT
V
calculated by the previous equation since capacitance
decreases with applied voltage. In general, a 1µF X7R ce-
IN
where I
is the output load current. The maximum av-
OUT
ramic capacitor is a good choice for C in most LTC3638
IN
erage diode current occurs with a shorted output at the
high line. For this worst-case condition, the diode current
will approach half of the programmed peak current. The
diode reverse voltage rating should be greater than the
maximum operating input voltage. When the OVLO pin is
used to limit the maximum operating input voltage, the
diode reverse voltage should be greater than the OVLO
pin setting, but may be lower the maximum input voltage
during overvoltage lockout.
applications.
To prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by:
VOUT
V
IN
VOUT
IRMS =IOUT(MAX)
•
•
–1
V
IN
This formula has a maximum at V = 2V , where I =
RMS
IN
OUT
I
/2.Thissimpleworst-caseconditioniscommonlyused
For high efficiency at full load, it is important to select a
catch diode with a low reverse recovery time and low for-
ward voltage drop. As a result, Schottky diodes are often
used as catch diodes. However, Schottky diodes generally
exhibit much higher leakage than silicon diodes. In sleep,
thecatchdiodeleakagecurrentwillappearasloadcurrent,
and may significantly reduce light load efficiency. Diodes
with low leakage often have larger forward voltage drops
at a given current, so a trade-off can exist between light
load and full load efficiency.
OUT
fordesignbecauseevensignificantdeviationsdonotoffer
muchrelief.Notethatripplecurrentratingsfromcapacitor
manufacturers are often based only on 2000 hours of life
which makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required.Severalcapacitorsmayalsobeparalleledtomeet
size or height requirements in the design.
The output capacitor, C , filters the inductor’s ripple
OUT
current and stores energy to satisfy the load current when
the LTC3638 is in sleep. The output ripple has a lower limit
The selection of Schottky diodes with high reverse voltage
ratings is limited relative to that of silicon diodes. There-
fore, for low reverse leakage and part availability, some
applications may prefer a silicon diode. If a silicon diode
is necessary, be sure to select a diode with a specified low
reverse recovery time to maximize efficiency.
of V /160 due to the 5mV typical hysteresis of the feed-
OUT
back comparator. The time delay of the comparator adds
an additional ripple voltage that is a function of the load
current. During this delay time, the LTC3638 continues to
switch and supply current to the output. The output ripple
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LTC3638
APPLICATIONS INFORMATION
can be approximated by:
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
termreliability.CeramiccapacitorshaveexcellentlowESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant input voltage ringing.
I
4•10–6 VOUT
PEAK
2
∆VOUT
≈
–I
LOAD
•
+
COUT
160
Theoutputrippleisamaximumatnoloadandapproaches
lower limit of V /160 at full load. Choose the output
OUT
capacitor C
to limit the output voltage ripple ∆V
OUT
OUT
using the following equation:
Input Voltage Steps
Iftheinputvoltagefallsbelowtheregulatedoutputvoltage,
thebodydiodeoftheinternalMOSFETwillconductcurrent
from the output supply to the input supply. If the input
voltagefallsrapidly, thevoltageacrosstheinductorwillbe
significant and may saturate the inductor. A large current
will then flow through the MOSFET body diode, resulting
in excessive power dissipation that may damage the part.
I
PEAK •2•10–6
VOUT
COUT
≥
∆VOUT
–
160
Thevalueoftheoutputcapacitormustalsobelargeenough
to accept the energy stored in the inductor without a large
change in output voltage during a single switching cycle.
Setting this voltage step equal to 1% of the output voltage,
the output capacitor must be:
If rapid voltage steps are expected on the input supply, put
a small silicon or Schottky diode in series with the V pin
IN
to prevent reverse current and inductor saturation, shown
below as D1 in Figure 5. The diode should be sized for a
reverse voltage of greater than the regulated output volt-
age, and to withstand repetitive currents higher than the
maximum peak current of the LTC3638.
2
L
OUT > •
2
IPEAK
100%
1%
C
•
V
OUT
Typically, a capacitor that satisfies the voltage ripple re-
quirementisadequatetofiltertheinductorripple. To avoid
overheating, the output capacitor must also be sized to
handle the ripple current generated by the inductor. The
worst-case ripple current in the output capacitor is given
LTC3638
D1
L
V
SW
IN
INPUT
SUPPLY
V
OUT
C
OUT
C
by I
= I
/2. Multiple capacitors placed in parallel
IN
RMS
PEAK
3638 F05
maybeneededtomeettheESRandRMScurrenthandling
requirements.
Figure 5. Preventing Current Flow to the Input
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR but
Ceramic Capacitors and Audible Noise
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. Their high ripple
current, high voltage rating, and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
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LTC3638
APPLICATIONS INFORMATION
thepowerissuppliedbyawalladapterthroughlongwires,
voltage, connect V
to SS and V
to GND. For 3.3V,
PRG1
to GND and V
PRG2
a load step at the output can induce ringing at the input,
connect V
to SS. For 1.8V, connect
PRG1
PRG2
V . At best, this ringing can couple to the output and be
both V
and V
to SS. For any of the fixed output
IN
PRG1
PRG2
mistaken as loop instability. At worst, a sudden inrush
voltage options, directly connect the V pin to V
.
FB
OUT
of current through the long wires can potentially cause
For the adjustable output mode (V
the output voltage is set by an external resistive divider
according to the following equation:
= V
= GND),
PRG2
PRG1
a voltage spike at V large enough to damage the part.
IN
For applications with inductive source impedance, such
as a long wire, a series RC network may be required in
R1
R2
parallel with C to dampen the ringing of the input supply.
VOUT = 0.8V • 1+
IN
Figure 6 shows this circuit and the typical values required
todampentheringing. RefertoApplicationNote88forad-
ditionalinformationonsuppressinginputsupplytransients.
The resistive divider allows the V pin to sense a fraction
FB
of the output voltage as shown in Figure 7. The output
Ceramic capacitors are also piezoelectric. The LTC3638’s
burst frequency depends on the load current, and in some
applications the LTC3638 can excite the ceramic capaci-
tor at audio frequencies, generating audible noise. This
noise is typically very quiet to a casual ear; however, if the
noise is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
voltage can range from 0.8V to V . Be careful to keep
IN
the divider resistors very close to the V pin to minimize
FB
noise pick-up on the sensitive V trace.
FB
V
OUT
R1
0.8V
V
FB
LTC3638
R2
V
PRG1
V
PRG2
L
LTC3638
IN
V
IN
3638 F07
LIN
CIN
R=
Figure 7. Setting the Output Voltage with External Resistors
3638 F06
C
IN
4 • C
IN
To minimize the no-load supply current, resistor values in
the megohm range may be used; however, large resistor
values should be used with caution. The feedback divider
is the only load current when in shutdown. If PCB leakage
currenttotheoutputnodeorswitchnodeexceedstheload
current, the output voltage will be pulled up. In normal
operation, this is generally a minor concern since the load
current is much greater than the leakage.
Figure 6. Series RC to Reduce VIN Ringing
Output Voltage Programming
The LTC3638 has three fixed output voltage modes and
an adjustable mode that can be selected with the V
PRG1
and V
pins. The fixed output modes use an internal
PRG2
To avoid excessively large values of R1 in high output volt-
feedback divider which enables higher efficiency, higher
noise immunity, and lower output voltage ripple for 5V,
3.3V, and 1.8V applications. To select the fixed 5V output
age applications (V
≥ 10V), a combination of external
OUT
and internal resistors can be used to set the output volt-
age. This has an additional benefit of increasing the noise
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LTC3638
APPLICATIONS INFORMATION
immunity on the V pin. Figure 8 shows the LTC3638
The RUN and OVLO pins can alternatively be configured
as precise undervoltage (UVLO) and overvoltage (OVLO)
FB
with the V pin configured for a 5V fixed output with an
FB
external divider to generate a higher output voltage. The
internal 5M resistance appears in parallel with R2, and
the value of R2 must be adjusted accordingly. R2 should
be chosen to be less than 200k to keep the output volt-
age variation less than 1% due to the tolerance of the
LTC3638’s internal resistor.
lockoutsontheV supplywitharesistivedividerfromV
IN
IN
toground. Asimpleresistivedividercanbeusedasshown
in Figure 10 to meet specific V voltage requirements.
IN
V
IN
R3
R4
R5
RUN
LTC3638
OVLO
V
OUT
R1
LTC3638
4.2M
V
5V
FB
3638 F10
R2
0.8V
Figure 10. Adjustable UV and OV Lockout
800k
SS
PRG1
PRG2
The current that flows through the R3-R4-R5 divider will
directly add to the shutdown, sleep, and active current of
the LTC3638, and care should be taken to minimize the
impact of this current on the overall efficiency of the ap-
plicationcircuit.Resistorvaluesinthemegohmrangemay
berequiredtokeeptheimpactonquiescentshutdownand
sleep currents low. To pick resistor values, the sum total
V
V
3638 F08
Figure 8. Setting the Output Voltage with
External and Internal Resistors
RUN Pin and Overvoltage/Undervoltage Lockout
The LTC3638 has a low power shutdown mode controlled
by the RUN pin. Pulling the RUN pin below 0.7V puts the
LTC3638 into a low quiescent current shutdown mode
of R3 + R4 + R5 (R
) should be chosen first based
TOTAL
on the allowable DC current that can be drawn from V .
IN
The individual values of R3, R4 and R5 can then be cal-
(I ~ 1.4µA). When the RUN pin is greater than 1.21V,
culated from the following equations:
Q
switching is enabled. Figure 9 shows examples of con-
1.21V
figurations for driving the RUN pin from logic.
R5=RTOTAL
R4=RTOTAL
•
•
Rising V OVLO Threshold
IN
V
IN
1.21V
–R5
SUPPLY
4.7M
1k
Rising V UVLO Threshold
LTC3638
RUN
LTC3638
RUN
IN
R3=RTOTAL –R5–R4
1k
3638 F09
For applications that do not need a precise external OVLO,
the OVLO pin should be tied directly to ground. The RUN
pin in this type of application can be used as an external
UVLO using the previous equations with R5 = 0Ω.
Figure 9. RUN Pin Interface to Logic
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LTC3638
APPLICATIONS INFORMATION
Similarly, for applications that do not require a precise
ramptimecanbesignificant.Therefore,theoutputvoltage
UVLO, theRUNpincanbetiedtoV . Inthisconfiguration,
ramp time from 0V to the regulated V
value is limited
IN
OUT
the UVLO threshold is limited to the internal V UVLO
to a minimum of
IN
thresholdsasshownintheElectricalCharacteristicstable.
The resistor values for the OVLO can be computed using
the previous equations with R3 = 0Ω.
2COUT
IPEAK
Ramp Time ≥
VOUT
Be aware that the OVLO pin cannot be allowed to exceed
its absolute maximum rating of 6V. To keep the voltage
on the OVLO pin from exceeding 6V, the following relation
should be satisfied:
Optimizing Output Voltage Ripple
After the peak current resistor and inductor have been
selected to meet the load current and frequency require-
ments,anoptionalcapacitor,C
canbeaddedinparallel
ISET
R5
with R
to reduce the output voltage ripple dependency
V
•
<6V
ISET
IN(MAX)
R3+R4+R5
on load current.
At light loads the output voltage ripple will be a maximum.
The peak inductor current is controlled by the voltage on
If this equation cannot be satisfied in the application,
connect a 4.7V Zener diode between the OVLO pin and
ground to clamp the OVLO pin voltage.
the I
pin. The current out of the I
pin is 5µA while
SET
SET
the LTC3638 is active and is reduced to 1µA during sleep
mode. The I current will return to 5µA on the first
Soft-Start
SET
switching cycle after sleep mode. Placing a parallel RC
Soft-start is implemented by ramping the effective refer-
ence voltage from 0V to 0.8V. To increase the duration of
the soft-start, place a capacitor from the SS pin to ground.
An internal 5µA pull-up current will charge this capacitor.
The value of the soft-start capacitor can be calculated by
the following equation:
network to ground on the I pin filters the I voltage
SET
SET
as the LTC3638 enters and exits sleep mode, which in
turn will affect the output voltage ripple, efficiency, and
load step transient performance.
Higher Current Applications
5µA
CSS =Soft-Start Time •
0.8V
For applications that require more than 250mA, the
LTC3638 provides a feedback comparator output pin
(FBO) for driving additional LTC3638s. When the FBO pin
The minimum soft-start time is limited to the internal
soft-start timer of 1ms. When the LTC3638 detects a
fault condition (input supply undervoltage/overvoltage or
overtemperature) or when the RUN pin falls below 1.1V,
the SS pin is quickly pulled to ground and the internal
soft-start timer is reset. This ensures an orderly restart
when using an external soft-start capacitor.
of a master LTC3638 is connected to the V pin of one
FB
or more slave LTC3638s, the master controls the burst
cycle of the slaves.
Figure 11 shows an example of a 5V, 500mA regulator
using two LTC3638s. The master is configured for a 5V
fixed output with external soft-start and V UVLO/OVLO
IN
levels set by the RUN and OVLO pins. Since the slave is
Note that the soft-start capacitor may not be the limiting
factor in the output voltage ramp. The maximum output
current, which is equal to half of the peak current, must
charge the output capacitor from 0V to its regulated value.
For small peak currents or large output capacitors, this
directly controlled by the master, its SS pin should be
floating, RUN should be tied to V , and OVLO should be
IN
tiedtoground.Furthermore,theslaveshouldbeconfigured
for a 1.8V fixed output (V
= V
= SS) to set the
PRG1
PRG2
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LTC3638
APPLICATIONS INFORMATION
L1
V
OUT
The junction temperature is given by:
T = T + T
5V
V
IN
SW
V
IN
500mA
C
C
OUT
IN
D1
LTC3638
J
A
R
R3
(MASTER)
V
Generally, the worst-case power dissipation is in dropout
at low input voltage. In dropout, the LTC3638 can provide
a DC current as high as the full 575mA peak current to the
output. At low input voltage, this current flows through a
higher resistance MOSFET, which dissipates more power.
FB
RUN
SS
R4
R5
C
SS
V
PRG1
V
PRG2
OVLO
FBO
V
IN
V
FB
Asanexample,considertheLTC3638indropoutataninput
voltage of 5V, a load current of 575mA and an ambient
temperatureof85°C.FromtheTypicalPerformancegraphs
LTC3638
(SLAVE)
L2
D2
SW
SS
RUN
of Switch On-Resistance, the R
of the top switch
DS(ON)
V
PRG1
V
PRG2
OVLO
at V = 5V and 100°C is approximately 3.2Ω. Therefore,
IN
the power dissipated by the part is:
FBO
3638 F11
2
2
P = (I
) • R
= (575mA) • 3.2Ω = 1.06W
D
LOAD
DS(ON)
Figure 11. 5V, 500mA Regulator
For the MSOP package the θ is 40°C/W. Thus, the junc-
JA
V pin threshold at 1.8V. The inductors L1 and L2 do not
FB
tion temperature of the regulator is:
necessarily have to be the same, but should both meet
40°C
W
the criteria described in the Inductor Selection section.
TJ = 85°C+1.06W•
=127°C
Thermal Considerations
which is below the maximum junction temperature of
150°C.
Inmostapplications,theLTC3638doesnotdissipatemuch
heat due to its high efficiency. But, in applications where
the LTC3638 is running at high ambient temperature with
low supply voltage and high duty cycles, such as dropout,
the heat dissipated may exceed the maximum junction
temperature of the part.
NotethatthewhiletheLTC3638isindropout,itcanprovide
output current that is equal to the peak current of the part.
This can increase the chip power dissipation dramatically
and may cause the internal overtemperature protection
circuitry to trigger at 180°C and shut down the LTC3638.
To prevent the LTC3638 from exceeding the maximum
junctiontemperature,theuserwillneedtodosomethermal
analysis. The goal of the thermal analysis is to determine
whetherthepowerdissipatedexceedsthemaximumjunc-
tion temperature of the part. The temperature rise from
ambient to junction is given by:
Pin Clearance/Creepage Considerations
TheLTC3638MSEpackagehasbeenuniquelydesignedto
meet high voltage clearance and creepage requirements.
Pins 2, 4, 13, and 15 are omitted to increase the spac-
ing between adjacent high voltage solder pads (V , SW,
IN
and RUN) to a minimum of 0.657mm which is sufficient
for most applications. For more information, refer to the
printed circuit board design standards described in IPC-
2221 (www.ipc.org).
T = P • θ
JA
R
D
Where P is the power dissipated by the regulator and
D
θ
is the thermal resistance from the junction of the die
JA
to the ambient temperature.
3638fa
17
For more information www.linear.com/LTC3638
LTC3638
APPLICATIONS INFORMATION
Design Example
also be rated for an average forward current of at least:
90V –12V
As a design example, consider using the LTC3638 in an
ID(AVG) = 250mA
= 217mA
application with the following specifications: V = 36V
IN
90V
to 72V (48V nominal), V
= 12V, I
= 250mA, f =
OUT
OUT
During a short-circuit, the average current in the diode
200kHz,andthatswitchingisenabledwhenV isbetween
IN
could be as high as I /2, or 288mA. For margin, select
PEAK
30V and 90V.
a catch diode with a reverse breakdown of at least 100V
First, calculate the inductor value based on the switching
frequency:
and an average current of 350mA or higher.
C
OUT
will be selected based on a value large enough to
12V
12V
48V
satisfy the output voltage ripple requirement. For a 1%
output ripple (120mV), the value of the output capacitor
can be calculated from:
L =
• 1–
≅ 78µH
200kHz •0.575A
Choose a 100µH inductor as a standard value. Next, verify
0.575A •2•10–6
that this meets the L
input voltage:
requirement at the maximum
MIN
COUT
≥
≅ 26µF
12V
120mV –
160
90V •150ns
0.575A
LMIN
=
•1.2= 28µH
C
OUT
also needs an ESR that will satisfy the output voltage
ripple requirement. The required ESR can be calculated
from:
Therefore, the minimum inductor requirement is satisfied
and the 100μH inductor value may be used.
120mV
0.575A
Next,C andC
areselected.Forthisdesign,C should
IN
IN
OUT
ESR<
≅ 208mΩ
be sized for a current rating of at least:
A 33µF ceramic capacitor has significantly less ESR than
208mΩ. The output voltage can now be programmed by
choosing the values of R1 and R2. Since the output volt-
age is higher than 10V, the LTC3638 should be set for a
5V fixed output with an external divider to divide the 12V
output down to 5V. R2 is chosen to be less than 200k
to keep the output voltage variation to less than 1% due
to the internal 5M resistor tolerance. Set R2 = 196k and
calculate R1 as:
12V
36V
36V
12V
I
RMS = 250mA •
•
–1≅118mARMS
The value of C is selected to keep the input from droop-
ing less than 360mV (1%) at low line:
IN
100µH•0.575A2
2•36V •360mV
CIN >
≅1.3µF
Since the capacitance of capacitors decreases with DC
bias, a 2.2µF capacitor should be chosen.
12V –5V
R1=
• 196kΩ5MΩ = 264kΩ
(
)
5V
The catch diode should have a reverse voltage rating of
greaterthantheovervoltagelockoutsettingof90V.Itshould
Choose a standard value of 267k for R1.
3638fa
18
For more information www.linear.com/LTC3638
LTC3638
APPLICATIONS INFORMATION
The undervoltage and overvoltage lockout requirements
PC Board Layout Checklist
on V can be satisfied with a resistive divider from V to
IN
IN
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3638. Check the following in your layout:
the RUN and OVLO pins (refer to Figure 10). Choose R3 +
R4 + R5 = 2.5M to minimize the loading on V . Calculate
IN
R3, R4 and R5 as follows:
1. Largeswitchedcurrentsflowinthepowerswitch,catch
diode, and input capacitor. The loop formed by these
components should be as small as possible. A ground
plane is recommended to minimize ground impedance.
1.21V •2.5MΩ
R5=
R4=
= 33.6k
V
IN_OV(RISING)
1.21V •2.5MΩ
–R5= 67.2k
V
2. Connect the (+) terminal of the input capacitor, C , as
IN
IN_UV(RISING)
close as possible to the V pin. This capacitor provides
IN
R3= 2.5MΩ–R4–R5= 2.4M
the AC current into the internal power MOSFET.
Since specific resistor values in the megohm range are
generally less available, it may be necessary to scale R3,
R4, and R5 to a standard value of R3. For this example,
choose R3 = 2.2M and scale R4 and R5 by 2.2M/2.4M.
Then, R4 = 61.6k and R5 = 30.8k. Choose standard values
of R3 = 2.2M, R4 = 62k, and R5 = 30.9k. Note that the fall-
ing thresholds for both UVLO and OVLO will be 10% less
than the rising thresholds, or 27V and 81V respectively.
3. Keep the switching node, SW, away from all sensitive
smallsignalnodes.Therapidtransitionsontheswitching
node can couple to high impedance nodes, in particular
V , and create increased output ripple.
FB
L1
V
V
SW
LTC3638
V
OUT
IN
IN
R3
R4
R5
R1
R2
V
FB
RUN
The I pin should be left open in this example to select
FBO
SET
D1
C
C
OUT
IN
OVLO
SS
I
SET
maximum peak current (575mA). Figure 12 shows a
V
PRG1
V
PRG2
complete schematic for this design example.
R
ISET
C
SS
GND
100µH
V
OUT
V
IN
12V
V
SW
IN
36V TO 72V
250mA
2.2M
62k
LTC3638
267k
V
FB
RUN
FBO
I
SET
SS
L1
2.2µF
33µF
OVLO
196k
V
PRG1
GND
D1
30.9k
V
PRG2
GND
V
OUT
C
C
OUT
3638 F12
IN
Figure 12. 36V to 72V Input to 12V Output, 250mA Regulator
R3
R5
V
IN
R4
R
ISET
C
SS
R2R1
GND
3638 F13
VIAS TO GROUND PLANE
VIAS TO INPUT SUPPLY (V
)
IN
VIAS TO OUTPUT SUPPLY (V
)
OUT
OUTLINE OF LOCAL GROUND PLANE
Figure 13. Example PCB Layout
3638fa
19
For more information www.linear.com/LTC3638
LTC3638
TYPICAL APPLICATIONS
Efficiency vs Input Voltage
L1
330µH
V
*
100
95
90
85
80
75
70
65
60
OUT
V
I
= 100mA
OUT
IN
V
SW
5V
IN
4V TO 140V
250mA
LTC3638
V
FB
RUN
FBO
C
IN
C
OUT
22µF
1µF
V
= 5V
OUT
SS
I
SET
250V
D1
V
V
OVLO
PRG1
PRG2
V
= 3.3V
OUT
GND
3638 F14
V
= 1.8V
OUT
60
C
C
: TDK C5750X7R2E105K
OUT
*V
= V FOR V < 5V
OUT IN IN
IN
: TDK C3216X5R0J226MT
0
30
90
120
150
L1: COILCRAFT MSS1278T-334KL
D1: DIODES INC PDS3200
V
INPUT VOLTAGE (V)
IN
3638 F14b
Figure 14. High Efficiency 250mA Regulator
L1
68µH
V
*
OUT
V
IN
V
SW
3.3V
250mA
IN
4V TO 140V
Soft-Start Waveform
LTC3638
30Ω LOAD
V
FB
RUN
FBO
SS
C
IN
1µF
D1
OUTPUT
VOLTAGE
500mV/DIV
I
SET
250V
220pF
V
V
PRG2
PRG1
C
OUT
OVLO
470nF
220k
100µF
GND
3638 F15
3638 F15b
10ms/DIV
C
C
: MURATA GRM55DR72E105KW01L
OUT
IN
: MURATA GRM43SR60J107ME20
L1: SUMIDA CDRH8D28NP-680NC
D1: VISHAY U1D
Figure 15. Low Output Voltage Ripple 250mA Regulator with 75ms Soft-Start
4V to 125V Input to –15V Output Positive-to-Negative Regulator
Maximum Load Current
vs Input Voltage
L1
220µH
250
200
150
100
50
V
IN
V
SW
V
= –5V
OUT
IN
4V TO 125V
C
IN
LTC3638
200k
102k
1µF
250V
V
FB
RUN
V
= –15V
OUT
FBO
C
10µF
25V
OUT
SS
I
SET
D1
V
V
OVLO
PRG1
PRG2
GND
V
OUT
–15V
V
IPEAK
2
3638 TA04a
IN
MAXIMUM LOAD CURRENT ≈
•
V
IN + VOUT
0
30
60
90
120
150
MAXIMUM INPUT VOLTAGE = 140 –|V
|
OUT
V
INPUT VOLTAGE (V)
IN
3638 TA04b
C
C
: KEMET C2225C105KARACTU
OUT
IN
: AVX 12103C106MAT
L1: TDK SLF12555-221MR72
D1: ST MICRO STTH102A
3638fa
20
For more information www.linear.com/LTC3638
LTC3638
TYPICAL APPLICATIONS
4V to 90V Input to 12V/500mA Output Regulator with Overvoltage Lockout
L1
Low Dropout Startup and
Shutdown
47µH
V
*
OUT
V
IN
V
SW
12V
500mA
IN
4V TO 90V
UP TO 140V
TRANSIENT
LTC3638
267k
(MASTER)
V
1M
IN
V
FB
RUN
OVLO
SS
V
V
V
/V
IN OUT
V
OUT
5V/DIV
C
C
IN1
OUT
1µF
47µF
16V
D1
I
SET
200V
FBO
PRG1
X5R
L1 CURRENT
500mA/DIV
13.7k
196k
PRG2
GND
L2 CURRENT
500mA/DIV
3638 TA05b
1s/DIV
L2
47µH
V
IN
SW
LTC3638
(SLAVE)
Overvoltage Lockout Operation
V
FB
RUN
TRANSIENT TO 140V
72V
OVLO
SS
C
IN2
V
IN
50V/DIV
V
D2
1µF
I
SET
200V
OUT
10V/DIV
V
V
FBO
PRG2
PRG1
GND
L1 CURRENT
500mA/DIV
3638 TA05a
L2 CURRENT
500mA/DIV
3638 TA05c
C
C
/C : VISHAY VJ2225Y105KXCA
200ms/DIV
IN1 IN2
: TAIYO YUDEN EMK325 BJ 476MM-T
OUT
L1/L2: WÜRTH 744 778 914 7
D1/D2: CENTRAL SEMI CMSH1-100M-LTN
*V
= V FOR V < 12V
IN IN
OUT
3638fa
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For more information www.linear.com/LTC3638
LTC3638
TYPICAL APPLICATIONS
6W LED Driver
Efficiency vs Input Voltage
100
95
90
85
80
L1
100µH
PWM OPEN
DIM
V
OPEN
V
IN
V
V
SW
OUT
IN
32V TO 140V
1M
LTC3638
1M
V
FB
RUN
FBO
C
4.7µF
50V
C
IN
OUT
24V LED
250mA
D1
1µF
OVLO
250V
V
I
V
V
DIM
SET
PRG1
PRG2
SS
27.4k
42.2k
GND
M1
PWM
3638 TA03a
3.3V
30
60
90
INPUT VOLTAGE (V)
IN
120
150
V
C
C
: TDK C5750X7R2E105K
: TDK C4532X7R1H475M
L1: TDK SLF10145T-101M
D1: TOSHIBA CRH01
M1: VISHAY SILICONIX Si2356DS
V
= 0.1V TO 1V FOR 10:1 ANALOG DIMMING
DIM
PWM = SQUARE WAVE FOR DIGITAL DIMMING
30V OVERVOLTAGE PROTECTION ON V
IN
OUT
3638 TA03b
OUT
36V to 140V to 36V/250mA with 75mA Input Current Limit
Maximum Load and Input Current
vs Input Voltage
L1
100µH
300
250
200
150
100
50
V
OUT
V
IN
36V
250mA*
V
SW
IN
36V TO 140V
LTC3638
220k
MAXIMUM LOAD CURRENT
R1
470k
V
FB
RUN
C
C
OUT
IN
D1
1µF
4.7µF
I
SS
SET
250V
50V
FBO
OVLO
V
V
35.7k
PRG1
PRG2
R2
4.02k
GND
MAXIMUM INPUT CURRENT
3638 TA06a
0
VOUT
4
R2
R1+R2
5µA •R1 VOUT
R2
R1+R2
INPUT CURRENT LIMIT =
•
• 1+
≈
•
40 50 60 70 80 90 100 110 120 130 140 150
V
4
IN
V
INPUT VOLTAGE (V)
IN
V
3638 TA06b
IN
*MAXIMUM LOAD CURRENT =
•75mA ≤250mA
36V
C
C
: TDK C5750X7R2E105K
: TDK C4532X7R1H475M
L1: TDK SLF12555T-101M1R1
D1: ROHM RF101L2S
IN
OUT
3638fa
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For more information www.linear.com/LTC3638
LTC3638
TYPICAL APPLICATIONS
Burst Frequency vs Load Current
100
10
WITH BURST FREQUENCY LIMIT
1
5V to 140V Input to 5V/250mA Output with 20kHz Minimum Burst Frequency
WITHOUT BURST FREQUENCY LIMIT
0.1
0.01
L1
100µH
V
OUT
V
IN
V
SW
5V
IN
V
= 48V
IN
5V TO 140V
250mA
D1
953k
100k
10Ω
LTC3638
C
+
IN
0.1
1
10
100
1000
V
C
OUT
22µF
1µF
V
FB
RUN
LOAD CURRENT (mA)
LTC6994-1
250V
3638 TA08b
2N7000
I
FBO
IN
OUT
SET
V
V
PRG2
PRG1
DIV
SET
Input Current vs Load Current
OVLO
SS
GND
200k
100
10
GND
V
= 48V
IN
3638 TA08a
WITH BURST FREQUENCY LIMIT
C
C
: AVX 2225PC105MAT1A
: KEMET C1206C226K9PAC
L1: COILTRONICS DR74-101-R
D1: DIODES INC MURS120-13-F
IN
OUT
1
0.1
0.01
WITHOUT BURST FREQUENCY LIMIT
0.1
1
10
100
1000
LOAD CURRENT (mA)
3638 TA08c
3638fa
23
For more information www.linear.com/LTC3638
LTC3638
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSE Package
Variation: MSE16 (12)
16-Lead Plastic MSOP with 4 Pins Removed
Exposed Die Pad
(Reference LTC DWG # 05-08-1871 Rev D)
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
0.889 ±0.127
(.035 ±.005)
1
8
0.35
REF
5.10
(.201)
MIN
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
3.20 – 3.45
(.126 – .136)
0.12 REF
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
DETAIL “B”
16
9
0.305 ±0.038
0.50
NO MEASUREMENT PURPOSE
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
(.0120 ±.0015)
(.0197)
1.0
(.039)
BSC
TYP
BSC
0.280 ±0.076
(.011 ±.003)
16 14 121110
9
RECOMMENDED SOLDER PAD LAYOUT
REF
DETAIL “A”
0.254
(.010)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0° – 6° TYP
4.90 ±0.152
(.193 ±.006)
GAUGE PLANE
0.53 ±0.152
(.021 ±.006)
1
3 5 6 7 8
1.0
DETAIL “A”
0.86
(.034)
REF
1.10
(.043)
MAX
0.18
(.007)
(.039)
BSC
SEATING
PLANE
0.17 – 0.27
(.007 – .011)
TYP
0.1016 ±0.0508
(.004 ±.002)
MSOP (MSE16(12)) 0213 REV D
0.50
(.0197)
BSC
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
3638fa
24
For more information www.linear.com/LTC3638
LTC3638
REVISION HISTORY
REV
DATE
DESCRIPTION
PAGE NUMBER
A
12/14 Clarified OVLO Pin Function
Clarified Related Parts List
6
24
3638fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
25
LTC3638
TYPICAL APPLICATION
12V/250mA Automotive Supply
Efficiency and Power Loss vs
Load Current
L1
220µH
100
90
80
70
60
50
40
30
20
10
0
V
OUT
EFFICIENCY
V
IN
12V*
V
SW
IN
4V TO 140V
250mA
LTC3638
267k
196k
V
V
V
= 24V
= 48V
= 120V
IN
IN
IN
V
FB
RUN
FBO
C
IN
1000
100
10
C
OUT
1µF
250V
X7R
SS
I
10µF
SET
16V
X7R
D1
V
V
OVLO
PRG1
POWER LOSS
PRG2
GND
*V
= V FOR V < 12V
IN IN
OUT
3638 TA07
L1: COILCRAFT MSS1246T-224KL
D1: DIODES INC SBR1U200P1-7
1
1000
0.1
1
10
100
LOAD CURRENT (mA)
3638 TA07b
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
V : 4V to 140V, V
LTC7138
140V, 400mA Micropower Step-Down Regulator
= 0.8V, I = 12μA, I = 1.4μA, MSE16 Package
IN
OUT(MIN)
= 0.8V, I = 12µA, I = 1.4µA, MS16E Package
OUT(MIN) Q SD
Q
SD
LTC3639
150V, 100mA Synchronous Micropower Step-Down V : 4V to 150V, V
IN
DC/DC Regulator
LTC3630
LTC3637
LTC3630A
LTC3810
65V, 500mA Synchronous Step-Down DC/DC
Regulator
V : 4V to 65V, V
= 0.8V, I = 12µA, I = 5µA,
IN
OUT(MIN) Q SD
3mm × 5mm DFN16, MSOP16E Packages
76V, 1A Synchronous Step-Down DC/DC Regulator V : 4V to 76V, V
= 0.8V, I = 12µA, I = 3µA,
IN
OUT(MIN)
Q
SD
3mm × 5mm DFN16, MSOP16E Packages
76V, 500mA Synchronous Step-Down DC/DC
Regulator
V : 4V to 76V, V = 0.8V, I = 12µA, I = 5µA,
IN
OUT(MIN)
Q
SD
3mm × 5mm DFN16, MSOP16E Packages
100V Synchronous Step-Down DC/DC Controller
V : 6.4V to 100V, V
= 0.8V, I = 2mA, I < 240µA,
OUT(MIN) Q SD
IN
SSOP28 Package
LTC3631/LTC3631-
3.3 LTC3631-5
45V (Transient to 60V), 100mA Synchronous Step- V : 4.5V to 45V, V
= 0.8V, I = 12µA, I < 3µA,
IN
OUT(MIN) Q SD
Down DC/DC Regulator
3mm × 3mm DFN8, MSOP8 Packages
LTC3642
LTC3632
LTC3891
45V (Transient to 60V), 50mA Synchronous Step-
Down DC/DC Regulator
V : 4.5V to 45V, V = 0.8V, I = 12µA, I < 3µA,
IN
OUT(MIN)
Q
SD
3mm × 3mm DFN8, MSOP8 Packages
50V (Transient to 60V), 20mA Synchronous Step-
Down DC/DC Regulator
V : 4.5V to 45V, V = 0.8V, I = 12µA, I < 3µA,
IN
OUT(MIN)
Q
SD
3mm × 3mm DFN8, MSOP8 Packages
60V Synchronous Step-Down DC/DC Controller with V : 4V to 60V, V
Burst Mode Operation
= 0.8V, I = 50µA, I < 14µA,
IN
OUT(MIN)
Q
SD
3mm × 4mm QFN20, TSSOP20E Packages
3638fa
LT 1214 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
26
(408)432-1900 FAX: (408) 434-0507 www.linear.com/LTC3638
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LINEAR TECHNOLOGY CORPORATION 2014
相关型号：
LTC3639HMSE#PBF
LTC3639 - High Efficiency, 150V 100mA Synchronous Step-Down Regulator; Package: MSOP; Pins: 16; Temperature Range: -40°C to 125°C
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LTC3642EDD#PBF
LTC3642 - High Efficiency, High Voltage 50mA Synchronous Step-Down Converter; Package: DFN; Pins: 8; Temperature Range: -40°C to 85°C
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LTC3642EDD-3.3#PBF
LTC3642 - High Efficiency, High Voltage 50mA Synchronous Step-Down Converter; Package: DFN; Pins: 8; Temperature Range: -40°C to 85°C
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