LTC3442
Micropower Synchronous
Buck-Boost DC/DC Converter with
Automatic Burst Mode Operation
DESCRIPTION
FEATURES
Regulated Output with Input Voltages Above,
Below, or Equal to the Output
n Single Inductor, No Schottky Diodes Required
n Manual or Programmable Automatic Burst Mode®
Operation
n Programmable Average Input Current Limit
n Up to 1.2A Continuous Output Current from a Single
Lithium-Ion Cell
n High Efficiency: Up to 95%
n Output Disconnect in Shutdown
n 2.4V to 5.5V Input Range
n 2.4V to 5.25V Output Range
n 35µA Quiescent Current in Burst Mode Operation
n Programmable Frequency from 300kHz to 2MHz
n 1.25V
MIN
Ratio of IOUT to IBURST
Ratio of IIN to IRLIM, IIN = 0.5A
When IC Is Enabled
When EA Is at Max Boost Duty Cycle
VSHDN = 5.5V
SHDN/SS Input Current
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3442E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
l
POWER LOSS
10
60
FIXED FREQUENCY
50
1
40
40
30
20
0.1
70
VOUT = 3.3V
600kHz
1
10
100
LOAD (mA)
1000
10000
3442 G01
30
20
0.1
VIN = 3.6V
VOUT = 3.3V
1
10
100
1000
LOAD CURRENT (mA)
0.1
10000
3442 G02
92
EFFICIENCY (%)
FIXED FREQUENCY
100
POWER LOSS (mW)
VIN = 5V
EFFICIENCY (%)
EFFICIENCY (%)
50
VIN = 2.5V
VIN = 2.5V
96
94
80
70
60
1000
Burst Mode
90 OPERATION
80 VIN = 3.3V
V
V
V
µA
1.4
2.4
1
Efficiency vs Frequency
100
VIN = 3.3V
Burst Mode
90 OPERATION V = 5V
IN
UNITS
µA
V
V
(TA = 25°C unless otherwise specified).
Efficiency and Power Loss
vs Load
100
MAX
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 PERFORMANCE CHARACTERISTICS
Efficiency vs Load
0.4
TYP
300
0.88
1.12
20,000
70,000
0.95
0.7
2.2
0.01
WITH SCHOTTKY DIODES
90
88
86
WITHOUT SCHOTTKY DIODES
84
82
VIN = 3.6V
VOUT = 3.3V
80
400 600 800 1000 1200 1400 1600 1800 2000
FREQUENCY (kHz)
3442 G03
3442fb
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3
LTC3442
TYPICAL PERFORMANCE CHARACTERISTICS
VOUT = 3.3V
6 1MHz
0.70
0.60
0.50
0.40
0.30
0.20
4
2
0
–2
–4
–8
2.5
0 .05 .10 .15 .20 .25 .30 .35 .40 .45 .50
INPUT CURRENT (A)
3.0
3.5
4.5
4.0
5
VOUT DROPS 10%
0
–5
–10
0.50
5.0
3.5
50
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
35
30
25
20
15
10
3.0
3.5
4.0
VIN (V)
4.5
5.0
0
5.5
2.5
3.0
3.5
4.0
VIN (V)
4.5
120
110
100
90
80
70
60
150
ENTER Burst Mode
OPERATION
175
200
RBURST (kΩ)
225
250
3442 G10
1.5
1.0
3.0
3.5
4.0
VIN (V)
4.5
5.0
Average Input Current Limit
vs Temperature (Normalized)
2.30
5%
2.29
4%
2.28
3%
2.27
2%
2.26
2.25
2.24
2.23
0%
–1%
–2%
–3%
2.21
–4%
–25
35
65
5
TEMPERATURE (°C)
95
125
3442 G11
VIN = VOUT = 3.3V
1%
2.22
2.20
–55
5.5
3442 G09
CHANGE FROM 25°C
MINIMUM START VOLTAGE (V)
LEAVE Burst Mode
OPERATION
2.0
Minimum Start Voltage
vs Temperature
150
140
2.5
3442 G08
Automatic Burst Mode Threshold
vs RBURST
2.00
Peak Current Clamp vs VIN
0.0
2.5
5.5
5.0
3442 G07
130
1.75
0.5
5
0.0
2.5
1.00 1.25 1.50
FREQUENCY (MHz)
3.0
40
INPUT CURRENT (A)
3.5
0.75
3442 TA01b
Burst Mode Quiescent Current
vs VIN
4.0
VIN QUIESCENT CURRENT (mA)
VOUT SHORTED
3442 G05
Quiescent Current vs VIN
(Fixed Frequency Mode)
LOAD CURRENT (mA)
10
VIN (V)
3442 G04
160
VIN = 5V
–6
0.10
0.00
% CHANGE (NORMALIZED)
RLIM VOLTAGE (V)
0.80
15
8
VIN = 3.6V
VOUT = 3.3V
RLIM = 133k
% CHANGE (NORMALIZED)
0.90
Average Input Current Limit
vs Frequency (Normalized)
Average Input Current Limit
vs VIN (Normalized)
Input Current Mirror Linearity
1.00
(TA = 25°C unless otherwise specified).
–5%
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3442 G12
3442fb
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LTC3442
TYPICAL PERFORMANCE CHARACTERISTICS
Frequency Change
vs Temperature (Normalized)
Switch Pins Before Entering
Boost Mode
Feedback Voltage
vs Temperature (Normalized)
1.0%
2.0%
0.8%
1.5%
VIN = VOUT = 3.3V
SW1
2V/DIV
0.6%
1.0%
CHANGE FROM 25°C
CHANGE FROM 25°C
(TA = 25°C unless otherwise specified).
0.5%
0.0%
–0.5%
–1.0%
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%
–1.5%
–0.8%
–2.0%
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
–1.0%
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3442 G13
3442 G14
Switch Pins Entering
Buck-Boost Mode
Switch Pins in Buck-Boost Mode
Output Ripple at 1A Load
SW1
2V/DIV
SW1
2V/DIV
3442 G15
VIN = 2.7V
VIN = 3.3V
SW2
2V/DIV
SW2
2V/DIV
VIN = 4.2V
1µs/DIV
50ns/DIV
50ns/DIV
3442 G16
VIN = 3.3V
VOUT = 3.3V AT 500mA
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
VIN = 4.2V
VOUT = 3.3V AT 500mA
Load Transient Response in Fixed
Frequency Mode, No Load to 1A
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
20µs/DIV
COUT = 100F
LOW ESR TANTALUM
3442 G21
3442fb
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5
LTC3442
TYPICAL PERFORMANCE CHARACTERISTICS
(TA = 25°C unless otherwise specified).
Pulsed Overload Using Average Input
Current Limit
Transition from Burst Mode
Operation to Fixed Frequency Mode
VOUT
2V/DIV
VOUT
50mV/DIV
RLIM PIN
0.5V/DIV
INDUCTOR
CURRENT
0.5A/DIV
INDUCTOR
CURRENT
0.5A/DIV
200µs/DIV
1ms/DIV
3442 G22
3442 G23
RLIM = 133k
CLIM = .001µF
COUT = 100µF
LOW ESR TANTALUM
PIN FUNCTIONS
SHDN/SS (Pin 1): Combined Soft-Start and Shutdown.
Applied voltage 1.4V to
enable the IC and >2.4V to ensure the error amp is not
clamped from soft-start. For Burst Mode operation, this
pin must be pulled up to within 0.5V of VIN. 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.
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.
SGND (Pin 3): Signal Ground for the IC.
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.
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.
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.
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.
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.
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.
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.
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.
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.
3442fb
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LTC3442
BLOCK DIAGRAM
SW1
SW2
4
6
SW D
SW A
8
SW B
SW C
–
GATE
DRIVERS
AND
ANTICROSS
CONDUCTION
REVERSE
AMP
–
RLIM 10
0.95V
+
AV = 6
+
AVERAGE
ILIM
Gm = 1/60k
–
3A
PEAK CURRENT
LIMIT
–
PWM
LOGIC
2.3V
–
PWM
COMPARATORS
UVLO
11 VC
+
+
AUTOMATIC
BURST MODE
CONTROL AND
VC HOLD
SLEEP
2
12 FB
–
VIN
–
1.22V
+
5A
+
ERROR
AMP
–
+
RT
OSC
7
VIN
SHDN/SS 1
VOUT
+
VIN 9
SHUTDOWN
SOFT-START
VCC
VREF
SS
BURST
1.22V
VREF
THERMAL
SHUTDOWN
SHUTDOWN
2
6
PGND
SGND
3442 BD
3442fb
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7
LTC3442
OPERATION
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.
LOW NOISE FIXED FREQUENCY OPERATION
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.
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.
Externally Programmable Current Limit
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:
(2 • VIN – VOUT )
70 • 0.86 +
40
RLIM(kΩ) =
IIN(AMPS)
CLIM(µF) ≥
0.1
RLIM(kΩ)
The programmable current limit feature is disabled in
Burst Mode operation.
3442fb
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LTC3442
OPERATION
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
PMOS A
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
SW1
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
V2 (≈1.55V)
D ON, C OFF
PWM AB SWITCHES BUCK REGION
V1 (≈0.9V)
0%
DUTY
CYCLE
3442 F02
VIN =
VOUT
1– (150ns • f)
The point at which the four switch region ends is given by:
VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V
INTERNAL
CONTROL
VOLTAGE, VCI
Figure 2. Switch Control vs Internal Control Voltage, VCI
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9
LTC3442
OPERATION
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
+
L
B
D
6
SW2
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 feedforward capacitor
across the upper resistor in the VOUT feedback divider
network (as in Type III compensation).
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:
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:
where n is typically 82% during Burst Mode operation.
IOUT(MAX)BURST ≈
0.2 • VIN
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
+
VIN
L
dI
dt
L
B
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:
D
–
6
SW2
C
n •ILOAD
35µA + ILOAD
Enter Burst Mode: I =
IINDUCTOR
A
EFFICIENCY ≅
Leave Burst Mode: I =
900mA
0mA
17.6
RBURST
T1
3442 F03
5
22.4
RBURST
where RBURST is in kΩ and IBURST is the load transition
current in Amps. Do not use values of RBURST greater
than 250k.
GND
Figure 3. Inductor Charge Cycle During Burst Mode Operation
3442fb
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LTC3442
OPERATION
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
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
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
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
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.)
Soft-Start
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above 0.7V typical, the IC
is enabled but the EA duty cycle is clamped from VC. A
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. To enable Burst Mode operation, SHDN/SS
must be raised to within 0.5V of VIN.
3442fb
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11
LTC3442
OPERATION
ERROR AMP
VIN
14µA
+
VOUT
1.22V
R1
FB
–
12
VC
SOFT-START
CLAMP
CP1
R2
11
VCI
TO PWM
COMPARATORS
SHDN/SS
RSS
ENABLE SIGNAL
1
CSS
+
CHIP
ENABLE
–
3442 F05
0.7V
Figure 5. Soft-Start Circuitry
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor 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:
VIN
1 SHDN/SS
FB 12
2 RT
VC 11
3 SGND
RLIM 10
4 SW1
VIN 9
5 PGND
6 SW2
VOUT 8
VIN
L BOOST >
VOUT
BURST 7
GND
RT
MULTIPLE
VIAS
3442 F06
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.
L BUCK >
VIN(MIN) • (VOUT – VIN(MIN) )
H
f • ∆IL • VOUT
VOUT • (VIN(MAX) – VOUT )
H
f • ∆IL • VIN(MAX)
where f = operating frequency, Hz
∆IL = maximum allowable inductor ripple current, A
VIN(MIN) = minimum input voltage, V
VIN(MAX) = maximum input voltage, V
VOUT = output voltage, V
IOUT(MAX) = maximum output load current
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LTC3442
APPLICATIONS INFORMATION
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.
Output Capacitor Selection
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=
(
%
COUT • VOUT • f
)
%
2
% RIPPLE_BUCK =
(
IOUT(MAX) • VIN(MAX) – VOUT • 100
COUT • VIN(MAX) • VOUT • f
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.
Input Capacitor Selection
)
IOUT(MAX) • VOUT – VIN(MIN) • 100
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.
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.
where COUT = output filter capacitor in Farads and
f = switching frequency in Hz.
Table 1. Inductor Vendor Information
SUPPLIER
Coilcraft
CoEv Magnetics
Murata
Sumida
TDK
TOKO
PHONE
(847) 639-6400
(800) 227-7040
(814) 237-1431
(800) 831-9172
USA: (847) 956-0666
Japan: 81(3) 3607-5111
(847) 803-6100
(847) 297-0070
FAX
(847) 639-1469
(650) 361-2508
(814) 238-0490
WEB SITE
www.coilcraft.com
www.circuitprotection.com/magnetics.asp
www.murata.com
USA: (847) 956-0702
Japan: 81(3) 3607-5144
(847) 803-6296
(847) 699-7864
www.sumida.com
www.component.tdk.com
www.tokoam.com
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
3442fb
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13
LTC3442
APPLICATIONS INFORMATION
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.
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
Boost:
Iq = [0.4 • (VIN + VOUT) • f] mA
Buck/Boost: Iq = [f • (1.2 • VIN + 0.4 • VOUT)] mA
where f = switching frequency in MHz. Therefore frequency
selection is a compromise between the optimal efficiency
and the smallest solution size.
Output Voltage < 2.4V
Closing the Feedback Loop
The LTC3442 can operate as a buck converter with output 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 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:
Output Voltage > 4.3V
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 Voltage > 4.5V
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.
Higher operating frequencies allow the use of a smaller
inductor and smaller input and output filter capacitors, thus reducing board area and component height.
1
Hz
2 • π • L • COUT
(in buck mode)
f FILTER —POLE =
2 • VOUT
VIN
Hz
• π • L • COUT
(in boost mode)
where L is in henries and COUT is in farads.
The output filter zero is given by:
f FILTER — ZERO =
1
2 • π • RESR • COUT
Hz
where RESR is the equivalent series resistance of the
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
Operating Frequency Selection
f FILTER —POLE =
f RHPZ =
VIN2
Hz
2 • π •IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zero
frequency.
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14
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LTC3442
APPLICATIONS INFORMATION
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:
fUG =
the output filter. Referring to Figure 8, the location of the
poles and zeros are given by:
1
fPOLE1 ≅
Hz
2 • π • 32e3 • R1• CP1
(which is extremely close to DC)
1
Hz
2 • π • R1• CP1
referring to Figure 7.
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
fZERO1 =
1
Hz
2 • π • RZ • CP1
fZERO2 =
1
Hz
2 • π • R1• CZ1
fPOLE2 =
1
Hz
2 • π • RZ • CP2
where resistance is in ohms and capacitance is in Farads.
VOUT
VOUT
+
ERROR
AMP
–
+
1.22V
R1
FB
CP1
CZ1
12
VC
R2
11
R1
FB
–
12
VC
1.22V
ERROR
AMP
CP1
RZ
11
R2
CP2
3442 F07
3442 F08
Figure 7. Error Amplifier with Type I Compensation
Figure 9. Error Amplifier with Type III Compensation
TYPICAL APPLICATIONS
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
CIN
10µF
Li-Ion
+
1M
0.01µF
SW2
LTC3442
VOUT
SHDN/SS
FB
RLIM
VC
RT
43.2k
SGND
BURST
340k
15k
220pF
470pF
PGND
BURST FIXED FREQ
2.2k
VOUT
3.3V
1.2A
COUT
22µF
200k
CIN: TAIYO YUDEN JMK212BJ106MG
COUT: TAIYO YUDEN JMK325BJ226MM
L1: TDK RLF7030T-3R3M4R
3442 TA02
3442fb
For more information www.linear.com/LTC3442
15
LTC3442
TYPICAL APPLICATIONS
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
L1
4.7µH
1nF
2Ω
Li-Ion
D1
MBRM120T3
2.5V TO 5.5V
USB/5V
CIN
10µF
SW2
LTC3442
VOUT
VIN
SW1
1M
143k
SHDN/SS
FB
RLIM
VC
RT
2N7002
0.01µF
USB
PRESENT
RSNUB**
1Ω
1nF
143k
BURST
SGND
340k
15k
220pF
470pF
PGND
64.9k
2.2k
COUT
22µF
200k
200k
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
High Efficiency Li-Ion Powered Constant Current LED Driver
with Open-LED Protection
100
98
VIN = 3.6V
750kHz
SW1
SW2
VIN
VOUT
VOUT
ILED = 500mA
LTC3442
*OFF ON
SHDN/SS
FB
1nF
10µF
RLIM
RT
SGND
VC
BURST
PGND
57.6k
OPEN LED VOLTAGE LIMIT = (R4 + R5) • 0.95/R4
4.7µF
R2
200k
LHXL-PW03
EFFICIENCY (%)
96
VIN
2.5V TO
4.2V
R4
2k
3442 TA03
LED Driver Efficiency vs LED Current
3.3µH
R5
7.87k
VOUT
3.3V
600mA
94
92
90
88
86
84
82
80
R3
95.3k
47pF
R2 = R1/1.5
ILED = 24 • (R1 + R2 + R3)/(R1 • R3) AMPS
1.0
0.1
LED CURRENT (A)
R1
301k
3442 TA04b
3442 TA04a
* NOTE: THE SHDN/SS VOLTAGE MUST BE NO MORE THAN 0.5V BELOW VIN WHEN ENABLED.
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16
For more information www.linear.com/LTC3442
LTC3442
TYPICAL APPLICATIONS
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
10µF
6.3V
SHDN/SS
FB
1nF
RLIM
RT
R4
2k
57.6k
VC
R2
20k
BURST
PGND
SGND
OPEN LED VOLTAGE LIMIT = (R4 + R5) • 0.95/R4
LOW HI
10µF
6.3V
R2
200k
95.3k
1nF
40.2k
2N7002
LHXL-PW03
R1
301k
R2 = R1/1.5
ILED = 24 • (R1 + R2 + R3)/(R1 • R3) AMPS
(OR: ILED = 40/R3 + .08)
3442 TA05
* NOTE: THE SHDN/SS VOLTAGE MUST BE NO MORE THAN 0.5V BELOW VIN WHEN ENABLED.
3442fb
For more information www.linear.com/LTC3442
17
LTC3442
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DE/UE Package
12-Lead Plastic DFN (4mm × 3mm)
(Reference LTC DWG # 05-08-1695 Rev D)
0.70 ±0.05
3.30 ±0.05
3.60 ±0.05
2.20 ±0.05
1.70 ±0.05
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
2.50 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10
(2 SIDES)
7
R = 0.115
TYP
0.40 ±0.10
12
R = 0.05
TYP
PIN 1
TOP MARK
(NOTE 6)
0.200 REF
3.30 ±0.10
3.00 ±0.10
(2 SIDES)
1.70 ±0.10
0.75 ±0.05
6
0.25 ±0.05
1
PIN 1 NOTCH
R = 0.20 OR
0.35 × 45°
CHAMFER
(UE12/DE12) DFN 0806 REV D
0.50 BSC
2.50 REF
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. 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
3442fb
18
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LTC3442
REVISION HISTORY
(Revision history begins at Rev B)
REV
DATE
DESCRIPTION
PAGE NUMBER
B
05/13
Modified the Absolute Maximum Ratings section and added new Order Information
Modified the Electrical Characteristics table and Note 2
2
2, 3
Simplified Block Diagram, update 1V to 0.95V
7
Changed Operation section, 1V to 0.7V for soft-start
12
3442fb
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.
For more
information
www.linear.com/LTC3442
19
LTC3442
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LTC3401/LTC3402
1A/2A (ISW), 3MHz Synchronous Step-Up DC/DC Converter
VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19µA/300µA,
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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,
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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,
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LT3436
3A (ISW), 1MHz, 34V Step-Up DC/DC Converter
VIN: 3V to 25V, VOUT(MAX) = 34V, IQ = 0.9mA, ISD < 6µA,
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LTC3440
600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
VIN: 2.5V to 5.5V, VOUT(MIN) = 5.5V, IQ = 25µA, ISD < 1µA,
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600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter
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LTC3443
1.2A (IOUT), 600kHz Synchronous Buck-Boost DC/DC Converter
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LT3467
1.1A (ISW), 1.3MHz, High Efficiency Step-Up DC/DC Converter
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3442fb
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1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LTC3442
(408) 432-1900 ● FAX: (408) 434-0507
●
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LT 0613 REV B • PRINTED IN USA
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