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High Current Step-Down Controller Regulates to 0.6V Output
from 3V Input – Design Note 322
Charlie Zhao and Wei Chen
The LTC3832 uses a synchronous switching architecture with external N-channel MOSFETs. An adjustable
current limit is provided by sensing the current through
the drain-source on-resistance of the top MOSFET,
eliminating the need for a current sense resistor. The
LTC3832 has a 300kHz free-running switching frequency
that can be programmed or synchronized externally
from 100kHz to 500kHz. All of these features promote
a DC/DC converter solution with high efficiency and
small size.
The LTC3832 also includes a thermal protection circuit which disables both gate drivers if the junction
temperature reaches 150°C. The chip resumes normal
operation when the temperature drops below 125°C.
Design Examples
Figure 1 shows the schematic diagram and photo of a
compact 1V/7A step-down DC/DC converter that accepts
2.2μF
10Ω
0.01μF
LTC3832-1
SS VCC/PVCC2
COMP
G1
15k
GND
MBR0520T1
VIN = 5V
VOUT = 1V
95
90
85
80
75
70
0
1
2
3
4
5
LOAD CURRENT (A)
Si7806DN
Si7806DN
4.32k
Figure 3 shows the schematic diagram of a high efficiency 3V to 8V to 1.8V/20A synchronous step-down
power supply. The switching frequency is set at 360kHz
by the external resistor connected to the FREQSET pin.
For normal operation, a pull-up resistor brings the
voltage high at the SHDN pin. A low voltage for more
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and SwitcherCAD is a trademark of Linear Technology Corporation. All other
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L1
3.2μH
VOUT
1V
7A
+
COUT
270μF
2V
DN322 F01
680pF
COUT: PANASONIC EEFUEOD271R
CIN: SANYO POSCAP 10TPB220M
L1: SUMIDA CDEP105-3R2MC-88
Figure 1. A Compact 5V to 1V Power Supply at 7A
10/03/322_conv
7
Figure 2. Efficiency for the Circuit in Figure 1
PVCC1
G2
6
DN322 F02
CIN
220μF
0.1μF
6.49k
FB
100
VIN
3V TO 8V
+
0.1μF
an input voltage of 3V to 8V. The design is based on
the LTC3832-1, an SO-8 package option. Its efficiency
is shown in Figure 2.
EFFICIENCY (%)
Introduction
The LTC ®3832 is a voltage mode, high efficiency, high
power step-down switching regulator controller that can
operate from input voltages as low as 3V. It integrates a
0.6V reference voltage, two powerful MOSFET drivers
and other features which make it possible to build cost
effective, high efficiency, high current power supplies
with output voltages as low as 0.6V.
+
D3
MBR0520LT1
DZ
MMSZ5242B
+
1μF
10Ω
+
PVCC1
PVCC2
VCC
2.2μF
CSS
0.01μF
1k
130k
D1
B320A
Q2
G2
+
PGND
COMP
RC
18.2k
CC
3000pF
LO
0.6μH
1k
LTC3832 IFB
SHDN
C1
150pF
Q1
IMAX
FREQSET
RUN
0.1μF
G1
SS
CIN
220μF
10μF
0.1μF
RQ
12k
VIN
3V TO 8V
COUT
180μF
w2
VOUT
1.8V
20A
RA
6.04k
1%
470pF
GND
SENSE +
SENSE –
NC
FB
RB
3.01k
1%
Q1, Q2: SILICONIX Si7892DP w2
CIN: SANYO POSCAP 10TPB220M
COUT: PANASONIC EEFUD0D181R
LO: TOKO FDH1055
DN322 F03
NC
Figure 3. High Efficiency 3.3V to 1.8V at 20A Synchronous Step-Down DC/DC Converter
The LTC3832 also includes a soft-start circuit. The
soft-start voltage ramp rate in this circuit is set by CSS.
Figure 4 shows the efficiency curve of the circuit. Up
to 90% efficiency is obtained with a 3.3V input. For
other output voltages, simply change the value of R A.
The output voltage can be as low as 0.6V (see Table 1).
For higher output currents, parallel more MOSFETs and
use a higher current inductor.
Table 1. RA Values for Different Output Voltages
VOUT (V)
RA (Ω)
0.6
0
1.0
2.00k
1.2
3.01k
1.5
4.53k
1.8
6.04k
2.5
9.53k
Data Sheet Download
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Linear Technology Corporation
100
VIN = 3.3V
VOUT = 1.8V
95
EFFICIENCY (%)
than 100μs at SHDN pin pulls the COMP and SS pins
to ground and shuts Q1 and Q2 off. The current limit
function is provided by connecting an external resistor
RQ from the IMAX pin to the drain of the top MOSFET
(Q1). The current limit threshold can be adjusted by the
value of RQ based on the RDS(ON) of Q1 and the load
requirement. The 0.1μF decoupling capacitor across RQ
filters the switching noise.
90
85
80
75
70
0
4
8
12
LOAD CURRENT (A)
16
20
DN322 F04
Figure 4. Efficiency for the Circuit
in Figure 3 with VIN = 3.3V
Conclusion
The LTC3832 is a voltage mode controller optimized
for high power, low voltage—as low as 0.6V—applications. It provides a set of features that promote high
efficiency while keeping solution costs low. LTC3832
designs may be simulated with SwitcherCAD™ III which
can be downloaded from www.linear.com.
For applications help,
call (408) 432-1900
dn322f_conv LT/TP 1003 316.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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