LTC3857-1_15 Datasheet, PDF (1/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

FEATURES
n _{Low Operating IQ: 50μA (One Channel On)}
n _{Wide Output Voltage Range: 0.8V ≤ VOUT ≤ 24V}
n _{Wide VIN Range: 4V to 38V}
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 _{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 IQ: <8μA}
n _{Internal LDO Powers Gate Drive from VIN or EXTVCC}
n _{No Current Foldback During Start-Up}
n _{Narrow SSOP Package}
APPLICATIONS
n _{Automotive Always-On Systems}
n _{Battery Operated Digital Devices}
n _{Distributed DC Power Systems}
LTC3857-1
Low I_Q, Dual, 2-Phase
Synchronous Step-Down
DESCRIPTION
Controller
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 50μA 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.
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.
TYPICAL APPLICATION
V_OUT1
3.3V
5A
High Efficiency Dual 3.3V/8.5V Step-Down Converter
3.3μH
0.1μF
V_IN
TG1
BOOST1
SW1
INTV_CC
TG2
BOOST2
SW2
1μF
0.1μF
V_IN
22μF ^{9V TO 38V}
50V
7.2μH
BG1
BG2
LTC3857-1
PGND
SENSE1⁺
SENSE2⁺
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
38571 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
38571fc
1