LTC1709-85
2-Phase, 5-Bit VID,
Current Mode, High Efficiency,
Synchronous Step-Down Switching Regulator
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DESCRIPTIO
FEATURES
The LTC®1709-85 is a 2-phase, VID programmable, syn-
chronous step-down switching regulator controller that
drivestwoallN-channelexternalpowerMOSFETstagesin
a fixed frequency architecture. The 2-phase controller
drives its two output stages out of phase at frequencies up
to 300kHz to minimize the RMS ripple currents in both
input and output capacitors. The 2-phase technique effec-
tively multiplies the fundamental frequency by two, im-
proving transient response while operating each channel
at an optimum frequency for efficiency. Thermal design is
also simplified.
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Output Stages Operate Antiphase Reducing Input
Capacitance Requirements and Power Supply
Induced Noise
Dual Input Supply Capability for Load Sharing
5-Bit VID Code (VRM 8.5): VOUT = 1.05V to 1.825V
True Remote Sensing Differential Amplifier
Power Good Output Indicator
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±1% Output Voltage Accuracy
Active Voltage Positioning Capable
Current Mode Control Ensures Current Sharing
OPTI-LOOP® Compensation Minimizes COUT
Three Operational Modes: PWM, Burst and Cycle Skip
Programmable Fixed Frequency: 150kHz to 300kHz
Wide VIN Range: 4V to 36V Operation
An operating mode select pin (FCB) can be used to select
among three modes including Burst Mode® operation for
highestefficiency.Aninternaldifferentialamplifierprovides
true remote sensing of the regulated supply’s positive and
negative output terminals as required in high current ap-
plications.
Adjustable Soft-Start Current Ramping
Internal Current Foldback and Short-Circuit Shutdown
Overvoltage Soft Latch Eliminates Nuisance Trips
Available in 36-Lead Narrow SSOP Package
The RUN/SS pin provides soft-start and optional timed,
short-circuit shutdown. Current foldback limits MOSFET
dissipation during short-circuit conditions when the
overcurrentlatchoffisdisabled.OPTI-LOOPcompensation
allows the transient response to be optimized for a wide
range of output capacitors and ESR values.
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APPLICATIO S
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Server/Desktop Computers
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Internet Servers
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Large Memory Arrays
DC Power Distribution Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
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OPTI-LOOP and Burst Mode are registered trademarks of Linear Technology Corporation.
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TYPICAL APPLICATIO
V
IN
5V TO 28V
+
10µF
35V
×4
0.1µF
V
IN
FCB
TG1
S
0.002Ω
0.002Ω
RUN/SS
BOOST1
SW1
0.47µF
220pF
3.3k
1µH
S
LTC1709-85
I
TH
BG1
SGND
PGND
+
SENSE1
SENSE1
PGOOD
–
5 VID BITS VID0–VID4
TG2
BOOST2
SW2
V
OUT
EAIN
0.47µF
1.05V TO 1.825V
40A
1µH
ATTENOUT
ATTENIN
BG2
V
V
V
INTV
DIFFOUT
CC
+
10µF
C
OUT
+
–
SENSE2
SENSE2
OS
1000µF
4V
+
–
OS
×2
170985 F01
Figure 1. High Current Dual Phase Step-Down Converter
170985f
1