LTC1265CS-5#TR [Linear]
LTC1265 - 1.2A, High Efficiency Step-Down DC/DC Converter; Package: SO; Pins: 14; Temperature Range: 0°C to 70°C;型号: | LTC1265CS-5#TR |
厂家: | Linear |
描述: | LTC1265 - 1.2A, High Efficiency Step-Down DC/DC Converter; Package: SO; Pins: 14; Temperature Range: 0°C to 70°C 开关 光电二极管 |
文件: | 总16页 (文件大小:230K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LTC1265/LTC1265-3.3/LTC1265-5
1.2A, High Efficiency
Step-Down DC/DC Converter
U
FEATURES
DESCRIPTIO
The LTC®1265 is a monolithic step-down current mode
DC/DC converter featuring Burst Mode TM operation at low
output current. The LTC1265 incorporates a 0.3Ω switch
(VIN =10V) allowing up to 1.2A of output current.
■
High Efficiency: Up to 95%
■
Current Mode Operation for Excellent Line and Load
Transient Response
■
Internal 0.3
Ω Power Switch (VIN = 10V)
■
■
■
■
■
■
■
Short-Circuit Protection
Under no load condition, the converter draws only 160µA.
In shutdown it typically draws a mere 5µA making this
converter ideal for current sensitive applications. In drop-
out the internal P-channel MOSFET switch is turned on
continuouslymaximizingthelifeofthebatterysource.The
LTC1265incorporatesautomaticpowersavingBurstMode
operation to reduce gate charge losses when the load
currents drop below the level required for continuous
operation.
Low Dropout Operation: 100% Duty Cycle
Low-Battery Detector
Low 160µA Standby Current at Light Loads
Active-High Micropower Shutdown: IQ < 15µA
Peak Inductor Current Independent of Inductor Value
Available in 14-pUin SO Package
APPLICATIO S
■
5V to 3.3V Conversion
Theinductorcurrentisuser-programmableviaanexternal
current sense resistor. Operation up to 700kHz permits
the use of small surface mount inductors and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
■
Distributed Power Systems
■
Step-Down Converters
■
Inverting Converters
■
Memory Backup Supply
■
Portable Instruments
■
Battery-Powered Equipment
■
Cellular Telephones
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TYPICAL APPLICATIO
V
IN
5.4V TO
12V
LTC1265-5 Efficiency
†††
+
C
IN
100
68µF
20V
0.1µF
L1*
R
**
SENSE
V
PWR V
SHDN
IN
IN
33µH
V
5V
1A
0.1Ω
OUT
V
= 6V
= 9V
IN
SW
95
90
85
80
75
70
V
IN
D1†
LTC1265-5
1k
††
+
C
OUT
I
TH
220µF
PGND
V
IN
= 12V
10V
3900pF
+
C
T
SENSE
130pF
1000pF
L = 33µH
V = 5V
OUT
–
SENSE
R
T
= 0.1Ω
* COILTRONICS CTX33-4
SENSE
SGND
C
= 130pF
** IRC LRC2010-01-R100-J
†
MBRS130LT3
†† AVX TPSE227K010
††† AVX TPSE686K020
0.01
0.10
LOAD CURRENT (A)
1.00
LTC1265-FO1
LTC1265 TA01
Figure 1. High Efficiency Step-Down Converter
1
LTC1265/LTC1265-3.3/LTC1265-5
W W U W
ABSOLUTE AXI U RATI GS
(Voltages Refer to GND Pin) (Note 1)
U W
U
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
Input Supply Voltage (Pins 1, 2, 13)..........–0.3V to 13V
DC Switch Current (Pin 14) .................................... 1.2A
Peak Switch Current (Pin 14) ................................. 1.6A
Switch Voltage (Pin 14) ..................................VIN – 13.0
Operating Temperature Range
LTC1265C ............................................... 0° to 70°C
LTC1265I ........................................ –40°C to 85°C
Junction Temperature (Note 2)............................. 125°C
Storage Temperature Range ....................–65° to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
PWR V
1
2
3
4
5
6
7
14
13
12
11
10
9
SW
IN
IN
V
PWR V
PGND
SGND
SHDN
IN
LTC1265CS
LB
OUT
LTC1265CS-5
LTC1265CS-3.3
LTC1265IS
LB
IN
C
T
I
N/C (V *)
FB
TH
–
+
SENSE
8
SENSE
S PACKAGE
14-LEAD PLASTIC SO
*ADJUSTABLE OUTPUT VERSION
= 125°C, θ = 110°C/W
T
JMAX
JA
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 0V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
LTC1265
MIN
TYP
MAX
UNITS
I
Feedback Current into Pin 9
Feedback Voltage
0.2
1
µA
FB
V
LTC1265C
●
●
1.22
1.20
1.25
1.25
1.28
1.30
V
V
FB
V
= 9V, LTC1265I
IN
V
Regulator Output Voltage
LTC1265-3.3: I
= 800mA
LOAD
●
●
3.22
4.9
3.3
5
3.40
5.2
V
V
OUT
LTC1265-5: I
= 800mA
LOAD
∆V
Output Voltage Line Regulation
Output Voltage Load Regulation
V
= 6.5V to 10V, I = 800mA
LOAD
–40
0
40
mV
OUT
IN
LTC1265-3.3: 10mA < I
LTC1265-5: 10mA < I
< 800mA
40
60
65
100
mV
mV
LOAD
< 800mA
LOAD
Burst Mode Operation Output Ripple
Input DC Supply Current (Note 3)
I
= 0mA
50
mV
P-P
LOAD
I
Active Mode: 3.5V < V < 10V
Sleep Mode: 3.5V < V < 10V
Sleep Mode: 5V < V < 10V (LTC1265-5)
Shutdown: V
1.8
160
160
5
2.4
230
230
15
mA
µA
µA
µA
Q
IN
IN
IN
= V , 3.5V < V < 10V
SHDN
IN
IN
V
Low-Battery Trip Point
Current into Pin 4
1.15
0.5
1.25
1.35
0.5
V
LBTRIP
I
I
µA
LBIN
Current Sunk by Pin 3
V
V
= 0.4V, V = 0V
LBIN
1.0
1.5
1.0
mA
µA
LBOUT
LBOUT
LBOUT
= 5V, V
= 10V
LBIN
–
V – V
Current Sense Threshold Voltage
LTC1265: V
V
LTC1265-3.3: V
= 5V, V = V /4 + 25mV (Forced)
25
150
25
150
25
mV
mV
mV
mV
mV
mV
8
7
SENSE
9
OUT
–
= 5V, V = V /4 – 25mV (Forced)
130
130
130
180
180
180
SENSE
9
OUT
–
= V
= V
OUT
+ 100mV (Forced)
– 100mV (Forced)
+ 100mV (Forced)
– 100mV (Forced)
SENSE
OUT
–
V
SENSE
–
OUT
LTC1265-5: V
V
= V
SENSE
SENSE
–
= V
150
OUT
R
ON Resistance of Switch
LTC1265C
LTC1265I
●
0.3
0.3
0.60
0.70
Ω
Ω
ON
–
I
t
C Pin Discharge Current
V
V
in Regulation, V
= 0V
= V
OUT
40
60
2
100
10
µA
µA
5
T
OUT
OUT
SENSE
Switch Off Time (Note 4)
C = 390pF, I
C = 390pF, I
= 800mA (LTC1265C)
= 800mA (LTC1265I)
●
●
4
3.5
5
5
6
7
µs
µs
OFF
T
LOAD
LOAD
T
2
LTC1265/LTC1265-3.3/LTC1265-5
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 10V, VSHDN = 0V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Shutdown Pin High
Shutdown Pin Low
Shutdown Pin Input Current
Min Voltage at Pin 10 for Device to be in Shutdown
Max Voltage at Pin 10 for Device to be Active
1.2
V
V
IH
IL
0.6
0.5
I
V
= 8V
SHDN
µA
10
Note 3: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 4: In applications where R
off time increases by approximately 40%.
is placed at ground potential, the
Note 2: T is calculated from the ambient temperature T and power
SENSE
J
A
dissipation P according to the following formulas:
D
LTC1265CS, LTC1265CS-3.3, LTC1265CS-5:
T = T + (P • 110°C/W)
J
A
D
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TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Input Voltage
(VOUT = 5V)
Efficiency vs Input Voltage
(VOUT = 3.3V)
Efficiency vs Load Current
100
98
96
94
92
90
88
86
84
82
80
100
98
96
94
92
90
88
86
84
82
80
100
95
90
85
80
75
70
LTC1265-3.3
R
= 0.1Ω
SENSE
T
COIL = CTX33-4
C
= 130pF
V
= 5V
IN
I
= 250mA
LOAD
I
= 250mA
V
= 9V
LOAD
IN
I
= 800mA
LOAD
V
IN
= 12V
I
= 800mA
LTC1265-3.3
= 3.3V
LOAD
V
OUT
LTC1265-5
R = 0.1Ω
SENSE
R
= 0.1Ω
SENSE
T
C
= 130pF
C = 130pF
T
COIL = CTX33-4
COIL = CTX33-4
4
5
6
7
8
9
10 11 12 13
0.01
0.10
1.00
11
4
5
6
7
8
9
10
12 13
INPUT VOLTAGE (V)
LOAD CURRENT (A)
INPUT VOLTAGE (V)
LTC1265 G03
1265 G01
1265 G02
Operating Frequency
vs (VIN – VOUT
Switch Leakage Current
Switch Resistance
)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1.2
1.0
0.8
0.6
0.4
0.2
0
300
270
V
IN
= 12V
0°C
25°C
240
210
70°C
T = 125°C
J
180
150
T = 70°C
J
120
90
60
30
0
T = 25°C
J
T = 0°C
J
0
0
1
2
3
4
5
6
7
8
9
10
3
10 11 12 13
INPUT VOLTAGE (V)
4
5
6
7
8
9
0
20
40
60
80
100
(V
–
V
) VOLTAGE (V)
OUT
IN
TEMPERATURE (°C)
1265 G04
1265 G05
1265 G06
3
LTC1265/LTC1265-3.3/LTC1265-5
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TYPICAL PERFOR A CE CHARACTERISTICS
DC Supply Current
Supply Current in Shutdown
Gate Charge Losses
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
8
7
6
5
4
3
2
1
0
DOES NOT INCLUDE
GATE CHARGE
SHUTDOWN = 3V
T
= 25C
A
V
IN
= 12V
ACTIVE MODE
V
= 9V
= 6V
IN
V
IN
SLEEP MODE
0
2
4
6
8
10
12
14
0
200
400
600
800
1000
3
4
5
6
7
8
9
10 11 12 13
FREQUENCY (kHz)
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
1265 G07
1265 G09
1265 G08
U
U
U
PI FU CTIO S
PWR VIN (Pins 1, 13): Supply for the Power MOSFET and
itsDriver.Mustdecouplethispinproperlytoground.Must
always tie Pins 1 and 13 together.
SENSE+ (Pin 8): The (+) Pin to the Current Comparator. A
built-in offset between Pins 7 and 8 in conjunction with
RSENSE sets the current trip threshold.
VIN (Pin 2): Main Supply for All the Control Circuitry in the
N/C,VFB (Pin 9): For the LTC1265 adjustable version, this
pin serves as the feedback pin from an external resistive
dividerusedtosettheoutputvoltage. OntheLTC1265-3.3
and LTC1265-5 versions, this pin is not used.
LTC1265.
LBOUT (Pin 3): Open-Drain Output of the Low-Battery
Comparator. This pin will sink current when Pin 4 (LBIN)
goes below 1.25V. During shutdown, this pin is high
impedance.
SHDN (Pin 10): Pulling this pin HIGH keeps the internal
switch off and puts the LTC1265 in micropower shut-
down. Do not float this pin.
LBIN(Pin4):The(–)InputoftheLow-BatteryComparator.
The (+) input is connected to a reference voltage of 1.25V.
SGND (Pin 11): Small-Signal Ground. Must be routed
separately from other grounds to the (–) terminal of COUT
.
CT (Pin 5): External capacitor CT from Pin 5 to ground sets
the switch off time. The operating frequency is dependent
on the input voltage and CT.
PGND (Pin 12): Switch Driver Ground. Connects to the
(–) terminal of CIN. Anode of the Schottky diode must be
connected close to this pin.
ITH (Pin 6): Feedback Amplifier Decoupling Point. The
current comparator threshold is proportional to Pin 6
voltage.
SENSE– (Pin 7): Connect to the (–) input of the current
comparator. For LTC1265-3.3 and LTC1265-5, it also
connects to an internal resistive divider which sets the
output voltage.
SW (Pin 14): Drain of the P-Channel MOSFET Switch.
Cathode of the Schottky diode must be connected close to
this pin.
4
LTC1265/LTC1265-3.3/LTC1265-5
U
U
W
FU CTIO AL DIAGRA
(Pin 9 connection shown for LTC1265-3.3 and LTC1265-5; change create LTC1265)
PWR V
1, 13
IN
+
–
SENSE
8
SENSE
7
14 SW
12 PGND
V
FB
–
+
9
ADJUSTABLE
VERSION
V
–
+
SLEEP
R
S
25mV TO 150mV
C
+
Q
5pF
V
OS
S
–
+
–
V
13k
TH2
6
G
I
TH
100k
V
–
+
TH1
T
V
2
3
LB
0UT
IN
+
–
REFERENCE
10
SHDN
5
C
T
OFF-TIME
CONTROL
A3
–
SENSE
FB
V
4
LB
IN
1265 FD
11 SGND
U
OPERATIO
(Refer to Functional Diagram)
The LTC1265 uses a constant off-time architecture to
switch its internal P-channel power MOSFET. The off time
is set by an external timing capacitor at CT (Pin 5). The
operatingfrequencyisthendeterminedbytheofftimeand
the voltage across the shunt reaches the comparator’s
threshold value, its output signal will change state, setting
theflipflopandturningtheinternalP-channelMOSFEToff.
The timing capacitor connected to Pin 5 is now allowed to
discharge at a rate determined by the off-time controller.
the difference between VIN and VOUT
.
The output voltage is set by an internal resistive divider
(LTC1265-3.3 and LTC1265-5) connected to SENSE–
(Pin 7) or an external divider returned to VFB (Pin 9 for
LTC1265). A voltage comparator V, and a gain block G,
compare the divided output voltage with a reference
voltage of 1.25V.
When the voltage on the timing capacitor has discharged
past VTH1, comparator T trips, sets the flip flop and causes
the switch to turn on. Also, the timing capacitor is re-
charged. The inductor current will again ramp up until the
current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage de-
creases slightly. This causes the output of the gain stage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
Tooptimizeefficiency,theLTC1265automaticallyswitches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is relatively light, the LTC1265 automati-
cally goes into Burst Mode operation. The current loop is
interrupted when the output voltage exceeds the desired
regulated value. The hysteretic voltage comparator V trips
when VOUT is above the desired output voltage, shutting
off the switch and causing the capacitor to discharge. This
When the load is heavy, the LTC1265 is in continuous
operation. During the switch ON time, current comparator
C monitors the voltage between Pins 7 and 8 connected
across an external shunt in series with the inductor. When
5
LTC1265/LTC1265-3.3/LTC1265-5
U
OPERATIO
(Refer to Functional Diagram)
capacitor discharges past VTH1 until its voltage drops
below VTH2. Comparator S then trips and a sleep signal is
generated. Thecircuitnowentersintosleepmodewiththe
power MOSFET turned off. In sleep mode, the LTC1265 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 2mA to 160µA. When the output capaci-
tor discharges by the amount of the hysteresis of the
comparatorV,theP-channelswitchturnsonagainandthe
process repeats itself. During Burst Mode operation the
To avoid the operation of the current loop interfering with
BurstModeoperation, abuilt-inoffsetVOS isincorporated
in the gain stage. This prevents the current from increas-
inguntiltheoutputvoltagehasdroppedbelowaminimum
threshold.
Using constant off-time architecture, the operating fre-
quency is a function of the voltage. To minimize the
frequencyvariationasdropoutisapproached,theoff-time
controller increases the discharge current as VIN drops
below VOUT + 2V. In dropout the P-channel MOSFET is
turned on continuously (100% duty cycle) providing low
dropout operation with VOUT VIN.
peak inductor current is set at 25mV/RSENSE
.
W U U
APPLICATIO S I FOR ATIO
The basic LTC1265 application circuit is shown in
Figure 1. External component selection is driven by the
U
150mV
25mV
2 • R
I
=
(Amps)
OUT(MAX)
–
R
SENSE
SENSE
loadrequirement, andbeginswiththeselectionofRSENSE
.
137.5mV
=
(Amps)
Once RSENSE is known, CT and L can be chosen. Next, the
R
SENSE
Schottky diode D1 is selected followed by CIN and COUT
.
Solving for RSENSE and allowing a margin of variations in
the LTC1265 and extended component values yields:
RSENSE Selection for Output Current
RSENSE is chosen based on the required output current.
Withthecurrentcomparatormonitoringthevoltagedevel-
oped across RSENSE, the threshold of the comparator
determines the peak inductor current. Depending on the
load current condition, the threshold of the comparator
liesbetween25mV/RSENSE and150mV/RSENSE. Themaxi-
mum output current of the LTC1265 is:
100mV
OUT(MAX)
R
=
(Ω)
SENSE
I
TheLTC1265isratedwithacapabilitytosupplyamaximum
of1.2Aofoutputcurrent.Therefore, theminimumvalueof
RSENSE that can be used is 0.083Ω. A graph for selecting
RSENSE versus maximum output is given in Figure 2.
150mV
I
RIPPLE
I
=
(Amps)
OUT(MAX)
–
0.5
0.4
0.3
0.2
0.1
0
R
2
SENSE
where IRIPPLE is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1265 is in Burst Mode
operation. In this mode the peak inductor current is set at
25mV/RSENSE.TofullybenefitfromBurstModeoperation,
the inductor current should be continuous during burst
periods. Hence, the peak-to-peak inductor ripple current
must not exceed 25mV/RSENSE
.
Toaccountforlightandheavyloadconditions,theIOUT(MAX)
is then given by:
0
0.2
0.4
0.6
0.8
1
MAXIMUM OUTPUT CURRENT (A)
1265 G10
Figure 2. Selecting RSENSE
6
LTC1265/LTC1265-3.3/LTC1265-5
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APPLICATIO S I FOR ATIO
U
Under short-circuit condition, the peak inductor current is
determined by:
2V, the LTC1265 reduces tOFF by increasing the discharge
current in CT. This prevents audible operation prior to
dropout. (See shelving effect shown in the Operating
Frequency curve under Typical Performance Character-
istics.)
150mV
I
=
(Amps)
SC(PK)
R
SENSE
In this condition, the LTC1265 automatically extends the
off time of the P-channel MOSFET to allow the inductor
current to decay far enough to prevent any current build-
up. The resulting ripple current causes the average short-
To maintain continuous inductor current at light load, the
inductor must be chosen to provide no more than 25mV/
RSENSE of peak-to-peak ripple current. This results in the
following expression for L:
circuit current to be approximately IOUT(MAX)
.
L ≥ 5.2(105)RSENSE(CT)VREG
CT and L Selection for Operating Frequency
Using an inductance smaller than the above value will
result in the inductor current being discontinuous. A
consequenceofthisisthattheLTC1265willdelayentering
Burst Mode operation and efficiency will be degraded at
low currents.
The LTC1265 uses a constant off-time architecture with
tOFF determined by an external capacitor CT. Each time the
P-channel MOSFET turns on, the voltage on CT is reset to
approximately 3.3V. During the off time, CT is discharged
by a current that is proportional to VOUT. The voltage on CT
is analogous to the current in inductor L, which likewise,
decays at a rate proportional to VOUT. Thus the inductor
value must track the timing capacitor value.
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically, there are two kinds of losses in an
inductor; core and copper losses.
The value of CT is calculated from the desired continuous
mode operating frequency:
Core losses are dependent on the peak-to-peak ripple
current and core material. However it is independent of
the physical size of the core. By increasing the induc-
tance, the peak-to-peak inductor ripple current will de-
crease, therefore reducing core loss. Utilizing low core
loss material, such as molypermalloy or Kool Mµ® will
allow user to concentrate on reducing copper loss and
preventing saturation.
V – V
V + V
IN
1
IN
OUT
D
C =
T
(Farads)
)
)
4
1.3(10 )f
where VD is the drop across the Schottky diode.
As the operating frequency is increased, the gate charge
losses will reduce efficiency. The complete expression for
operating frequency is given by:
Although higher inductance reduces core loss, it in-
creases copper loss as it requires more windings. When
space is not at a premium, larger wire can be used to
reduce the wire resistance. This also prevents excessive
heat dissipation.
V – V
V + V
IN
1
OFF
IN
OUT
D
(Hz)
f ≈
)
)
t
where:
V
V
REG
OUT
CATCH DIODE SELECTION
4
(sec)
t
= 1.3(10 )C
OFF
T
)
)
Losses in the catch diode depend on forward drop and
switching times. Therefore Schottky diodes are a good
choice for low drop and fast switching times.
VREG is the desired output voltage (i.e. 5V, 3.3V). VOUT is
the measured output voltage. Thus VREG/VOUT = 1
in regulation.
The catch diode carries load current during the off time.
The average diode current is therefore dependent on the
Kool Mµ is a registered trademark of Magnetics, Inc.
Note that as VIN decreases, the frequency decreases.
When the input-to-output voltage differential drops below
7
LTC1265/LTC1265-3.3/LTC1265-5
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APPLICATIO S I FOR ATIO
P-channel switch duty cycle. At high input voltages, the
worst-case RMS ripple current in the output capacitor is
given by:
diode conducts most of the time. As VIN approaches VOUT
,
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short circuited. Under this condition, the diode must
safely handle ISC(PK) at close to 100% duty cycle. Most
LTC1265 circuits will be well served by either a 1N5818 or
a MBRS130LT3 Schottky diode. An MBRS0520 is a good
choice for IOUT(MAX) ≤ 500mA.
150mV
I
≈
(A
)
RMS
RMS
2(R
)
SENSE
Generally, once the ESR requirement for COUT has been
met, the RMS current rating far exceeds the IRIPPLE(P-P)
requirement.
ESR is a direct function of the volume of the capacitor.
ManufacturerssuchasNichicon,AVXandSpragueshould
be considered for high performance capacitors. The
OS-CON semiconductor dielectric capacitor available
from Sanyo has the lowest ESR for its size at a somewhat
higher price.
CIN
In continuous mode, the input current of the converter is
a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor must be
used. In addition, the capacitor must handle a high RMS
current. The CIN RMS current is given by:
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Alumi-
num electrolyte and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are both available
in surface mount configuration and are surge tested for
useinswitchingpowersupplies. Anexcellentchoiceisthe
AVX TPS series of surface mount tantalums, available in
case heights ranging from 2mm to 4mm. Consult the
manufacturer for other specific recommendations.
1
/
2
I
[V
(V – V )]
OUT OUT IN OUT
(A
)
I
≈
RMS
RMS
V
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst case is commonly used for
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or to choose a capacitor rated at a higher temperature
than required. Do not underspecify this component. An
additional 0.1µF ceramic capacitor is also required on
PWR VIN for high frequency decoupling.
When the capacitance of COUT is made too small, the
outputrippleatlowfrequencieswillbelargeenoughtotrip
the voltage comparator. This causes Burst Mode opera-
tion to be activated when the LTC1265 would normally be
in continuous operation. The effect will be most pro-
nounced with low value of RSENSE and can be improved at
higher frequencies with lower values of L.
COUT
The selection of COUT is based upon the effective series
resistance(ESR)forproperoperationoftheLTC1265.The
required ESR of COUT is:
Low-Battery Detection
ESRCOUT < 50mV/IRIPPLE
The low-battery comparator senses the input voltage
through an external resistive divider. This divided voltage
connects to the (–) input of a voltage comparator (Pin 4)
which is compared with a 1.25V reference voltage. Ne-
glecting Pin 4 bias current, the following expression is
used for setting the trip limit:
where IRIPPLE is the ripple current of the inductor. For the
case where the IRIPPLE is 25mV/RSENSE, the required ESR
of COUT is:
ESRCOUT < 2(RSENSE
)
To avoid overheating, the output capacitor must be sized
to handle the ripple current generated by the inductor. The
R4
R3
1 +
= 1.25
V
LB_TRIP
)
8
LTC1265/LTC1265-3.3/LTC1265-5
W U U
APPLICATIO S I FOR ATIO
Theoutput,Pin3,isanN-channelopendrainthat goeslow
when the battery voltage is below the threshold set by R3
and R4. In shutdown, the comparator is disabled and Pin
3 is in a high impedance state.
U
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW (Pin 14)
can never exceed VIN (Pins 1, 2, 13) by more than 0.3V.
Normally this situation should never occur. It could,
however, if the output is held up while the VIN supply is
pulled down. A condition where this could potentially
occur is when a battery is supplying power to an LTC1265
regulator and also to one or more loads in parallel with the
the regulator’s VIN. If the battery is disconnected while the
LTC1265 regulator is supplying a light load and one of the
parallel circuits has a heavy load, the input capacitor of the
LTC1265 regulator could be pulled down faster than the
output capacitor, causing the absolute maximum ratings
to be exceeded. The result is often a latchup which can be
destructive if VIN is reapplied quickly. Battery disconnect
is possible as a result of mechanical stress, bad battery
contacts or use of a lithium-ion battery with a built-in
internal disconnect. The user needs to assess his/her
application to determine whether this situation could
occur. If so, additional protection is necessary.
V
IN
R4
4
LTC1265
3
–
+
R3
1.25V REFERENCE
LTC1265 F03
Figure 3. Low-Battery Comparator
LTC1265 ADJUSTABLE APPLICATIONS
The LTC1265 develops a 1.25V reference voltage between
the feedback (Pin 9) terminal and signal ground (see
Figure 4). By selecting resistor R1, a constant current is
caused to flow through R1 and R2 to set overall output
voltage. The regulated output voltage is determined by:
Prevention against latchup can be accomplished by
simply connecting a Schottky diode across the SW and
VIN pins as shown in Figure 5. The diode will normally be
reverse biased unless VIN is pulled below VOUT at which
timethediodewillclampthe(VOUT –VIN)potentialtoless
thanthe0.6Vrequiredforlatchup.Notethatalowleakage
Schottky should be used to minimize the effect on no-
load supply current. Schottky diodes such as MBR0530,
BAS85 and BAT84 work well. Another more serious
effect of the protection diode leakage is that at no load
withnothingtoprovideasinkforthisleakagecurrent, the
R2
R1
1 +
= 1.25
V
OUT
)
)
For most applications a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1265.
V
OUT
LATCHUP
PROTECTION
SCHOTTKY
R2
9
LTC1265
V
FB
PWR
IN
SGND
11
100pF
V
OUT
SW
R1
V
+
LTC1265
LTC1265 F04
1265 F05
Figure 4. LTC1265 Adjustable Configuration
Figure 5. Preventing Absolute Maximum
Ratings from Being Exceeded
9
LTC1265/LTC1265-3.3/LTC1265-5
U
W U U
APPLICATIONS INFORMATION
output voltage can potentially float above the maximum
allowable tolerance. To prevent this from occuring, a
resistor must be connected between VOUT and ground
with a value low enough to sink the maximum possible
leakage current.
Now consider the case of a 1A regulator with VIN = 4V and
TA = 65°C. Starting with the same 0.55Ω assumption for
RDSON, the TJ calculation will yield 125°C. But from the
graph, this will increase the RDSON to 0.76Ω, which when
used in the above calculation yields an actual TJ > 148°C.
ThereforetheLTC1265wouldbeunsuitablefora4Vinput,
1A output regulator operating at TA = 65°C.
THERMAL CONSIDERATIONS
In a majority of applications, the LTC1265 does not
dissipate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turnedoncontinuously(DC),theuserwillneedtodosome
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated by the regulator
exceeds the maximum junction temperature of the part.
The temperature rise is given by:
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1265. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
1. Are the signal and power grounds segregated? The
LTC1265 signal ground (Pin 11) must return to the (–)
plate of COUT. The power ground (Pin 12) returns to the
anode of the Schottky diode, and the (–) plate of CIN,
whose leads should be as short as possible.
TR = P(θJA)
where P is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
2. Does the (+) plate of the CIN connect to the power VIN
(Pins 1,13) as close as possible? This capacitor pro-
vides the AC current to the internal P-channel MOSFET
and its driver.
The junction temperature is simply given by:
TJ = TR + TA
3. Is the input decoupling capacitor (0.1µF) connected
closely between power VIN (Pins 1,13) and power
ground (Pin 12)? This capacitor carries the high fre-
quency peak currents.
As an example, consider the LTC1265 is in dropout at an
input voltage of 4V with a load current of 0.5A. From the
Typical Performance Characteristics graph of Switch Re-
sistance, the ON resistance of the P-channel is 0.55Ω.
Therefore power dissipated by the part is:
4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch (Pin 14)?
P = I2(RDSON) = 0.1375W
5. Does the LTC1265 SENSE– (Pin 7) connect to a point
close to RSENSE and the (+) plate of COUT? In adjustable
applications, the resistive divider, R1 and R2, must be
connected between the (+) plate of COUT and signal
ground.
For the SO package, the θJA is 110°C/W.
Therefore the junction temperature of the regulator when
it is operating in ambient temperature of 25°C is:
TJ = 0.1375(110) + 25 = 40.1°C
6. AretheSENSE– andSENSE+ leadsroutedtogetherwith
minimum PC trace spacing? The 1000pF capacitor
between Pins 7 and 8 should be as close as possible to
the LTC1265.
Remembering that the above junction temperature is
obtained from a RDSON at 25°C, we need to recalculate the
junction temperature based on a higher RDSON since it
increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
7. Is SHDN (Pin 10) actively pulled to ground during
normal operation? The SHDN pin is high impedance
and must not be allowed to float.
10
LTC1265/LTC1265-3.3/LTC1265-5
W U U
APPLICATIO S I FOR ATIO
U
PWR V
1
IN
V
IN
2
14
13
12
11
10
9
V
IN
SW
D1
PWR V
IN
+
LTC1265
C
IN
0.1µF
3
4
L
PGND
SGND
SHDN
LB
LB
OUT
1k
1000pF
IN
3900pF
5
6
R1
SHDN
C
T
C
OUT
+
I
TH
N/C (V
SENSE
)
FB
R
SENSE
7
8
R2
+
–
SENSE
V
OUT
1000pF
OUTPUT DIVIDER REQUIRED
WITH ADJUSTABLE VERSION ONLY
BOLD LINES INDICATE
HIGH PATH CURRENTS
LTC1265 F06
Figure 6. LTC1265 Layout Diagram (See Board Layout Checklist)
Troubleshooting Hints
3.3V
Since efficiency is critical to LTC1265 applications, it is
very important to verify that the circuit is functioning
correctlyinbothcontinuousandBurstModeoperation.As
the LTC1265 is highly tolerant of poor layout, the output
voltage will still be regulated. Therefore, monitoring the
output voltage will not tell you whether you have a good or
bad layout. The waveform to monitor is the voltage on the
timing capacitor Pin 5.
2.4V
0V
TIME
(a) CONTINUOUS MODE OPERATION
LTC1265 F07a
IncontinuousmodethevoltageontheCT pinisasawtooth
with approximately 0.9VP-P swing. This voltage should
never dip below 2V as shown in Figure 7a.
SLEEP MODE
3.3V
2.4V
When the load currents are low (ILOAD < IBURST) Burst
Mode operation occurs. The voltage on CT pin now falls to
ground for periods of time as shown in Figure 7b. During
this time the LTC1265 is in sleep mode with quiescent
current reduced to 160µA.
0V
TIME
(b) Burst Mode OPERATION
LTC1265 F07b
The inductor current should also be monitored. If the
circuit is poorly decoupled, the peak inductor current will
be haphazard as in Figure 8a. A well decoupled LTC1265
has a clean inductor current as in Figure 8b.
Figure 7. CT Waveforms
11
LTC1265/LTC1265-3.3/LTC1265-5
W U U
U
APPLICATIO S I FOR ATIO
(a) POORLY DECOUPLED LTC1265
(b) WELL DECOUPLED LTC1265
Figure 8. Inductor Waveforms
V
IN
Design Example
5V
As a design example, assume VIN = 5V, VOUT = 3.3V, IMAX
= 0.8A and f = 250kHz. With this information we can easily
calculate all the important components.
+
C
IN
V
PWR V
SHDN
IN
IN
0.1µF
22µH
V
3.3V
0.8A
0.125Ω
OUT
SW
From (1),
LTC1265-3.3
D1
1k
+
I
TH
C
OUT
PGND
RSENSE = 100mV/0.8 = 0.125Ω
From (2) and assuming VD = 0.4V,
CT 100pF
3900pF
100pF
+
SENSE
C
T
1000pF
–
SENSE
LTC1265 F09
Using (3), the value of the inductor is:
L ≥ 5.2(105)(0.125)(100pF)3.3V = 22µH
SGND
For the catch diode, a MBRS130LT3 or 1N5818 will be
sufficient in this application.
Figure 9. Design Example Circuit
CIN will require an RMS current rating of at least 0.4A at
temperature, and COUT will require an ESR of (from 5):
100
95
90
85
80
75
70
L = DALE LPT4545-220 (22µH)
V
C
= 3.3V
OUT
T
ESRCOUT < 0.25Ω
= 100pF
The inductor ripple current is given by:
V
+ V
D
L
OUT
t
= 0.22A
I
=
OFF
RIPPLE
)
)
At light loads the peak inductor current is at:
IPEAK = 25mV/0.125 = 0.2A
Therefore, at load current less than 0.1A the LTC1265 will
be in Burst Mode operation. Figure 9 shows the complete
circuit and Figure 10 shows the efficiency curve with the
above calculated component values.
0.01
0.1
LOAD CURRENT (mA)
1.0
1265 G11
Figure 10. Design Example Efficiency Curve
12
LTC1265/LTC1265-3.3/LTC1265-5
U
TYPICAL APPLICATIO S
High Efficiency 5V to 3.3V Converter
V
IN
5V
+
C
*
2
1, 13
IN
L1†
47µH
††
0.1µF
100µF
R
SENSE
V
PWR V
IN
IN
10V
V
3.3V
1A
0.1Ω
OUT
4
14
LB
SW
IN
+
C
**
OUT
MBRS130LT1
LTC1265-3.3
PGND
220µF
3
5
12
11
10
9
LB
10V
OUT
270pF
C
SGND
SHDN
NC
T
SHDN
3900pF
1k
6
7
I
THR
8
–
+
SENSE
SENSE
*AVX TPSD107K010
**AVX TPSE227K010
1000pF
†COILCRAFT D03316-473
††DALE WSL2010-0.1-1%
LTC1265 TA02
Positive-to-Negative (–5V) Converter
*AVX TPSD226K025
V
IN
3.5V TO 7.5V
**AVX TPSD107K010
C
*
IN
2
1, 13
SHDN
D1
+
†L1 SELECTION
22µF
25V
× 2
L1†
50µH
0.1µF
TP0610L
14
V
PWR V
IN
IN
PART NO.
MANUFACTURER
4
LB
IN
SW
DO3316-473
CTX50-4
LPT4545-500LA
CD74-470
COILCRAFT
COILTRONICS
DALE
LTC1265-5
3
5
12
11
V
OUT
LB
PGND
OUT
SUMIDA
–5V
220pF
††IRC LRC2010-01-R100-J
C
SGND
T
D1= MBRS130LT3
2200pF
V
(V)
I
(mA)
1k
100k
IN
OUT(MAX)
6
7
10
8
C
**
OUT
I
SHDN
THR
3.5
4.0
5.0
6.0
7.0
7.5
360
430
540
630
720
740
100µF
+
10V
–
+
SENSE
SENSE
††
1000pF
R
SENSE
0.1Ω
LTC1265 TA03
13
LTC1265/LTC1265-3.3/LTC1265-5
U
TYPICAL APPLICATIO S
5V Buck-Boost Converter
V
IN
V
(V)
I
(mA)
OUT(MAX)
IN
3.5V TO 7.5V
C
*
2
1, 13
+
IN
33µF
3.5
4.0
5.0
6.0
7.0
7.5
240
275
365
490
610
665
L1A††
0.1µF
100µF
10V*
V
PWR V
IN
IN
16V
33µH
4
14
V
OUT
LB
SW
IN
5V
2
1
LTC1265
1N5818
3
5
12
11
10
9
LB
PGND
SGND
SHDN
OUT
75pF
4
•
L1B††
33µH
75k
25k
L1A
L1A
C
L1B
L1B
T
2
3
+
3
SHDN
C
*
OUT
TOP VIEW
4
3300pF
100µF
1k
•
6
7
1
10V
V
I
FB
THR
8
–
+
SENSE
SENSE
SANYO OS-CON CAPACITOR
IRC LRC2010-01-R162-J
L1A, L2A SELECTION
*
100pF
0.01µF
R
**
**
SENSE
†
0.162Ω
PART NO.
MANUFACTURER
CTX33-4
LPT4545-330LA
COILTRONICS
DALE
LTC1265 F09
9V to 12V and – 12V Outputs
MBRS130LT3
V
OUT
V
(V)
I
(mA)
OUT(MAX)
IN
–12V
V
IN
C
*
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
40
4V TO 12V
OUT
+
C *
IN
68µF
60
2
1, 13
+
0.1µF
68µF
33µF**
L1A††
50µH
20V
80
20V
25V
V
PWR V
IN
IN
100
115
130
150
165
180
1N914
4
14
V
OUT
LB
IN
SW
12V
2
1
LTC1265
MBRS130LT3
3
5
12
11
10
9
LB
PGND
SGND
SHDN
OUT
75pF
4
SI19430DY
•
301k
34k
C
I
L1B††
50µH
T
L1A
L1A
L1B
L1B
2
3
+
SHDN
C
*
OUT
3
3300pF
TOP VIEW
68µF
1k
6
7
•
20V
4
1
V
FB
THR
8
–
+
SENSE
SENSE
*AVX TPSE686K020
**AVX TPSE336K025
100pF
0.01µF
R
*
†IRC LRC2010-01-R162-J
††L1A,L2A SELECTION
SENSE
0.162Ω
PART NO.
MANUFACTURER
LTC1265 TA05
CTX50-4
LPT4545-500LA
COILTRONICS
DALE
14
LTC1265/LTC1265-3.3/LTC1265-5
U
TYPICAL APPLICATIO S
2.5mm Max Height 5V-to-3.3V (500mA)
*AVX TAJB156K010
**AVX TAJB226K06
V
IN
3.5V TO 12.5V
2
1, 13
C
*
+
†IRC LRC2010-01-R200-J
††SUMIDA CLS62-180
IN
0.1µF
15µF
V
PWR V
IN
IN
10V × 2
4
14
LB
IN
SW
MBRS0520LT1
LTC1265-3.3
PGND
3
5
12
11
10
9
LB
OUT
51pF
C
T
SGND
SHDN
N/C
L1††
18µH
SHDN
3300pF
1k
C
**
6
7
OUT
I
22µF
THR
+
6.3V × 2
8
–
+
SENSE
SENSE
†
1000pF
R
SENSE
0.20Ω
V
OUT
3.3V
LTC1265 TA06
500mA
Logic Selectable 0V/3.3V/5V 700mA Regulator
*DALE 593D68X0020E2W
**DALE 593D107X0010D2W
†IRC LRC2010-01-R15-J
††L1 SELECTION
PART NO.
MANUFACTURER
DO3316-333
CTX33-4
LPT4545-330LA
CD74-330
COILCRAFT
COILTRONICS
DALE
V
IN
3.5V TO 12.5V
C
*
2
1, 13
+
IN
SUMIDA
0.1µF
68µF
†††V
= 0V: V
= 5V: V
= 3.3V/5V
= 0V
V
PWR V
IN
IN
20V
SHDN
OUT
OUT
4
14
0V: V
5V: V
= 5V
= 3.3V
OUT
OUT
LB
SW
IN
MBRS130LT3
100pF
LTC1265
3
5
12
11
10
9
LB
PGND
SGND
SHDN
OUT
75pF
C
T
L1††
33µH
45.3k
†††
V
SHDN
3300pF
1k
C
**
6
7
OUT
100µF
V
I
FB
THR
+
10V
8
–
+
SENSE
SENSE
56.2k
75k
†
1000pF
R
SENSE
0.15Ω
V
OUT
0V/3.3V/5V
700mA
LTC1265 TA07
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 represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
15
LTC1265/LTC1265-3.3/LTC1265-5
U
TYPICAL APPLICATIO S
4-NiCad Battery Charger
*DALE 593D226X0025D2W
**DALE 593D107X0016E2W
†DALE WSL2010-0.10-1%
††L1 SELECTION
V
IN
PART NO.
MANUFACTURER
8V TO 12.5V
2
1, 13
+
C
*
IN
22µF, 25V
0.1µF
DO3316-104
CTX100-4P
CD105-101
COILCRAFT
COILTRONICS
SUMIDA
V
PWR V
IN
IN
4
14
LB
SW
IN
MBRS130LT3
LTC1265
51Ω
3
5
12
11
10
9
LB
PGND
SGND
SHDN
OUT
C
T
L1††
100µH
FAST CHARGE: = 0V
TRICKLE CHARGE: > 2V
30k
CHARGER
ON/OFF
100pF
270pF
C
**
OUT
6
7
V
I
100µF
FB
THR
+
10V
8
1k
–
+
VN2222L
SENSE
SENSE
138k
†
1000pF
3300pF
R
SENSE
0.10Ω
MBRS130LT3
V
OUT
4 NICAD
1A FAST CHARGE
0.1A TRICKLE CHARGE
LTC1265 TA08
U
PACKAGE DESCRIPTIO
Dimension in inches (millimeters) unless otherwise noted.
S Package
14-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.337 – 0.344*
(8.560 – 8.738)
0.010 – 0.020
14
13
12
11 10
9
8
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
(0.254 – 0.508)
0.008 – 0.010
(0.203 – 0.254)
0° – 8° TYP
0.228 – 0.244
(5.791 – 6.197)
0.150 – 0.157**
(3.810 – 3.988)
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
TYP
0.016 – 0.050
(0.406 – 1.270)
S14 1298
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
1
2
3
4
5
6
7
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
Dual Version of LTC1147
Nonsynchronous, 8-Pin, V ≤ 16V
LTC1143
Dual Step-Down Switching Regulator Controller
Step-Down Switching Regulator Controller
Step-Down Switching Regulator Controller
Step-Down Switching Regulator with Internal 0.5A Switch
LTC1147
IN
LTC1148HV
LTC1174
Synchronous, V ≤ 20V
IN
V
≤ 18.5V, Comparator/Low Battery Detector
IN
LTC1474/LTC1475 Low Quiescent Current Step-Down Regulators
Monolithic, I = 40µA, 400mA, MS8
Q
LTC1574
Step-Down Switching Regulator with Internal 0.5A Switch
and Schottky Diode
V ≤ 18.5V, Comparator
IN
LTC1622
LTC1627
LTC1772
Low Input Voltage Step-Down DC/DC Controller
Constant Frequency, 2V to 10V V , MS8
IN
Monolithic Synchronous Step-Down Switching Regulator Constant Frequency, I
to 500mA, 2.65V to 8.5V V
IN
OUT
Constant Frequency Step-Down DC/DC Controller
SOT-23, 2.2V to 9.8V V
IN
126535fb LT/LCG 0800 2K REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1995
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
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