FEATURES
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LTC3728
Dual, 550kHz, 2-Phase
Synchronous Step-Down Switching Regulator
DESCRIPTIO
The LTC
®
3728 is a dual high performance step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant fre-
quency current mode architecture allows phase-lockable
frequency of up to 550kHz. Power loss and noise due to
the ESR of the input capacitors are minimized by operating
the two controller output stages out of phase.
OPTI-LOOP compensation allows the transient response
to be optimized over a wide range of output capacitance and
ESR values. The precision 0.8V reference and power good
output indicator are compatible with future microproces-
sor generations, and a wide 3.5V to 30V (36V maximum)
input supply range encompasses all battery chemistries.
A RUN/SS pin for each controller provides both soft-start
and optional timed, short-circuit shutdown. Current
foldback limits MOSFET dissipation during short-circuit
conditions when overcurrent latchoff is disabled. Output
overvoltage protection circuitry latches on the bottom
MOSFET until V
OUT
returns to normal. The FCB mode pin
can select among Burst Mode, constant frequency mode
and continuous inductor current mode or regulate a
secondary winding. The LTC3728 includes a power good
output pin that indicates when both outputs are within
7.5% of their designed set point.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Protected by U.S. Patents, including 5481178, 5929620, 6177787, 6144194, 6100678,
5408150, 6580258, 6304066, 5705919.
V
IN
5.2V TO 28V
Dual, 180° Phased Controllers Reduce Required
Input Capacitance and Power Supply Induced Noise
OPTI-LOOP
®
Compensation Minimizes C
OUT
±1%
Output Voltage Accuracy
Power Good Output Voltage Indicator
Phase-Lockable Fixed Frequency 250kHz to 550kHz
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 3.5V to 36V Operation
Very Low Dropout Operation: 99% Duty Cycle
Adjustable Soft-Start Current Ramping
Foldback Output Current Limiting
Latched Short-Circuit Shutdown with Defeat Option
Output Overvoltage Protection
Remote Output Voltage Sense
Low Shutdown I
Q
: 20µA
5V and 3.3V Regulators
3 Selectable Operating Modes: Constant Frequency,
Burst Mode
®
Operation and PWM
Available in 32-Pin 5mm
×
5mm QFN and
28-Pin SSOP Packages
APPLICATIO S
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Notebook and Palmtop Computers
Telecom Systems
Portable Instruments
Battery-Operated Digital Devices
DC Power Distribution Systems
TYPICAL APPLICATIO
+
4.7µF
M1
D3
V
IN
PGOOD INTV
CC
TG1
TG2
BOOST2
SW2
LTC3728
BG2
PGND
SENSE2
+
1000pF
SENSE1
–
V
OSENSE1
C
C1
220pF
R
C1
15k
I
TH1
C
SS1
0.1µF
SENSE2
–
V
OSENSE2
I
TH2
C
SS2
0.1µF
C
C2
220pF
R
C2
15k
R4
63.4k
1%
C
B2
, 0.1µF
D4
1µF
M2
L1
3.2µH
C
B1
, 0.1µF
BOOST1
SW1
BG1
f
IN
500kHz
PLLIN
SENSE1
+
R
SENSE1
0.01Ω
V
OUT1
5V
5A
1000pF
+
R2
105k
C
OUT1
1%
47µF
6V
SP
M1, M2: FDS6982S
R1
20k
1%
RUN/SS1 SGND RUN/SS2
Figure 1. High Efficiency Dual 5V/3.3V Step-Down Converter
U
U
U
C
IN
22µF
50V
L2
3.2µH
R
SENSE2
0.01Ω
V
OUT2
3.3V
5A
R3
20k
1%
C
OUT
56µF
6V
SP
+
3728 F01
3728fb
1
LINER [ LINEAR TECHNOLOGY ]
