LTC3857 Datasheet, PDF (1/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857
Features
Low I_Q, Dual, 2-Phase
Synchronous Step-Down
Description
Controller
n _{Low Operating IQ: 50µA (One Channel On)}
n _{Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 24V}
n _{Wide VIN Range: 4V to 38V (40V Abs Max)}
n _{RSENSE or DCR Current Sensing}
n _{Out-of-Phase Controllers Reduce Required Input}
Capacitance and Power Supply Induced Noise
n _OPTI-LOOP^® _{Compensation Minimizes COUT}
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 _{Selectable Current Limit}
n _{Very Low Dropout Operation: 99% Duty Cycle}
n _{Adjustable Output Voltage Soft-Start or Tracking}
n _{Power Good Output Voltage Monitors}
n _{Output Overvoltage Protection}
n _{Low Shutdown IQ : <8µA}
n _{Internal LDO Powers Gate Drive from VIN or EXTVCC}
n _{No Current Foldback During Start-up}
n _{Small Low Profile (0.75mm) 5mm × 5mm QFN Package}
Applications
n _{Automotive Always-On Systems}
n _{Battery Operated Digital Devices}
n _{Distributed DC Power Systems}
The LTC^®3857 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 50μA no-load quiescent current extends operating run
time in battery-powered systems. The LTC3857 features a
precision 0.8V reference and power good output indicators. 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 leaded 28-lead SSOP package with a fixed current
limit and one PGOOD output, without phase modulation
or a clock output, see the LTC3857-1 data sheet.
L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, PolyPhase, 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.
Typical Application
High Efficiency Dual 3.3V/8.5V Step-Down Converter
4.7µF
V_IN
22µF ^{9V TO 38V}
50V
V_IN
TG1
INTV_CC
TG2
3.3µH
0.1µF
BOOST1
BOOST2
0.1µF
7.2µH
SW1
SW2
BG1
BG2
LTC3857
PGND
SENSE1⁺
SENSE2⁺
V_OUT1
3.3V
5A
0.007Ω
62.5k
150µF
680pF
SENSE1^–
V_FB1
I_TH1
SENSE2^–
V_FB2
I_TH2
TRACK/SS1 SGND TRACK/SS2
20k 15k
0.1µF
0.1µF
680pF
15k
0.010Ω
193k
20k
V_OUT2
8.5V
3.5A
150µF
3857 TA01
Efficiency and Power Loss
vs Output Current
100
V_IN = 12V
90 V_OUT = 3.3V
FIGURE 13 CIRCUIT
80
10000
1000
70
60
100
50
40
10
30
20
1
10
0
0.00001 0.0001 0.001 0.01 0.1
OUTPUT CURRENT (A)
0.1
1 10
3857 TA01b
3857fa