LTC3857-1
1
38571fc
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
Low I
Q
, Dual, 2-Phase
Synchronous Step-Down
Controller
High Efficiency Dual 3.3V/8.5V Step-Down Converter
n
Low Operating I
Q
: 50μA (One Channel On)
n
Wide Output Voltage Range: 0.8V ≤ V
OUT
≤ 24V
n
Wide V
IN
Range: 4V to 38V
n
R
SENSE
or DCR Current Sensing
n
Out-of-Phase Controllers Reduce Required Input
Capacitance and Power Supply Induced Noise
n
OPTI-LOOP
®
Compensation Minimizes C
OUT
n
Phase-Lockable Frequency (75kHz-850kHz)
n
Programmable Fixed Frequency (50kHz-900kHz)
n
Selectable Continuous, Pulse-Skipping or Low Ripple
Burst Mode
®
Operation at Light Loads
n
Very Low Dropout Operation: 99% Duty Cycle
n
Adjustable Output Voltage Soft-Start or Tracking
n
Power Good Output Voltage Monitor
n
Output Overvoltage Protection
n
Low Shutdown I
Q
: <8µA
n
Internal LDO Powers Gate Drive from V
IN
or EXTV
CC
n
No Current Foldback During Start-Up
n
Narrow SSOP Package
n
Automotive Always-On Systems
n
Battery Operated Digital Devices
n
Distributed DC Power Systems
0.1µF
62.5k
3.3µH
680pF
150µF
1µF
22µF
50V
0.007Ω
20k
15k
V
OUT1
3.3V
5A
150µF
0.1µF
193k
7.2µH
680pF
0.010Ω
20k
15k
V
OUT2
8.5V
3.5A
TG1 TG2
BOOST1 BOOST2
SW1 SW2
BG1 BG2
SGND
PGND
SENSE1
+
SENSE2
+
SENSE1
–
SENSE2
–
V
FB1
V
FB2
I
TH1
I
TH2
V
IN
INTV
CC
TRACK/SS1 TRACK/SS2
V
IN
9V TO 38V
38571 TA01
0.1µF
0.1µF
LTC3857-1
Efficiency and Power Loss
vs Output Current
The LTC
®
3857-1 is a high performance dual step-down
switching regulator controller that drives all N-channel
synchronous power MOSFET stages. A constant frequency
current mode architecture allows a phase-lockable fre-
quency of up to 850kHz. Power loss and noise due to the
ESR of the input capacitor ESR are minimized by operating
the two controller output stages out of phase.
The 50A no-load quiescent current extends operating life
in battery-powered systems. The LTC3857-1 features a
precision 0.8V reference and a power good output indicator.
A wide 4V to 38V input supply range encompasses a wide
range of intermediate bus voltages and battery chemistries.
Independent TRACK/SS pins for each controller ramp the
output voltages during start-up. Current foldback limits
MOSFET heat dissipation during short-circuit conditions.
The PLLIN/MODE pin selects among Burst Mode opera-
tion, pulse-skipping mode, or continuous inductor current
mode at light loads.
For a leadless 32-pin QFN package with the additional fea-
tures of adjustable current limit, clock out, phase modula-
tion and two PGOOD outputs, see the LTC3857 data sheet.
OUTPUT CURRENT (A)
0.00001 0.0001
40
EFFICIENCY (%)
POWER LOSS (mW)
50
60
70
80
0.001 0.01 0.1 1 10
3857 TA01b
30
20
10
0
90
100
10
100
1000
1
0.1
10000
V
IN
= 12V
V
OUT
= 3.3V
FIGURE 13 CIRCUIT
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, µModule, Linear Technology and the Linear logo
are registered trademarks and No R
SENSE
and UltraFast are 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, 5408150, 6580258, 5705919,
6100678.
