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LTC3831_15 Datasheet, PDF (7/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination | |||
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LTC3831
PIN FUNCTIONS
FREQSET (Pin 11): Frequency Set. Use this pin to adjust
the free-running frequency of the internal oscillator. With
the pin ï¬oating, the oscillator runs at about 200kHz. A
resistor from FREQSET to ground speeds up the oscillator;
a resistor to VCC slows it down.
IMAX (Pin 12): Current Limit Threshold Set. IMAX sets the
threshold for the internal current limit comparator. If IFB
drops below IMAX with TG on, the LTC3831 goes into cur-
rent limit. IMAX has an internal 12μA pull-down to GND.
Connect this pin to the main VIN supply at the drain of
Q1, through an external resistor to set the current limit
threshold. Connect a 0.1μF decoupling capacitor across
this resistor to ï¬lter switching noise.
IFB (Pin 13): Current Limit Sense. Connect this pin to the
switching node at the source of Q1 and the drain of Q2
through a 1k resistor. The 1k resistor is required to prevent
voltage transients from damaging IFB.This pin is used
for sensing the voltage drop across the upper N-channel
MOSFET, Q1.
VCC (Pin 14): Power Supply Input. All low power internal
circuits draw their supply from this pin. This pin requires
a 4.7μF bypass capacitor to GND.
PVCC2 (Pin 15): Power Supply Input for BG. Connect this
pin to the main high power supply.
BG (Pin 16): Bottom Driver Output . Connect this pin to
the gate of the lower N-channel MOSFET, Q2. This output
swings from PGND to PVCC2. It remains low when TG is high
or during shutdown mode. To prevent output undershoot
during a soft-start cycle, BG is held low until TG ï¬rst goes
high (FFBG in the Block Diagram).
BLOCK DIAGRAM
SHDN
FREQSET
COMP
SS
QC
100μs DELAY
INTERNAL
OSCILLATOR
LOGIC AND
THERMAL SHUTDOWN
DISABLE GATE DRIVE
POWER DOWN
â
PWM
+
SQ
RQ
12μA
QSS
FFBG
SQ
POR R
ENABLE
BG
ERR
MIN
MAX
+â
VREF
VREF â 3%
VREF + 3%
CC â
IFB
+
DISABLE
2.2V
ILIM
1.2V
12μA
IMAX
V
PVCC1
VCC1 + 2.5V
VREF + 3%
VREF
VREF â 3%
VCC
PVCC1
TG
PVCC2
BG
PGND
FB
R+
24k
750Ω
750Ω
24k
Râ
GND
3830 BD
3831fb
7
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