LTC3831 Datasheet, PDF (7/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
PI FU CTIO S
FREQSET (Pin 11): Frequency Set. Use this pin to adjust
the free-running frequency of the internal oscillator. With
the pin floating, the oscillator runs at about 200kHz. A
resistor from FREQSET to ground speeds up the oscillator;
a resistor to V_CC slows it down.
I_MAX (Pin 12): Current Limit Threshold Set. I_MAX sets the
threshold for the internal current limit comparator. If I_FB
drops below I_MAX with TG on, the LTC3831 goes into
current limit. I_MAX has an internal 12µA pull-down to GND.
Connect this pin to the main V_IN supply at the drain of Q1,
through an external resistor to set the current limit thresh-
old. Connect a 0.1µF decoupling capacitor across this
resistor to filter switching noise.
I_FB (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 I_FB.This pin is used for
sensing the voltage drop across the upper N-channel
MOSFET, Q1.
V_CC (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.
PV_CC2 (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 PV_CC2. It remains low when TG is
high or during shutdown mode. To prevent output under-
shoot during a soft-start cycle, BG is held low until TG first
goes high (FFBG in the Block Diagram).
BLOCK DIAGRA
SHDN
FREQSET
COMP
SS
Q_C
100µs DELAY
INTERNAL
OSCILLATOR
LOGIC AND
THERMAL SHUTDOWN
DISABLE GATE DRIVE
POWER DOWN
–
PWM
+
SQ
RQ
12µA
QSS
FFBG
SQ
POR R
ENABLE
BG
ERR
MIN
MAX
+–
V_REF
V_REF – 3%
CC ^–
V_REF + 3%
I_FB
+
2.2V
DISABLE
I_LIM
1.2V
12µA
I_MAX
V
PV_CC1
V_CC1 + 2.5V
V_REF + 3%
V_REF
V_REF – 3%
V_CC
PV_CC1
TG
PV_CC2
BG
PGND
FB
R⁺
24k
750Ω
750Ω
24k
R^–
GND
3830 BD
3831f
7