LT1944 [Linear]
1.4A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect; 1.4A ， 1.5MHz的同步升压型DC / DC转换器输出断接
| 型号： | LT1944 |
| 厂家： | 
Linear |
| 描述： | 1.4A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect |
| 文件： | 总16页 (文件大小：339K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3458
1.4A, 1.5MHz Synchronous
Step-Up DC/DC Converter
with Output Disconnect
U
FEATURES
DESCRIPTIO
The LTC®3458 is a high efficiency, current mode, fixed
frequency, step up DC/DC converter with true output
disconnect and inrush current limiting. The LTC3458 is
rated for a 7.5V output and includes a 0.3Ω N-channel
MOSFET switch and a 0.4Ω P-channel MOSFET synchro-
nous rectifier. The LTC3458 is well suited for battery
powered applications and includes programmable output
voltage, switching frequency and loop compensation. The
oscillator frequency can be set up to 1.5MHz or synchro-
nized to an external clock.
■
High Efficiency: Up to 93%
■
Inrush Current Limiting and Output Disconnect
■
Programmable Output Voltages up to 7.5V
■
1.5V to 6V Input Range
■
Programmable/Synchronizable Fixed Frequency
Operation up to 1.5MHz
Programmable Automatic Burst Mode® Operation
■
■
Current Mode Control with Programmable Soft-Start
Period and Peak Current Limit
700mA at 7V from 5V Input
■
■
0.3Ω N-Channel and 0.4Ω P-Channel 1.4A Switches
Quiescent current is only 15µA during Burst Mode opera-
tion maximizing battery life in portable applications. The
BurstModecurrentthreshold,peakcurrentlimit,andsoft-
start are externally programmable. Other features include
<1µA shutdown current, antiringing control, and thermal
limit. The LTC3458 is available in a low profile (0.75mm),
3mm × 4mm 12-pin DFN package.
at 5VOUT
■
Ultralow Quiescent Currents: 15µA Sleep, <1µA in
Shutdown
■
3mm × 4mm Thermally Enhanced DFN Package
U
APPLICATIO S
■
Point-of-Load Regulators
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
■
USB VBUS Power
■
LCD Bias
OLED Displays
■
U
TYPICAL APPLICATIO
USB to 7V at 1MHz
COEV
10µH
DQ7545
USB to 7VOUT
1000
100
95
90
85
80
75
70
SW
V
IN
USB
4.35V to 5.25V
5.25V
IN
LTC3458
2.2µF
V
OUT
V
GND/PGND
SHDN
7V
OUT
FB
4.35V
500mA
IN
10pF
1.5M
316k
10
ON OFF
POWER LOSS
COMP
SS
SYNC
0.01µF
33k
R
I
T
22µF
X5R
0.01µF
10pF
0.1
1000
BURST
0.1
1
10
100
LIM
LOAD CURRENT (mA)
560pF
200k 124k
133k
3458 TA01b
3458 TA01a
3458f
1
LTC3458
W W
U W
U
W
U
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
TOP VIEW
ORDER PART
NUMBER
VIN, SS, SYNC Voltages ................................. –0.3 to 7V
BURST, SHDN, VOUT Voltages ....................... –0.3 to 8V
Operating Temperature Range
(Notes 2, 3) .........................................–40°C to 85°C
Storage Temperature Range ..................–65°C to 125°C
SW Voltage
SW
1
2
3
4
5
6
12 V
OUT
V
11 BURST
10 SS
IN
LTC3458EDE
SYNC
SHDN
13
9
8
7
GND
COMP
FB
I
LIM
R
T
DE PART MARKING
3458
DC ........................................................... –0.3V to 8V
Pulsed <100ns ...................................... –0.3V to 10V
DE12 PACKAGE
12-LEAD (4mm × 3mm) PLASTIC DFN
EXPOSED PAD IS PGND (PIN 13),
MUST BE SOLDERED TO PCB
TJMAX = 125°C, θJA = 45°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER
CONDITIONS
T = 0°C to 85°C
T = –40°C to 0°C
A
MIN
TYP
MAX
UNITS
Minimum V Operating Voltage
1.4
1.4
1.5
1.7
V
V
IN
A
Output Voltage Adjust Range
Feedback Voltage
●
2.0
7.5
V
0°C to 85°C, V
–40°C to 0°C
= 3.3V
1.21
1.20
1.23
1.25
1.25
V
V
OUT
Undervoltage (Exit Burst Mode Operation)
Feedback Input Current
Below Feedback Voltage
–4%
1
V
V
= 1.23V
50
nA
FB
Quiescent Current - Burst Mode Operation
V
V
Current at 3.3V
15
5
30
10
µA
µA
IN
Current at 5V
OUT
Quiescent Current - Shutdown
V
V
Current at 3.3V
0.5
1
1
3
µA
µA
IN
Current at 0V
OUT
Quiescent Current - Active
NMOS Switch Leakage
PMOS Switch Leakage
NMOS Switch On Resistance
PMOS Switch On Resistance
Fixed NMOS Current Limit
Maximum Duty Cycle
V
Current Switching
1
3
5
5
mA
µA
µA
Ω
IN
●
●
0.05
0.05
0.3
0.4
1.6
90
V
V
= 5V
OUT
OUT
= 5V
Ω
R
= 124k
●
●
●
●
1.4
80
A
ILIM
V
= 3.3V, f
= 1MHz
OSC
%
IN
Minimum Duty Cycle
0
%
Frequency Accuracy
R = 200k
0.85
1
100
7
1.15
MHz
µA/V
µA
µA
V
T
Error Amplifier Transconductance
Error Amplifier Source Current
Error Amplifier Sink Current
SYNC Input High
7
●
●
1.5
SYNC Input Low
0.35
V
3458f
2
LTC3458
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 5V, RT = 200k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SHDN Input High
●
●
1.25
V
V
A
V
SHDN Input Low
0.3
BURST Mode Peak Current
BURST Threshold Voltage
R
ILIM
= 124k
0.4
1.10
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 2: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Note 3: The LTC3458 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
U W
(TA = 25°C unless otherwise specified)
TYPICAL PERFOR A CE CHARACTERISTICS
ILIMIT, IBURST, TZERO Currents
Current Limit Accuracy
Burst Mode Quiescent Current
20
15
10
5
2000
1800
1600
1400
1200
1000
800
1.8
1,7
1.6
V
V
= 3.3V
OUT
R
ILIMIT
= 124k
IN
= 5V
I
LIMIT
I
V
OUT
= 7V
VIN
L = 10µH
= 124k
R
ILIM
600
I
BURST PEAK
400
1.5
1.4
I
VOUT
200
I
ZERO
0
0
–200
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
(V)
–40
–15
10
35
60
85
–45 –30 –15
0
15 30 45 60 75 90
V
IN
TEMPERATURE (°C)
TEMPERATURE (°C)
3458 G01
3458 G02
3458 G03
Typical Burst Mode Threshold and
Hysteresis vs RBURST
Maximum Load Current in Burst
Oscillator Programming Resistor
250
200
150
100
50
600
160
140
120
100
80
R
ILIM
= 124k
550
500
450
400
350
300
250
200
150
100
5V
OUT
7.5V
OUT
3.3V
OUT
OUT OF
BURST
INTO
BURST
60
40
20
0
0
3.5
(V)
400
600
1000 1200
1400
1.5 2.0 2.5 3.0
4.0 4.5 5.0 5.5
800
100
150
R
250
50
300
200
(kΩ)
V
OSCILLATOR FREQUENCY (kHz)
IN
BURST
3458 G04
3458 G05
3458 G06
3458f
3
LTC3458
U W
(TA = 25°C unless otherwise specified)
TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Accuracy
Efficiency vs Frequency
N-Channel and P-Channel RDS(ON)
95
93
91
89
87
85
0.5
0.4
0.3
0.2
0.1
0
1.05
1.03
V
V
= 3.3V
V
V
= 3.3V
= 5V
RT = 200k
IN
OUT
IN
OUT
= 5V at 100mA
P-CHANNEL
1.01
N-CHANNEL
0.99
0.97
0.95
500
700
900
1100
1300
1500
–40
–15
10
35
60
85
–45 –30 –15
0
15 30 45 60 75 90
TEMPERATURE (°C)
FREQUENCY (kHz)
TEMPERATURE (°C)
3458 G07
3458 G08
3458 G09
SHDN Pin Threshold and
Hysteresis
Maximum Load Current
SYNC Pin Threshold
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
1200
1000
800
600
400
200
0
1.0
0.9
1.8 to 5.5V at 700kHz
IN
ILIM
5V
OUT
R
= 124k
3.3V
7.5V
OUT
OUT
OPERATING
SHUTDOWN
0.8
0.7
0.6
0.5
1.5
3.5
4.5 5.0
2.0 2.5 3.0
4.0
5.5
–45 –30 –15
0
15 30 45 60 75 90
–45 –30 –15
0
15 30 45 60 75 90
V
IN
(V)
TEMPERATURE (°C)
TEMPERATURE (°C)
3458 G11
3458 G12
3458 G10
FB Voltage
1.25
1.24
1.23
1.22
1.21
1.20
–45 –30 –15
0
15 30 45 60 75 90
TEMPERATURE (°C)
3458 G13
3458f
4
LTC3458
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Fixed Frequency (FF)
Discontinuous Current
Fixed Frequency (FF)
Continuous Current
I
L
200mA/DIV
SW
2V/DIV
SW
2V/DIV
I
L
100mA/DIV
0mA
0mA
V
V
= 3.3V
OUT
L = 10µH
200ns/DIV
V
V
= 3.3V
OUT
L = 10µH
200ns/DIV
IN
IN
= 7V
= 7V
Over-Current with 1.5A ILIMIT
Burst Mode Operation
V
OUT
100mV/DIV
SW
2V/DIV
SW
5V/DIV
I
L
0.5A/DIV
I
L
200mA/DIV
0mA
0mA
V
V
= 3.3V
1µs/DIV
V
V
= 3.3V
OUT
L = 10µH
50µs/DIV
IN
IN
= 7V
= 7V
OUT
L = 10µH
R
= 133k
C
C
= 22µF
ILIM
OUT
= 22pF
FF
Burst Mode Operation Close-Up
Soft-Start into 50Ω Load
V
OUT
V
OUT
100mV/DIV
2V/DIV
V
IN
2V/DIV
SW
5V/DIV
SS
200mV/DIV
I
L
I
L
200mA/DIV
200mA/DIV
0mA
V
V
= 3.3V
= 7V
2µs/DIV
5ms/DIV
V
V
= 3.3V
= 7V
IN
OUT
L = 10µH
IN
OUT
L = 10µH
C
C
= 22µF
OUT
= 22pF
FF
3458f
5
LTC3458
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Sync Operation at 1.33MHz
FF Mode 100-300mA Load Step
SW
5V/DIV
V
OUT
200mV/DIV
SYNC
2V/DIV
COMP
500mV/DIV
I
L
200mA/DIV
I
L
0.5A/DIV
0mA
0mA
500ns/DIV
V
V
= 3.3V
= 5V
200µs/DIV
V
V
= 3.3V
= 7V
OSC
IN
OUT
IN
OUT
R
R
= 33K
= 200k
Z
CC1 = 270pF
CC2 = 10pF
C
L = 10µH
F = 1MHz
= 22µF
OUT
Auto Mode 10mA to 100mA Load
Step
Burst Mode Operation 10mA to
50mA Load Step
V
V
OUT
OUT
200mV/DIV
200mV/DIV
50mA
10mA
BURST
BURST
1V/DIV
500mV/DIV
100mA
10mA
LOAD
LOAD
I
I
L
L
200mA/DIV
200mA/DIV
500µs/DIV
V
V
= 3.3V
200µs/DIV
V
V
= 3.3V
OUT
L = 10µH
IN
IN
= 5V
= 5V
OUT
L = 10µH
C
= 22µF
C
R
= 0.015µF
= 133k
OUT
BURST
BURST
10mA to 200mA Load Step
Showing UV Trip
Forced BURST to FF Mode
Switch with 50mA Load
V
OUT
V
OUT
200mV/DIV
200mV/DIV
–4%
FIXED
FREQUENCY
V
= 3.3V
OUT
IN
BURST
V
= 5V
L = 10µH
R = 33k
CC1 = 270pF
CC2 = 10pF
C = 22µF
V
= 3.3V
IN
V
= 5V
Z
OUT
L = 10µH
I
L
I
L
500mA/DIV
200mA/DIV
C
BURST
R
= 0.015µF
= 133k
OUT
BURST
200µs/DIV
200µs/DIV
3458f
6
LTC3458
U
U
U
PI FU CTIO S
SW (Pin 1): Switch Pin for Inductor Connection. During
discontinuous conduction mode an antiring resistor con-
nects SW to VIN to reduce noise.
GND (Pin 9): Signal Ground Pin.
SS (Pin 10): Connect a capacitor between this pin and
ground to set soft-start period. 5µA of current is sourced
from SS during soft-start.
VIN (Pin 2): Input Supply Pin. Connect this to the input
supply and decouple with 1µF minimum.
t(msec) = CSS (µF )• 200
SYNC (Pin 3): Oscillator Synchronization Pin. A clock
pulse width of 100ns to 2µs is required to synchronize the
internal oscillator. This pin is disabled when grounded.
BURST (Pin 11): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
BurstModeoperationisentered,accordingtotheformula:
SHDN (Pin 4): Shutdown Pin. Grounding this pin shuts
down the IC. Connect to >1.25V to enable.
10
IBURST
RBURST
=
ILIM (Pin 5): Adjustable Peak Current Limit. Connect a
resistor from ILIM to GND to program the peak inductor
current according to the following formula:
where RBURST is in kΩ and IBURST is in amps.
200
COUT •VOUT
ILIMIT
=
CBURST
=
R
ILIM
10,000
where ILIMIT is in amps and RT is in kΩ.
where CBURST(MIN) and COUT are in µF.
RT (Pin 6): Connect a resistor to ground to program the
oscillator frequency, according to the formula:
To force fixed frequency PWM mode, connect BURST to
VOUT through a 50k resistor.
1
VOUT (Pin 12): Output of the Synchronous Rectifier and
fOSC
=
0.2 + 0.004 •RT
where fOSC is in MHz and RT is in kΩ.
Internal Gate Drive Source for the Power Switches.
⎛
⎞
R2
R1
VOUT = 1.23 1+
⎜
⎟
⎝
⎠
FB (Pin 7): Connect Resistor Divider Tap Here. The output
voltage can be adjusted from 2V to 7.5V. Feedback refer-
ence voltage is typically 1.23V.
Exposed Pad (PGND) (Pin 13): Must be soldered to PCB
ground, for electrical contact and optimum thermal
performance.
COMP (Pin 8): gm Error Amp Output. A frequency com-
pensation network is connected from this pin to ground to
compensate the loop. See the section “Compensating the
Feedback Loop” for guidelines.
3458f
7
LTC3458
W
BLOCK DIAGRA
BURST
11
SW
1
V
CC
2
V
CC
ANTIRING
OSC/SYNC
SYNC
SLOPE
3
6
–
+
V
UNDER
MODE
SELECT
BURST MODE
CONTROL
MAX
DUTY
P-DRIVE
N-DRIVE
R
T
SW1
V
BEST
I
ZERO
I
ZERO
DETECT
P-DRIVE
N-DRIVE
PWM
AND
V
12
OUT
BURST MODE
P-DRIVE
4%
UNDERVOLTAGE
DRIVE LOGIC
UNDER
PGND
ERROR AMPLIFIER/
SLEEP TO
ALL BLOCKS
SLEEP
MODE
BURST COMPARATOR
SLEEP
CONTROL
BURST ACTIVE
–
+
7
FB
V
CC
(DISABLED IN
BURST MODE)
PEAK CURRENT
COMPARATOR
I
FIXED
COMP/LIMIT_PEAK
BIAS
FREQUENCY
BURST MODE
MUX
MODE
I
BURST_PEAK
SLOPE
REFERENCE/
BIAS
CURRENTS
I
I
, I ,
COMP LIMIT
BURST_PEAK
SLOPE COMP
I_SENSE
UVLO
,
TSD
V
SD
SOFT-START
THERMAL SD
BEST
N-DRIVE
SDB
TO ALL BLOCKS
PGND
9
5
13
PGND
10
8
4
3458 BD
GND
I
SS
COMP
SHDN
LIM
W U U
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APPLICATIO S I FOR ATIO
Detailed Description
LTC3458 Programmable Functions
The LTC3458 provides high efficiency, low noise power
forboostapplicationswithoutputvoltagesupto7.5V. The
true output disconnect feature eliminates inrush current,
and allows VOUT to go to zero during shutdown. The
current mode architecture with adaptive slope compensa-
tion provides ease of loop compensation with excellent
transient load response. The low RDS(ON), low gate charge
synchronous switches eliminate the need for an external
Schottky rectifier, and provide efficient high frequency
pulse width modulation (PWM) control. High efficiency is
achieved at light loads when Burst Mode operation is
entered, where the IC’s quiescent current is a low 15µA
typicalonVIN.TheLTC3458isdesignedtoprovidecustom
performance in a variety of applications with program-
mable feedback, current limit, oscillator frequency, soft-
start, and Burst Mode threshold.
Current Limit/Peak Burst Current. The programmable
current limit circuit sets the maximum peak current in the
internal N-channel MOSFET switch. This clamp level is
programmed using a resistor to ground on ILIM. In Burst
Mode operation, the current limit is automatically set to
~1/4 of the programmed current limit for optimal effi-
ciency. A 124k RILIM resistor is recommended in most
applications unless a lower limit is needed to prevent the
external inductor from saturating.
200
R
ILIM
=
I is in amps and R is in kΩ.
1
4
I
BURSTPEAK ≈ •ILIM
3458f
8
LTC3458
U
W U U
APPLICATIO S I FOR ATIO
ErrorAmp. Theerroramplifierisatransconductancetype,
with its positive input internally connected to the 1.23V
reference, anditsnegativeinputconnectedtoFB. Asimple
compensation network is placed from COMP to ground.
Internal clamps limit the minimum and maximum error
amp output voltage for improved large signal transient
response. During sleep (in Burst Mode), the compensa-
tion pin is high impedance, however clamps limit the
voltageontheexternalcompensationnetwork,preventing
the compensation capacitor from discharging to zero
during the sleep time.
Current Sensing. Lossless current sensing converts the
peak current signal to a voltage to sum in with the internal
slope compensation. This summed signal is compared to
theerroramplifieroutputtoprovideapeakcurrentcontrol
command for the PWM. The slope compensation in the IC
is adaptive to the input and output voltage, therefore the
converterprovidestheproperamountofslopecompensa-
tion to ensure stability, but not an excess to cause a loss
of phase margin in the converter.
Output Disconnect and Inrush Limiting. The LTC3458 is
designed to allow true output disconnect by eliminating
body diode conduction of the internal P-channel MOSFET
rectifier. This allows V0UT to go to zero volts during
shutdown, drawing no current from the input source. It
also allows for inrush current limiting at turn-on, minimiz-
ing surge currents seen by the input supply. Note that to
obtaintheadvantagesofoutputdisconnect, theremustbe
no external Schottky diodes connected between SW and
Oscillator. The frequency of operation is set through a
resistor from RT to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency is
calculated using the following formula:
1
fOSC
=
0.2 + 0.004 •RT
VOUT
.
where fOSC is in MHz and RT is in kΩ
Shutdown. The part is shut down by pulling SHDN below
0.3V, andmadeactivebypullingthepinabove1.25V. Note
that SHDN can be driven above VIN or VOUT, as long as it
is limited to less than 8V.
The oscillator can be synchronized with an external clock
applied to the SYNC pin. When synchronizing the oscilla-
tor, the free running frequency must be set to approxi-
mately 30% lower than the desired synchronized fre-
quency.
Synchronous Rectifier. To prevent the inductor current
from running away, the P-channel MOSFET synchronous
rectifier is only enabled when VOUT > (VIN + 0.25V).
Soft-Start. The soft-start time is programmed with an
external capacitor to ground on SS. An internal current
source charges it with a nominal 5µA. The voltage on the
SS pin (in conjunction with the external resistor on ILIM) is
used to control the peak current limit until the voltage on
the capacitor exceeds ~1V, at which point the external
resistor sets the peak current. In the event of a com-
manded shutdown, severe short-circuit, or a thermal
shutdown, the capacitor is discharged automatically.
Thermal Shutdown. If the die temperature reaches ap-
proximately 150°C, the part will go into thermal shutdown
and all switches will be turned off and the soft-start
capacitorwillbereset. Thepartwillbeenabledagainwhen
the die temperature has dropped by 10°C (nominal).
Zero Current Amplifier. The zero current amplifier moni-
tors the inductor current to the output and shuts off the
synchronous rectifier once the current is below 50mA
typical, preventing negative inductor current.
t(msec) = CSS (µF) • 200
Other LTC3458 Features and Functions
Burst Mode Operation
Antiringing Control. The antiringing control places a
resistoracrosstheinductortodamptheringingonSWpin
discontinuous conduction mode. The LC ringing
(L=inductor,CSW =CapacitanceonSWpin)islowenergy,
but can cause EMI radiation.
BurstModeoperationcanbeautomaticorusercontrolled.
In automatic operation, the IC will automatically enter
Burst Mode operation at light load and return to fixed
frequency PWM mode for heavier loads. The user can
program the average load current at which the mode
3458f
9
LTC3458
W U U
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APPLICATIO S I FOR ATIO
transition occurs using a single resistor. During Burst
Mode operation, the oscillator is shut down, since the on
time is determined by the time it takes the inductor current
to reach a fixed peak current, and the off time is deter-
minedbythetimeittakesfortheinductorcurrenttoreturn
to zero.
COUT •VOUT
10,000
CBURST
=
where CBURST(MIN) and COUT are in µF.
Note: the BURST pin only sources current based on
current delivered to VOUT through the P-channel MOSFET.
Ifcurrentintheinductorisallowedtogonegative(thiscan
occuratverylightloadsandhighstep-upratios), theburst
threshold may become inaccurate, preventing the IC from
entering Burst Mode operation. For RBURST values greater
than200k,alargerthanrecommendedinductorvaluemay
be needed to ensure positive inductor current and auto-
matic Burst Mode operation.
In Burst Mode operation, the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
15µA of quiescent current. In this mode the output ripple
voltage has a variable frequency component with load
current and will be typically 2% peak-to-peak. This maxi-
mizes efficiency at very light loads by minimizing switch-
ing and quiescent losses. Burst Mode ripple can be re-
duced slightly by using more output capacitance (22µF or
greater). ThiscapacitordoesnotneedtobealowESRtype
if low ESR ceramics are also used. Another method of
reducing Burst Mode ripple is to place a small feed-
forward capacitor across the upper resistor in the VOUT
feedback divider network.
Intheeventthatasuddenloadtransientcausesthevoltage
level on FB to drop by more than 4% from the regulation
value, an internal pull-up is applied to BURST, forcing the
part quickly out of Burst Mode operation. For optimum
transient response when going between Burst Mode op-
eration and PWM mode, Burst can be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
drop. Note that Burst Mode operation is inhibited during
start-up and soft-start.
During Burst Mode operation, COMP is disconnected
from the error amplifier in an effort to hold the voltage on
the external compensation network where it was before
entering Burst Mode operation. To minimize the effects of
leakage current and stray resistance, voltage clamps limit
the minimum and maximum voltage on COMP during
Burst Mode operation. This minimizes the transient expe-
rienced when a heavy load is suddenly applied to the
converter after being in Burst Mode operation for an
extended period of time.
Manual Control
For applications requiring fixed frequency operation at all
load currents, connect the BURST pin to VOUT through a
50kΩ resistor. To force Burst Mode operation, ground the
BURST pin.
Forapplicationswherealargeloadstepcanbeanticipated,
the circuit below can be used to reduce the voltage
transient on VOUT. Automatic operation is achieved when
the external PMOS is off and fixed frequency operation is
commanded when the external PMOS is on. In shutdown,
the PMOS should be off.
For automatic operation, an RC network should be con-
nected from BURST to ground. The value of the resistor
will control the average load current (IBURST) at which
Burst Mode operation will be entered and exited (there is
hysteresis to prevent oscillation between modes). The
equation given for the capacitor on BURST is for the
minimum value, to prevent ripple on the BURST pin from
causing the part to oscillate in and out of Burst Mode
operationatthecurrentwherethemodetransitionoccurs.
V
IN
HIGH: AUTO MODE
LOW: FIXED FREQUENCY
PMOS
BURST
0.01µF
133k
10
RBURST
=
IBURST
3458 FO2
where RBURST is in kΩ and IBURST is in amps.
Figure 1
3458f
10
LTC3458
W U U
APPLICATIO S I FOR ATIO
U
COMPONENT SELECTION
Some example inductor part types are:
Coilcraft: DO1608 and MSS5131 Series
TDK: RLF5018T and SLF7045 Series
Murata: LQH4C and LQN6C Series
Sumida: CDRH4D28 and CDRH6D28 Series
COEV: DQ7545 Series
Inductor Selection
The high frequency operation of the LTC3458 allows for
the use of small surface mount inductors. Since the
internalslopecompensationcircuitreliesontheinductor’s
current slope and frequency, Table 1 should be used to
select an inductor value for a given frequency of operation
(± 25%). The recommended value will yield optimal tran-
sient performance while maintaining stable operation.
Inductor values larger than listed in Table 1 are permis-
sible to reduce the current ripple.
TOKO: D62CB and D63LCB Series
..
WURTH: WE-PD2 Series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
tochargeintothecapacitoreachcycle. Themaxrippledue
to charge is given by:
Table 1. Recommended Inductor Values
Frequency
1.5MHz
1.25MHz
1MHz
Inductor Value(µH)
3.3 to 4.7
IP •V
COUT •VOUT • f
IN
4.7 to 6.8
VRBULK
=
6.8 to 10
750Hz
10 to 15
where IP = peak inductor current and f = switching
frequency.
500kHz
15 to 22
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
For high efficiency, choose an inductor with high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I2R losses, and must be
able to handle the peak inductor current without saturat-
ing. Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents in the
1A to 3A region. To minimize radiated noise, use a
toroidal or shielded inductor. (Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily). See Table 2 for a list of inductor
manufacturers.
VRCESR = IP • CESR
where CESR = Capacitor Series Resistance.
The ESL (equivalent series inductance) is also an impor-
tant factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the VOUT pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
outputvoltageripple.Forsurfacemountapplications,AVX
TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo
Yuden X5R type ceramic capacitors are recommended.
For through-hole applications, Sanyo OS-CON capacitors
offer low ESR in a small package size.
Table 2. Inductor Vendor Information
Supplier
Coilcraft
TDK
Phone
Website
(847) 639-6400
(847) 803-6100
www.coilcraft.com
www.component.tdk.com
Murata
USA: (814) 237-1431
(800) 831-9172
In all applications, a minimum of 4.7µF (generally 22µF is
recommended), low ESR ceramic capacitor should be
placed as close to the VOUT pin as possible, and grounded
to a local ground plane.
www.murata.com
Sumida
USA: (847) 956-0666
Japan: 81-3-3607-5111
www.japanlink.com/sumida
www.coev.net
COEV
Toko
(800) 227-7040
(847) 297-0070
(202) 785-8800
www.tokoam.com
..
Wurth
www.we-online.com
3458f
11
LTC3458
W U U
U
APPLICATIO S I FOR ATIO
Input Capacitor Selection
Compensating the Feedback Loop
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
In most applications >1µF per amp of peak input current
is recommended. See Table 3 for a list of capacitor
manufacturers for input and output capacitor selection.
The LTC3458 uses current mode control, with internal
adaptiveslopecompensation.Currentmodecontrolelimi-
nates the 2nd order filter due to the inductor and output
capacitor exhibited in voltage mode controllers, and sim-
plifies the power loop to a single pole filter response. The
product of the modulator control to output DC gain, and
the error amp open-loop gain gives the DC gain of the
system:
Table 3. Capacitor Vendor Information
Supplier
AVX
Phone
(803) 448 - 9411
Website
www.avxcorp.com
www.sanyovideo.com
www.component.tdk.com
Sanyo
TDK
(619) 661 - 6322
(847) 803 - 6100
VREF
VOUT
GDC = GCONTROL •GEA
•
•GCURRENT_SENSE
Murata
USA: (814) 237-1431
(800) 831-9172
www.murata.com
www.t-yuden.com
2•V
IOUT
IN
GCONTROL
=
,
Taiyo Yuden
(408) 573 - 4150
1
RDS(ON)
GEA ≈ 1,000
GCURRENT_SENSE
=
Operating Frequency Selection
Thereareseveralconsiderationsinselectingtheoperating
frequency of the converter. The first is staying clear of
sensitive frequency bands, which cannot tolerate any
spectral noise. For example in products incorporating RF
communications the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed.Thesecondconsiderationisthephysicalsizeof
the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
Thetradeoffisinefficiency,sincetheswitchinglossesdue
to gate charge increase proportional with frequency.
The output filter pole is given by:
IOUT
fFILTER_POLE
=
,
π •VOUT •COUT
where COUT is the output filter capacitor.
The output filter zero is given by:
1
fFILTER_ZERO
=
,
2π •RESR •COUT
where RESR is the output capacitor equivalent series
resistance.
Thermal Considerations
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
For the LTC3458 to deliver its full output power, it is
imperative that a good thermal path be provided to dissi-
pate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to
conductheatawayfromtheICandintoacopperplanewith
asmuchareaaspossible. Ifthejunctiontemperaturerises
above ~150°C, the part will go into thermal shutdown, and
all switching will stop until the temperature drops.
2
V
IN
fRHPZ
=
2π •IOUT •VOUT •L
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
3458f
12
LTC3458
W U U
APPLICATIO S I FOR ATIO
rolled off before the RHP zero frequency.
U
V
1.25V
OUT
+
–
The typical error amp compensation is shown in Figure 2.
The equations for the loop dynamics are as follows:
R1
R2
ERROR
AMP
FB
7
1
COMP
8
fPOLE1
fZERO1
fPOLE2
≈
=
≈
which is close to DC
2π •10e6 •CC1
CC1
Z
1
CC2
R
2π •RZ •CC1
1
2π •RZ •CC2
3458 F01
Figure 2
3458f
13
LTC3458
U
TYPICAL APPLICATIO S
Lithium-Ion to 5V, 500mA at 850kHz
WURTH
12µH
774775112
Li-Ion to 5VOUT
100
95
90
85
80
75
70
65
4.2V
SW
IN
V
IN
Li-Ion
2.5V to 4.2V
LTC3458
2.2µF
V
3.6V
OUT
IN
V
GND/PGND
SHDN
5V
OUT
450mA
2.5V
IN
10pF
1M
FB
ON OFF
COMP
SS
SYNC
324k
0.01µF
33k
R
T
22µF
X5R
0.01µF
10pF
0.1
1
10
100
1000
BURST
I
LIM
LOAD CURRENT (mA)
560pF
243k 124k
133k
3458 TA03b
3458 TA03a
Two Cell to 5VOUT, 200mA at 850kHz
WURTH
12µH
774775112
Two Alkaline to 5VOUT
100
95
90
85
80
75
70
65
SW
V
IN
2 ALKALINE
1.8V to 3.3V
LTC3458
2.2µF
V
OUT
3.3V
IN
V
GND/PGND
SHDN
5V
OUT
200mA
10pF
1M
FB
ON OFF
1.8V
IN
COMP
SS
SYNC
324k
0.01µF
33k
R
T
22µF
X5R
0.01µF
10pF
BURST
I
LIM
0.1
1
10
100
1000
560pF
243k 124k
133k
LOAD CURRENT (mA)
3458 TA04a
3458 TA04b
Lithium-Ion Battery to 7VOUT, 250mA at 1MHz
COEV
10µH
DQ7545
Li-Ion to 7VOUT
100
95
90
85
80
75
70
65
SW
V
IN
Li-Ion
2.5V to 4.2V
LTC3458
2.2µF
V
OUT
4.2V
3.6V
IN
V
GND/PGND
SHDN
7V
OUT
FB
250mA
10pF
1.5M
316k
IN
ON OFF
2.5V
IN
COMP
SS
SYNC
0.01µF
33k
R
T
22µF
X5R
0.01µF
10pF
BURST
I
LIM
0.1
1
10
100
1000
560pF
200k 124k
133k
LOAD CURRENT (mA)
3458 TA05b
3458 TA05a
3458f
14
LTC3458
U
PACKAGE DESCRIPTIO
DE/UE Package
12-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1695)
0.58 ±0.05
3.40 ±0.05
2.24 ±0.05 (2 SIDES)
1.70 ±0.05
PACKAGE OUTLINE
0.25 ± 0.05
0.50
BSC
3.30 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
0.38 ± 0.10
4.00 ±0.10
(2 SIDES)
R = 0.115
TYP
7
12
R = 0.20
TYP
3.00 ±0.10 1.70 ± 0.10
(2 SIDES)
(2 SIDES)
PIN 1
TOP MARK
PIN 1
NOTCH
(UE12/DE12) DFN 0802
6
0.25 ± 0.05
1
0.75 ±0.05
0.200 REF
0.50
BSC
3.30 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE A VARIATION OF VERSION
(WGED) IN JEDEC PACKAGE OUTLINE M0-229
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
4. EXPOSED PAD SHALL BE SOLDER PLATED
3458f
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
LTC3458
U
TYPICAL APPLICATIO
Dual Lumiled Application with BURST Pin Current Regulation
L1
2-Lumileds in Series
SW
V
IN
Li-Ion
f
= 850kHz
Z1
100
90
80
70
60
50
C
OSC
IN
2.2µF
LTC3458
2.7V to 4.2V
V
OUT
6.4V TO 6.8V
V
GND/PGND
SHDN
OUT
150mA, 6.4V
250mA, 6.6V
FB
ON OFF
COMP
SS
SYNC
D1
D2
350mA, 6.8V
0.01µF
33k
C
OUT
R
T
2.2µF
0.01µF
BURST
I
LIM
NOTE: LUMILED CURRENT REGULATION
~10% OVER V RANGE
0.01µF
243k
124k
R
BURST
IN
2.5 3.0 3.5
4.0 4.5 5.0 5.5
2.0
C
, C : TAIYO YUDEN JMK107BJ225MA
D1, D2: LUXEON EMITTER LUMILED WHITE
LXHLMW1D (2.9V AT 350mA)
R
BURST
: 35.7k FOR 350mA,
47.5k FOR 250mA,
82.5k FOR 150mA
3458 TA06a
IN OUT
INPUT VOLTAGE (V)
3458 TA06b
L1: Wurth 12µH 774775112
Z1: CENTRAL SEMI 6.8V ZENER DIODE SOT-23 CMPZ5235B
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550mA I , 600kHz/1.1MHz, High Efficiency Step-Up
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1.5A I , 1.25MHz, High Efficiency Step-Up DC/DC Converter
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SW
IN
LTC3400/
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600mA I , 1.2MHz, Synchronous Step-Up DC/DC Converter
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LTC3402
LTC3425
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IN
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600mA, 500kHz, Synchronous Step-Up DC/DC Converter
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IN
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85mA I , Constant Off-Time, High Efficiency Step-Up DC/DC
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V : 2.3V to 10V, V
ThinSOT
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OUT(MAX) Q SD
SW
IN
3458f
LT/TP 0904 1K • PRINTED IN USA
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16
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