LINEAR LTC3442EDE

LTC3442
Micropower Synchronous
Buck-Boost DC/DC Converter with
Automatic Burst Mode Operation
DESCRIPTIO
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FEATURES
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Regulated Output with Input Voltages Above,
Below, or Equal to the Output
Single Inductor, No Schottky Diodes Required
Manual or Programmable Automatic Burst Mode®
Operation
Programmable Average Input Current Limit
Up to 1.2A Continuous Output Current from a Single
Lithium-Ion Cell
High Efficiency: Up to 95%
Output Disconnect in Shutdown
2.4V to 5.5V Input Range
2.4V to 5.25V Output Range
35µA Quiescent Current in Burst Mode Operation
Programmable Frequency from 300kHz to 2MHz
<1µA Shutdown Current
Small, Thermally Enhanced 12-Lead (4mm × 3mm)
DFN Package
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APPLICATIO S
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The LTC®3442 is a highly efficient, fixed frequency, BuckBoost DC/DC converter, which operates from input voltages above, below, and equal to the output voltage. The
topology incorporated in the IC provides a continuous
transfer function through all operating modes, making the
product ideal for a single Lithium-Ion or multicell alkaline
applications where the output voltage is within the battery
voltage range.
The device includes two 0.10Ω N-channel MOSFET
switches and two 0.10Ω P-channel switches. Operating
frequency and average input current limit can each be
programmed with an external resistor. Quiescent current
is only 35µA in Burst Mode operation, maximizing battery
life in portable applications. Automatic Burst Mode operation allows the user to program the load current for Burst
Mode operation, or to control it manually.
Other features include 1µA shutdown current, programmable soft-start, peak current limit and thermal shutdown.
The LTC3442 is available in a low profile, thermally enhanced 12-lead (4mm × 3mm × 0.75mm) DFN package.
PDA/‘SMART’ Phones
Handheld Computers
MP3 Players
Handheld Instruments
Digital Cameras
Wireless Handsets
USB Peripherals
, LTC and LT are registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners. Burst Mode is a registered
trademark of Linear Technology Corporation. Protected by U.S. Patents including 6404251,
6166527.
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TYPICAL APPLICATIO
Efficiency vs VIN
4.7µH
100
300mA LOAD
SW2
VIN
VOUT
1M
Li-Ion
340k
LTC3442
SHDN/SS
FB
RLIM
VC
0.01µF
2.2k
22µF
15k
RT
71.5k
SGND
1A LOAD
80
70
220pF
470pF
10µF
VOUT
3.3V
1.2A
90
EFFICIENCY (%)
VIN
2.5V TO
4.2V
SW1
60
VOUT = 3.3V
L = 4.7µH
F = 600kHz
BURST
PGND
0.01µF
200k
50
200k
2.5
3442 TA01a
3.0
3.5
4.0
VIN (V)
4.5
5.0
5.5
3442 • TA01b
3442f
1
LTC3442
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
VIN, VOUT Voltage........................................... – 0.3 to 6V
SW1, SW2 Voltage
DC ................................................................. – 0.3 to 6V
Pulsed <100ns ............................................... – 0.3 to 7V
SHDN/SS, BURST Voltage ............................. – 0.3 to 6V
Operating Temperature (Note 2) ............. – 40°C to 85°C
Maximum Junction Temperature (Note 4) ............ 125°C
Storage Temperature Range ................. – 65°C to 125°C
ORDER PART
NUMBER
TOP VIEW
SHDN/SS
1
12 FB
RT
2
11 VC
SGND
3
10 RLIM
SW1
4
9
VIN
PGND
5
8
VOUT
SW2
6
7
BURST
13
LTC3442EDE
DE PART
MARKING
DE12 PACKAGE
12-LEAD (4mm × 3mm) PLASTIC DFN
TJMAX = 125°C
θJA = 53°C/W 1-Layer Board
θJA = 43°C/W 4-Layer Board
θJC = 4.3°C/W
EXPOSED PAD IS PGND (PIN 13)
MUST BE SOLDERED TO PCB
3442
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 3.6V, RT = 64.9k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
Input Start-Up Voltage
●
Output Voltage Adjust Range
●
2.4
●
1.19
Feedback Voltage
TYP
MAX
UNITS
2.3
2.4
V
5.25
V
1.22
1.25
V
Feedback Input Current
VFB = 1.22V
1
50
nA
Quiescent Current – Burst Mode Operation
VFB = 1.22V, BURST = 0V (Note 3)
35
60
µA
Quiescent Current – Shutdown
SHDN = 0V, VOUT = 0V, Not Including Switch Leakage
0.1
1
µA
Quiescent Current – Active
BURST = VIN (Note 3)
600
1100
µA
NMOS Switch Leakage
Switches B and C
0.1
2
µA
PMOS Switch Leakage
Switches A and D
0.1
3
µA
NMOS Switch On Resistance
Switches B and C
0.10
Ω
PMOS Switch On Resistance
Switches A and D
0.10
Ω
●
Input Current Limit
3
A
Reverse Current Limit
0.5
A
Burst Mode Operation Current Limit
0.9
A
88
%
%
Max Duty Cycle
Boost (% Switch C On)
Buck (% Switch A In)
●
●
Min Duty Cycle
●
Frequency Accuracy
●
2
70
100
570
670
0
%
770
kHz
Error Amp AVOL
90
dB
Error Amp Source Current
11
µA
Error Amp Sink Current
300
µA
Burst Threshold (Falling)
0.88
V
Burst Threshold (Rising)
1.12
V
3442f
2
LTC3442
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = VOUT = 3.6V, RT = 64.9k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
Burst Current Ratio
Ratio of IOUT to IBURST
20,000
Input Current Ratio
Ratio of IIN to IRLIM, IIN = 0.5A
70,000
RLIM Threshold
When IC is Enabled
When EA is at Max Boost Duty Cycle
SHDN/SS Input Current
VSHDN = 5.5V
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LTC3442E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
●
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100
Burst Mode
90 OPERATION
70
VIN = 2.5V
60
VIN = 5V
VIN = 2.5V
FIXED FREQUENCY
40
100
70
POWER LOSS
60
10
FIXED FREQUENCY
50
40
30
VOUT = 3.3V
600kHz
1
10
100
LOAD (mA)
1000
10000
30
Average Input Current Limit
vs Frequency (Normalized)
15
0.40
0.30
0.20
VIN = 5V
% CHANGE (NORMALIZED)
0.50
4
2
0
–2
–4
10
VOUT SHORTED
5
VOUT DROPS 10%
0
–5
–6
0.10
0.00
3442 G03
VOUT = 3.3V
6 1MHz
0.60
VIN = 3.6V
VOUT = 3.3V
80
400 600 800 1000 1200 1400 1600 1800 2000
FREQUENCY (kHz)
8
VIN = 3.6V
VOUT = 3.3V
RLIM = 133k
0.70
WITHOUT SCHOTTKY DIODES
86
Average Input Current Limit
vs VIN (Normalized)
% CHANGE (NORMALIZED)
RLIM VOLTAGE (V)
0.80
88
3442 G02
Input Current Mirror Linearity
0.90
90
82
0.1
10000
10
100
1000
LOAD CURRENT (mA)
3442 G01
1.00
µA
84
VIN = 3.6V
VOUT = 3.3V
1
1
WITH SCHOTTKY DIODES
92
1
20
0.1
0.01
94
80
EFFICIENCY (%)
EFFICIENCY (%)
80 VIN = 3.3V
V
V
96
EFFICIENCY (%)
Burst Mode
90 OPERATION V = 5V
IN
1.4
2.4
Efficiency vs Frequency
1000
POWER LOSS (mW)
VIN = 3.3V
0.7
2.2
(TA = 25°C unless otherwise specified).
Efficiency and Power Loss vs Load
Efficiency vs Load
100
0.4
V
Note 3: Current Measurements are performed when the outputs are not
switching.
Note 4: 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.
Continuous operation above the specified maximum operating junction
temperature may result in device degradation or failure.
TYPICAL PERFOR A CE CHARACTERISTICS
20
0.1
UNITS
0.95
SHDN/SS Threshold
50
MAX
0 .05 .10 .15 .20 .25 .30 .35 .40 .45 .50
INPUT CURRENT (A)
3442 G04
–8
2.5
3.0
3.5
4.0
4.5
5.0
VIN (V)
3442 G05
–10
0.50
0.75
1.00 1.25 1.50
FREQUENCY (MHz)
1.75
2.00
3442 TA01b
3442f
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LTC3442
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TYPICAL PERFOR A CE CHARACTERISTICS
Burst Mode Quiescent Current
vs VIN
Quiescent Current vs VIN
(Fixed Frequency Mode)
45
VIN QUIESCENT CURRENT (µA)
2.0 MHz
3.0
1.5 MHz
2.5
1.0 MHz
2.0
1.5
0.5 MHz
1.0
NO SWITCHING
0.5
3.0
40
INPUT CURRENT (A)
3.5
VIN QUIESCENT CURRENT (mA)
Peak Current Clamp vs VIN
3.5
50
4.0
35
30
25
20
15
10
2.0
1.5
1.0
0.5
0.0
2.5
0
3.0
3.5
4.0
VIN (V)
4.5
5.0
2.5
5.5
3.0
3.5
4.0
VIN (V)
4.5
5.0
5.5
Automatic Burst Mode Threshold
vs RBURST
150
2.29
4%
2.28
3%
2.27
2%
120
110
100
90
80
70
ENTER Burst Mode
OPERATION
60
150
175
200
RBURST (kΩ)
225
CHANGE FROM 25°C
5%
MINIMUM START VOLTAGE (V)
2.30
130
2.26
2.25
2.24
2.23
35
65
5
TEMPERATURE (°C)
95
125
1.5%
0.8%
SW1
2V/DIV
CHANGE FROM 25°C
0.4%
SW2
2V/DIV
0.2%
0.0%
–2.0%
–0.4%
50ns/DIV
VIN = 2.9V
VOUT = 3.3V AT 500mA
–0.6%
–2.0%
–55 –35 –15
–0.8%
5 25 45 65 85 105 125
TEMPERATURE (°C)
3442 G13
Switch Pins Before Entering
Boost Mode
VIN = VOUT = 3.3V
0.6%
–1.5%
5 25 45 65 85 105 125
TEMPERATURE (°C)
3442 G12
Feedback Voltage vs Temperature
(Normalized)
1.0%
–1.0%
–5%
–55 –35 –15
3442 G11
2.0%
–0.5%
VIN = VOUT = 3.3V
–2%
–3%
–25
5.5
0%
–4%
Frequency Change vs Temperature
(Normalized)
0.0%
5.0
–1%
2.21
3442 G10
0.5%
4.5
1%
2.22
2.20
–55
250
1.0%
4.0
VIN (V)
Average Input Current Limit
vs Temperature (Normalized)
Minimum Start Voltage
vs Temperature
LEAVE Burst Mode
OPERATION
3.5
3442 G09
160
140
3.0
3442 G08
3442 G07
CHANGE FROM 25°C
2.5
5
0.0
2.5
LOAD CURRENT (mA)
(TA = 25°C unless otherwise specified).
–1.0%
–55 –35 –15
3442 G15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3442 G14
3442f
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LTC3442
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TYPICAL PERFOR A CE CHARACTERISTICS
(TA = 25°C unless otherwise specified).
Switch Pins Entering
Buck-Boost Mode
Switch Pins in Buck-Boost Mode
Output Ripple at 1A Load
SW1
2V/DIV
SW1
2V/DIV
VIN = 2.7V
VIN = 3.3V
SW2
2V/DIV
SW2
2V/DIV
VIN = 4.2V
1µs/DIV
50ns/DIV
VIN = 3.3V
VOUT = 3.3V AT 500mA
Load Transient Response in Fixed
Frequency Mode, No Load to 1A
Load Transient Response in Auto
Burst Mode Operation, No Load to 1A
VOUT
100mV/DIV
VOUT
100mV/DIV
LOAD
0.5A/DIV
LOAD
0.5A/DIV
100µs/DIV
VIN = 3.6V
VOUT = 3.3V
COUT = 47µF, X5R CERAMIC
3442 G17
50ns/DIV
VIN = 4.2V
VOUT = 3.3V AT 500mA
3442 G16
3442 G19
3442 G18
VOUT
20mV/DIV
AC COUPLED
Burst Mode Operation
VOUT
50mV/DIV
INDUCTOR
CURRENT
0.5A/DIV
100µs/DIV
VIN = 3.6V
VOUT = 3.3V
COUT = 47µF, X5R CERAMIC + 100µF
LOW ESR TANTALUM
3442 G20
Transition from Burst Mode
Operation to Fixed Frequency Mode
20µs/DIV
COUT = 100µF
LOW ESR TANTALUM
3442 G21
Pulsed Overload Using Average
Input Current Limit
VOUT
2V/DIV
VOUT
50mV/DIV
RLIM PIN
0.5V/DIV
INDUCTOR
CURRENT
0.5A/DIV
INDUCTOR
CURRENT
0.5A/DIV
200µs/DIV
COUT = 100µF
LOW ESR TANTALUM
3442 G22
1ms/DIV
3442 G23
RLIM = 133k
CLIM = .001µF
3442f
5
LTC3442
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PI FU CTIO S
SHDN/SS (Pin 1): Combined Soft-Start and Shutdown.
Applied voltage <0.4V shuts down the IC. Tie to >1.4V to
enable the IC and >2.4V to ensure the error amp is not
clamped from soft-start. An RC network from the shutdown command signal to this pin will provide a soft-start
function by limiting the rise time of the VC pin.
BURST (Pin 7): Used to set the Automatic Burst Mode
Operation Threshold. Place a resistor and capacitor in
parallel from this pin to ground. See the Applications
Information section for component value selection. For
manual control, ground the pin to force Burst Mode
operation, connect to VOUT to force fixed frequency mode.
RT (Pin 2): Programs the Frequency of the Internal Oscillator. Place a resistor from this pin to ground. See the
Applications Information section for component value
selection.
VOUT (Pin 8): Output of the Synchronous Rectifier. A filter
capacitor is placed from VOUT to GND. A ceramic bypass
capacitor is recommended as close to the VOUT and GND
pins as possible.
SGND (Pin 3): Signal Ground for the IC.
VIN (Pin 9): Input Supply Pin. Internal VCC for the IC. A
10µF ceramic capacitor is recommended as close to VIN
and SGND as possible.
SW1 (Pin 4): Switch Pin Where Internal Switches A and B
are Connected. Connect inductor from SW1 to SW2. An
optional Schottky diode can be connected from SW1 to
ground for a moderate efficiency improvement. Minimize
trace length to reduce EMI.
PGND (Pin 5, 13): Power Ground for the Internal NMOS
Power Switches. The exposed pad must be soldered to
PCB ground to provide both electrical contact and a good
thermal contact to the PCB.
SW2 (Pin 6): Switch Pin Where Internal Switches C and D
are Connected. An optional Schottky diode can be connected from SW2 to VOUT for a moderate efficiency improvement. Minimize trace length to reduce EMI.
RLIM (Pin 10): Sets the Average Input Current Limit
Threshold. Place a resistor and capacitor in parallel from
this pin to ground. See the Applications Information
section for component value selection.
VC (Pin 11): Error Amp Output. A frequency compensation
network is connected from this pin to FB to compensate
the loop. During Burst Mode operation, VC is internally
connected to a hold circuit.
FB (Pin 12): Feedback Pin. Connect resistor divider tap
here. The output voltage can be adjusted from 2.4V to
5.25V. The feedback reference voltage is typically 1.22V.
3442f
6
LTC3442
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W
SI PLIFIED BLOCK DIAGRA
SW1
SW2
4
6
SW D
SW A
8
SW C
–
SW B
GATE
DRIVERS
AND
ANTICROSS
CONDUCTION
VOUT
+
VIN 9
REVERSE
AMP
–
RLIM 10
AV = 6
1V
+
+
AVERAGE
ILIM
Gm = 1/60k
–
3A
–
PWM
LOGIC
2.3V
–
11 VC
UVLO
+
+
PWM
COMPARATORS
–
VIN
AUTOMATIC
BURST MODE
CONTROL AND
VC HOLD
SLEEP
RT
2
12 FB
+
–
5A
1.22V
ERROR
AMP
–
+
+
PEAK CURRENT
LIMIT
OSC
7
VIN
SHDN/SS 1
SHUTDOWN
SOFT-START
VREF
VCC
SS
BURST
1.22V
VREF
THERMAL
SHUTDOWN
SHUTDOWN
2
6
PGND
SGND
3442 BD
3442f
7
LTC3442
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OPERATIO
The LTC3442 provides high efficiency, low noise power
for applications such as portable instrumentation. The
LTC proprietary topology allows input voltages above,
below or equal to the output voltage by properly phasing
the output switches. The error amp output voltage on VC
determines the output duty cycle of the switches. Since VC
is a filtered signal, it provides rejection of frequencies
from well below the switching frequency. The low RDS(ON),
low gate charge synchronous switches provide high
frequency pulse width modulation control at high efficiency. Schottky diodes across the synchronous switch D
and synchronous switch B are not required, but provide a
lower voltage drop during the break-before-make time
(typically 15ns). Schottky diodes will improve peak efficiency by typically 1% to 2%. High efficiency is achieved
at light loads when Burst Mode operation is entered and
the IC’s quiescent current drops to a low 35µA.
Internal Current Limit
There are three different current limit circuits in the
LTC3442. Two have internally fixed thresholds which vary
inversely with VIN, the third is externally programmable,
and does not vary with input voltage.
The first circuit is a high speed peak current limit amplifier
that will shut off switch A if the current exceeds 5A typical.
The delay to output of this amplifier is typically 50ns.
A second amplifier will begin to source current into the FB
pin to drop the output voltage once the peak input current
exceeds 3A typical. This method provides a closed loop
means of clamping the input current. During conditions
where VOUT is near ground, such as during a short-circuit
or during startup, this threshold is cut in half, providing a
foldback feature. For this current limit feature to be most
effective, the Thevenin resistance from FB to ground
should be greater than 100kΩ.
LOW NOISE FIXED FREQUENCY OPERATION
Externally Programmable Current Limit
Oscillator
The frequency of operation is programmed by an external
resistor from RT to ground, according to the following
equation:
f (kHz) =
43, 300
RT(kΩ)
Error Amp
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier (from FB to VC) to obtain stability of the converter. For improved bandwidth, an additional RC feedforward network can be placed across the upper feedback
divider resistor. The voltage on SHDN/SS clamps the
error amp output, VC, to provide a soft-start function.
The third current limit circuit is programmed by an external resistor on RLIM. This circuit works by mirroring the
input current in switch A, averaging it by means of the
external RC network on RLIM, and comparing the resulting
voltage with an internal reference. If the voltage on RLIM
starts to exceed 0.95V, a Gm amplifier will clamp VC,
lowering VOUT to maintain control of the input current.
This allows the user to program a maximum average input
current, for applications such as USB, where the current
draw from the bus must be limited to 500mA. The resistor
and capacitor values are determined by the following
equations:
(
RLIM(kΩ) =
CLIM(µF) ≥
⎛
2 • VIN – VOUT
70 • ⎜ 0.86 +
⎜
40
⎝
)⎞⎟
⎟
⎠
IIN(AMPS)
0.1
RLIM(kΩ)
3442f
8
LTC3442
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OPERATIO
Reverse Current Limit
During fixed frequency operation, the LTC3442 operates
in forced continuous conduction mode. The reverse current limit amplifier monitors the inductor current from
the output through switch D. Once the negative inductor
current exceeds 500mA typical, the IC will shut off switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four
internal switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation for the
LTC3442 as a function of the internal control voltage, VCI.
Depending on the control voltage, the IC will operate in
either buck, buck/boost or boost mode. The VCI voltage is
a level shifted voltage from the output of the error amp (VC)
(see Figure 5). The four power switches are properly
phased so the transfer between operating modes is continuous, smooth and transparent to the user. When VIN
approaches VOUT the buck/boost region is reached where
VIN
VOUT
9
8
SW1
SW2
4
6
NMOS B
Buck Region (VIN > VOUT)
Switch D is always on and switch C is always off during this
mode. When the internal control voltage, VCI, is above
voltage V1, output A begins to switch. During the off-time
of switch A, synchronous switch B turns on for the
remainder of the time. Switches A and B will alternate
similar to a typical synchronous buck regulator. As the
control voltage increases, the duty cycle of switch A
increases until the maximum duty cycle of the converter in
buck mode reaches DMAX_BUCK, given by:
DMAX_BUCK = 100 – D4SW %
where D4SW = duty cycle % of the four switch range.
D4SW = (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or buck/boost region
is reached.
PMOS D
PMOS A
the conduction time of the four switch region is typically
150ns. Referring to Figures 1 and 2, the various regions of
operation will now be described.
NMOS C
3442 F01
Figure 1. Simplified Diagram of Output Switches
88%
DMAX
BOOST
V4 (≈2.05V)
Buck/Boost or Four Switch (VIN ~ VOUT)
When the internal control voltage, VCI, is above voltage V2,
switch pair AD remain on for duty cycle DMAX_BUCK, and
the switch pair AC begins to phase in. As switch pair AC
phases in, switch pair BD phases out accordingly. When
the VCI voltage reaches the edge of the buck/boost range,
at voltage V3, the AC switch pair completely phase out the
BD pair, and the boost phase begins at duty cycle D4SW.
The input voltage, VIN, where the four switch region begins
is given by:
A ON, B OFF
BOOST REGION
PWM CD SWITCHES
DMIN
BOOST
DMAX
BUCK
V3 (≈1.65V)
FOUR SWITCH PWM
BUCK/BOOST REGION
VOUT
1 – (150ns • f)
V2 (≈1.55V)
The point at which the four switch region ends is given by:
D ON, C OFF
PWM AB SWITCHES BUCK REGION
V1 (≈0.9V)
0%
DUTY
CYCLE
VIN =
3442 F02
VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V
INTERNAL
CONTROL
VOLTAGE, VCI
Figure 2. Switch Control vs Internal Control Voltage, VCI
3442f
9
LTC3442
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OPERATIO
Switch A is always on and switch B is always off during this
mode. When the internal control voltage, VCI, is above
voltage V3, switch pair CD will alternately switch to provide a boosted output voltage. This operation is typical to
a synchronous boost regulator. The maximum duty cycle
of the converter is limited to 88% typical and is reached
when VCI is above V4.
VIN
VOUT
9
8
A
4
SW1
–
dI ≈ – VOUT
L
dt
D
+
L
6
SW2
B
C
IINDUCTOR
Boost Region (VIN < VOUT)
900mA
0mA
T2
3442 F04
5
GND
BURST MODE OPERATION
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
Burst Mode operation occurs when 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 35µA of quiescent current from VIN. In this mode the
output ripple has a variable frequency component that
depends upon load current, and will typically be about 2%
peak-to-peak. Burst Mode operation ripple can be reduced
slightly by using more output capacitance (47µF or greater).
Another method of reducing Burst Mode operation ripple
is to place a small feed-forward capacitor across the upper
resistor in the VOUT feedback divider network (as in Type
III compensation).
During the period where the device is delivering energy to
the output, the peak switch current will be equal to 900mA
typical and the inductor current will terminate at zero
current for each cycle. In this mode the typical maximum
average output current is given by:
0.2 • VIN
IOUT(MAX)BURST ≈
A
VOUT + VIN
Note that the peak efficiency during Burst Mode operation
is less than the peak efficiency during fixed frequency
VIN
VOUT
9
8
4
+
SW1
dI ≈ VIN
dt
L
L
B
D
–
6
SW2
C
IINDUCTOR
A
because the part enters full-time 4-switch mode (when
servicing the output) with discontinuous inductor current
as illustrated in Figures 3 and 4. During Burst Mode
operation, the control loop is nonlinear and cannot utilize
the control voltage from the error amp to determine the
control mode, therefore full-time 4-switch mode is required to maintain the Buck/Boost function. The efficiency
below 1mA becomes dominated primarily by the quiescent current. The Burst Mode operation efficiency is given
by:
EFFICIENCY ≅
n • ILOAD
35µA + ILOAD
where n is typically 82% during Burst Mode operation.
Automatic Burst Mode Operation Control
Burst Mode operation can be automatic or manually
controlled with a single pin. In automatic mode, the IC will
enter Burst Mode operation at light load and return to fixed
frequency operation at heavier loads. The load current at
which the mode transition occurs is programmed using a
single external resistor from the BURST pin to ground,
according to the following equations:
17.6
RBURST
22.4
Leave Burst Mode: I =
RBURST
Enter Burst Mode: I =
900mA
0mA
T1
3442 F03
5
GND
where RBURST is in kΩ and IBURST is the load transition
Figure 3. Inductor Charge Cycle During Burst Mode Operation
3442f
10
LTC3442
U
OPERATIO
current in Amps. Do not use values of RBURST greater than
250k.
For automatic operation, a filter capacitor should also be
connected from BURST to ground to prevent ripple on
BURST from causing the IC to oscillate in and out of Burst
Mode operation. The equation for the minimum capacitor
value is:
CBURST(MIN) ≥
COUT • VOUT
60, 000
where CBURST(MIN) and COUT are in µF
In the event that a load transient causes the feedback pin
to drop by more than 4% from the regulation value while
in Burst Mode operation, the IC will immediately switch to
fixed frequency mode and an internal pull-up will be
momentarily applied to BURST, rapidly charging the
BURST cap. This prevents the IC from immediately reentering Burst Mode operation once the output achieves
regulation.
Manual Burst Mode Operation
For manual control of Burst Mode operation, the RC
network connected to BURST can be eliminated. To force
fixed frequency mode, BURST should be connected to
VOUT. To force Burst Mode operation, BURST should be
grounded. When commanding Burst Mode operation
manually, the circuit connected to BURST should be able
to sink up to 2mA.
For optimum transient response with large dynamic loads,
the operating mode should be controlled manually by the
host. By commanding fixed frequency operation prior to a
sudden increase in load, output voltage droop can be
minimized. Note that if the load current applied during
forced Burst Mode operation (BURST pin is grounded)
exceeds the current that can be supplied, the output
voltage will start to droop and the IC will automatically
come out of Burst Mode operation and enter fixed frequency mode, raising VOUT. Once regulation is achieved,
the IC will then enter Burst Mode operation once again,
and the cycle will repeat, resulting in about 4% output
ripple. Note that Burst Mode operation is inhibited during
soft-start.
Burst Mode Operation to Fixed Frequency Transient
Response
In Burst Mode operation, the compensation network is not
used and VC is disconnected from the error amplifier.
During long periods of Burst mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fixed frequency mode of operation, even
at the same load current. To prevent this, the LTC3442
incorporates an active clamp circuit that holds the voltage
on VC at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fixed frequency mode operation. For optimum transient
3442f
11
LTC3442
U
OPERATIO
response, Type 3 compensation is also recommended to
broad band the control loop and roll off past the two pole
response of the output LC filter. (See Closing the Feedback
Loop.)
detailed diagram of this function is shown in Figure 5. The
components RSS and CSS provide a slow ramping voltage
on SHDN/SS to provide a soft-start function. To ensure
that VC is not being clamped, SHDN/SS must be raised
above 2.4V.
Soft-Start
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above 1V typical, the IC is
enabled but the EA duty cycle is clamped from VC. A
ERROR AMP
VIN
14µA
+
VOUT
1.22V
R1
FB
–
12
VC
SOFT-START
CLAMP
TO PWM
COMPARATORS
CP1
R2
11
VCI
SHDN/SS
RSS
ENABLE SIGNAL
1
CSS
+
3442 F05
CHIP
ENABLE
–
1V
Figure 5. Soft-Start Circuitry
3442f
12
LTC3442
U
U
W
U
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
where f = operating frequency, Hz
∆IL = maximum allowable inductor ripple current, A
VIN
1 SHDN/SS
FB 12
VIN(MIN) = minimum input voltage, V
2 RT
VC 11
VIN(MAX) = maximum input voltage, V
3 SGND
RLIM 10
4 SW1
VIN 9
VOUT = output voltage, V
VIN
VOUT 8
5 PGND
VOUT
IOUT(MAX) = maximum output load current
Figure 6. Recommended Component Placement. Traces Carrying
High Current Should be Short and Wide. Trace Area at FB and VC
Pins are Kept Low. Lead Length to Battery Should be Kept Short.
VOUT and VIN Ceramic Capacitors Close to the IC Pins.
For high efficiency, choose a ferrite inductor with a high
frequency core material to reduce core loses. 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 saturating. Molded chokes or
chip inductors usually do not have enough core to support
the peak inductor currents in the 1A to 2A region. To
minimize radiated noise, use a shielded inductor. See
Table 1 for a suggested list of inductor suppliers.
Inductor Selection
Output Capacitor Selection
The high frequency operation of the LTC3442 allows the
use of small surface mount inductors. The inductor ripple
current is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
The bulk value of the output filter capacitor is set to reduce
the ripple due to charge into the capacitor each cycle. The
steady state ripple due to charge is given by:
% RIPPLE_BOOST =
BURST 7
6 SW2
GND
RT
LBOOST >
2
VIN(MIN)
MULTIPLE
VIAS
• ( VOUT – VIN(MIN) )
f • ∆IL • V
OUT
LBUCK >
3442 F06
COUT • VOUT 2 • f
H
%
% RIPPLE_BUCK =
2
VOUT • ( VIN(MAX) – VOUT )
f • ∆IL • VIN(MAX)
IOUT(MAX) • (VOUT – VIN(MIN) ) •100
IOUT(MAX) • (VIN(MAX) – VOUT ) •100
H
COUT • VIN(MAX) • VOUT • f
%
where COUT = output filter capacitor in Farads and
f = switching frequency in Hz.
Table 1. Inductor Vendor Information
SUPPLIER
PHONE
FAX
WEB SITE
Coilcraft
(847) 639-6400
(847) 639-1469
www.coilcraft.com
CoEv Magnetics
(800) 227-7040
(650) 361-2508
www.circuitprotection.com/magnetics.asp
Murata
(814) 237-1431
(800) 831-9172
(814) 238-0490
www.murata.com
Sumida
USA: (847) 956-0666
Japan: 81(3) 3607-5111
USA: (847) 956-0702
Japan: 81(3) 3607-5144
www.sumida.com
TDK
(847) 803-6100
(847) 803-6296
www.component.tdk.com
TOKO
(847) 297-0070
(847) 699-7864
www.tokoam.com
3442f
13
LTC3442
U
W
U U
APPLICATIO S I FOR ATIO
The output capacitance is usually many times larger than
the minimum value in order to handle the transient response requirements of the converter. For a rule of thumb,
the ratio of the operating frequency to the unity-gain
bandwidth of the converter is the amount the output
capacitance will have to increase from the above calculations in order to maintain the desired transient response.
voltages as low as 0.4V. The part is specified at 2.4V
minimum to allow operation without the requirement of a
Schottky diode. Synchronous switch D is powered from
VOUT and the RDS(ON) will increase at low output voltages,
therefore a Schottky diode is required from SW2 to VOUT
to provide the conduction path to the output. Note that
Burst Mode operation is inhibited at output voltages below
1.6V typical.
The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden or
TDK ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. See Table 2 for
contact information.
A Schottky diode from SW2 to VOUT is required for output
voltages over 4.3V. The diode must be located as close to
the pins as possible in order to reduce the peak voltage on
SW2 due to the parasitic lead and trace inductance.
Input Capacitor Selection
Input Voltage > 4.5V
Since VIN is the supply voltage for the IC, as well as the
input to the power stage of the converter, it is recommended to place at least a 4.7µF, low ESR ceramic bypass
capacitor close to the VIN and SGND pins. It is also
important to minimize any stray resistance from the converter to the battery or other power source.
For applications with input voltages above 4.5V which
could exhibit an overload or short-circuit condition, a
2Ω/1nF series snubber is required between SW1 and
GND. A Schottky diode from SW1 to VIN should also be
added as close to the pins as possible. For the higher input
voltages, VIN bypassing becomes more critical; therefore,
a ceramic bypass capacitor as close to the VIN and SGND
pins as possible is also required.
Optional Schottky Diodes
The Schottky diodes across the synchronous switches B
and D are not required (VOUT < 4.3V), but provide a lower
drop during the break-before-make time (typically 15ns)
improving efficiency. Use a surface mount Schottky diode
such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will
compromise efficiency. For applications with an output
voltage above 4.3V, a Schottky diode is required from
SW2 to VOUT.
Output Voltage > 4.3V
Operating Frequency Selection
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capacitors,
thus reducing board area and component height. However, higher operating frequencies also increase the IC’s
total quiescent current due to the gate charge of the four
switches, as given by:
Buck:
Iq = (0.8 • VIN • f) mA
Output Voltage < 2.4V
Boost:
Iq = [0.4 • (VIN + VOUT) • f] mA
The LTC3442 can operate as a buck converter with output
Buck/Boost: Iq = [f • (1.2 • VIN + 0.4 • VOUT)] mA
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
FAX
WEB SITE
AVX
(803) 448-9411
(803) 448-1943
www.avxcorp.com
Murata
(814) 237-1431
(800) 831-9172
(814) 238-0490
www.murata.com
Sanyo
(619) 661-6322
(619) 661-1055
www.sanyovideo.com
Taiyo Yuden
(408) 573-4150
(408) 573-4159
www.t-yuden.com
TDK
(847) 803-6100
(847) 803-6296
www.component.tdk.com
3442f
14
LTC3442
U
W
U U
APPLICATIO S I FOR ATIO
VOUT
where f = switching frequency in MHz. Therefore frequency selection is a compromise between the optimal
efficiency and the smallest solution size.
+
1.22V
ERROR
AMP
–
12
Closing the Feedback Loop
1
2 • π • L • COUT
Hz
VIN
2 • VOUT • π • L • COUT
R2
11
3442 F07
Figure 7. Error Amplifier with Type I Compensation
fUG =
1
Hz
2 • π • R1 • CP1
referring to Figure 7.
(in buck mode)
f FILTER—POLE =
CP1
VC
The LTC3442 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck/boost), but is usually no greater than
15. The output filter exhibits a double pole response, as
given by:
f FILTER—POLE =
R1
FB
Hz
(in boost mode)
Most applications demand an improved transient response
to allow a smaller output filter capacitor. To achieve a
higher bandwidth, Type III compensation is required,
providing two zeros to compensate for the double-pole
response of the output filter. Referring to Figure 8, the
location of the poles and zeros are given by:
where L is in henries and COUT is in farads.
The output filter zero is given by:
1
f FILTER— ZERO =
Hz
2 • π • RESR • COUT
where RESR is the equivalent series resistance of the
output cap.
A troublesome feature in boost mode is the right-half
plane zero (RHP), given by:
2
VIN
f RHPZ =
Hz
2 • π • IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorporated to stabilize the loop, but at a cost of reduced
bandwidth and slower transient response. To ensure
proper phase margin using Type I compensation, the loop
must be crossed over a decade before the LC double pole.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
1
fPOLE1 ≅
Hz
2 • π • 32e3 • R1 • CP1
(which is extremely close to DC)
1
fZERO1 =
Hz
2 • π • RZ • CP1
1
fZERO2 =
Hz
2 • π • R1 • CZ1
1
fPOLE2 =
Hz
2 • π • RZ • CP2
where resistance is in ohms and capacitance is in farads.
VOUT
+
ERROR
AMP
–
1.22V
R1
CZ1
FB
12
VC
CP1
RZ
R2
11
CP2
3442 F08
Figure 8. Error Amplifier with Type III Compensation
3442f
15
LTC3442
U
TYPICAL APPLICATIO S
1MHz Li-Ion to 3.3V at 1.2A Converter with
Manual Mode Control (and Peak Current Limit Only)
L1
3.3µH
2.5V TO 4.2V
SW1
VIN
1M
CIN
10µF
SW2
LTC3442
VOUT
SHDN/SS
FB
RLIM
VC
RT
Li-Ion
0.01µF
15k
BURST
2.2k
COUT
22µF
220pF
470pF
PGND
SGND
43.2k
340k
VOUT
3.3V
1.2A
200k
CIN: TAIYO YUDEN JMK212BJ106MG
COUT: TAIYO YUDEN JMK325BJ226MM
L1: TDK RLF7030T-3R3M4R
BURST FIXED FREQ
3442 TA02
Multi-Input 3.3V at 600mA Boost Converter for Portable Applications
with Automatic Burst Mode Operation and Average Input Current Limit for USB Powered Devices
2Ω
L1
4.7µH
1nF
Li-Ion
D1
MBRM120T3
2.5V TO 5.5V
USB/5V
CIN
10µF
SW1
VIN
SHDN/SS
1M
RLIM
143k
2N7002
USB
PRESENT
SW2
LTC3442
VOUT
RT
0.01µF
RSNUB**
1Ω
1nF
143k
SGND
64.9k
340k
FB
VC
BURST
15k
2.2k
220pF
470pF
PGND
200k
200k
VOUT
3.3V
600mA
COUT
22µF
0.01µF
**A SNUBBER RESISTOR IS REQUIRED TO PREVENT
CIN: TAIYO YUDEN JMK212BJ106MG
RINGING IF THERE IS SIGNIFICANT INPUT INDUCTANCE, COUT: TAIYO YUDEN JMK325BJ476MM
L1: TDK RLF7030T-4R7M3R4
SUCH AS FROM A USB CABLE
3442 TA03
3442f
16
LTC3442
U
TYPICAL APPLICATIO S
High Efficiency Li-Ion Powered Constant Current LED Driver
with Open-LED Protection
3.3µH
R5
7.87k
VIN
2.5V TO
4.2V
SW1
SW2
VIN
VOUT
VOUT
ILED = 500mA
LTC3442
OFF ON
SD/SS
FB
1nF
10µF
RLIM
RT
SGND
R4
2k
57.6k
OPEN LED VOLTAGE LIMIT = (R4+R5)*0.95/R4
VC
LHXL-PW03
4.7µF
R2
200k
BURST
PGND
R3
95.3k
47pF
R1
301k
R2 = R1/1.5
ILED = 24 • (R1+R2+R3)/(R1 • R3) AMPS
3442 TA04a
LED Driver Efficiency
vs LED Current
100
98
VIN = 3.6V
750kHz
96
EFFICIENCY (%)
94
92
90
88
86
84
82
80
0.1
1.0
LED CURRENT (A)
3442 TA04b
3442f
17
LTC3442
U
PACKAGE DESCRIPTIO
UE/DE Package
12-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1695)
0.65 ±0.05
3.50 ±0.05
1.70 ±0.05
2.20 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
3.30 ±0.05
(2 SIDES)
0.50
BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ±0.10
(2 SIDES)
7
R = 0.115
TYP
0.38 ± 0.10
12
R = 0.20
TYP
PIN 1
TOP MARK
(NOTE 6)
3.00 ±0.10
(2 SIDES)
1.70 ± 0.10
(2 SIDES)
PIN 1
NOTCH
(UE12/DE12) DFN 0603
0.200 REF
0.75 ±0.05
6
0.25 ± 0.05
3.30 ±0.10
(2 SIDES)
0.00 – 0.05
1
0.50
BSC
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING PROPOSED TO BE A VARIATION OF VERSION
(WGED) IN JEDEC PACKAGE OUTLINE M0-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. 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
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3442f
18
LTC3442
U
TYPICAL APPLICATIO S
High Current LED Driver with Low/High Current Range for Pulsed
Applications; LED Current is 0.5A with 1.5A Pulse
3.3µH
R5
7.87k
VIN
2.7V TO
4.2V
SW1
SW2
VIN
VOUT
VOUT
ILED = 500mA/1.5A
LTC3442
OFF ON
SD/SS
FB
10µF
6.3V
1nF
RLIM
R2
20k
VC
10µF
6.3V
R2
200k
RT
R4
2k
57.6k
LHXL-PW03
BURST
PGND
SGND
OPEN LED VOLTAGE LIMIT = (R4+R5) • 0.95/R4
LOW HI
95.3k
1nF
40.2k
2N7002
R1
301k
R2 = R1/1.5
ILED = 24 • (R1+R2+R3)/(R1 • R3) AMPS
(OR: ILED = 40/R3 + .08)
3442 TA05
3442f
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LTC3442
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT 1613
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VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA,
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LT1618
1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter
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LT1930/LT1930A
1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up DC/DC Converter
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LT1935
2A (ISW), 1.2MHz, 38V Step-Up DC/DC Converter
VIN: 2.3V to 16V, VOUT(MAX) = 38V, IQ = 3mA,
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LT1946/LT1946A
1.5A (ISW), 1.2MHz/2.7MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 2.45V to 16V, VOUT(MAX) = 34V, IQ = 3.2mA,
ISD < 1µA, MS8 Package
LT1961
1.5A (ISW), 1.25MHz, High Efficiency Step-Up DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 35V, IQ = 0.9mA,
ISD = 6µA, MS8E Package
®
LTC3400/LTC3400B 600mA (ISW), 1.2MHz Synchronous Step-Up DC/DC Converter
VIN: 0.85V to 5V, VOUT(MAX) = 5V,
IQ = 19µA/300µA, ISD < 1µA, ThinSOT Package
LTC3401/LTC3402
VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38µA,
ISD < 1µA, MS Package
1A/2A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
LTC3405/LTC3405A 300mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter
VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 20µA,
ISD ≤ 1µA, MS10 Package
LTC3406/LTC3406B 600mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20µA,
ISD ≤ 1µA, ThinSOT Package
LTC3407
600mA (IOUT), 1.5MHz Dual Synchronous Step-Down
DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA,
ISD ≤ 1µA, MS Package
LTC3411
1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60µA,
ISD ≤ 1µA, MS Package
LTC3412
2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60µA,
ISD ≤ 1µA, TSSOP16E Package
LTC3421
3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12µA,
ISD < 1µA, QFN Package
LTC3425
5A (ISW), 8MHz Multiphase Synchronous Step-Up
DC/DC Converter
VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12µA,
ISD < 1µA, QFN Package
LTC3429
600mA (ISW), 500kHz Synchronous Step-Up DC/DC Converter
VIN: 0.5V to 4.4V, VOUT(MIN) = 5V, IQ = 20µA,
ISD < 1µA, QFN Package
LTC3436
3A (ISW), 1MHz, 34V Step-Up DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA,
ISD < 6µA, TSSOP-16E Package
LTC3440
600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 5.5V, IQ = 25µA,
ISD < 1µA, MS, DFN Packages
LTC3441
600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 5.5V, IQ = 25µA,
ISD < 1µA, DFN Package
LTC3443
1.2A (IOUT), 600kHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(MIN) = 5.25V, IQ = 28µA,
ISD < 1µA, MS Package
LT3467
1.1A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter
VIN: 2.6V to 16V, VOUT(MAX) = 40V, IQ = 1.2mA,
ISD < 1µA, ThinSOT Package
ThinSOT is a trademark of Linear Technology Corporation.
3442f
20
Linear Technology Corporation
LT/TP 1204 1K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004