LTC3851A-1
9
3851a1fa
pin FuncTions
(MSE/UD)
MODE/PLLIN (Pin 1/Pin 15): Forced Continuous Mode,
Burst Mode or Pulse-Skipping Mode Selection Pin and
External Synchronization Input to Phase Detector Pin.
Connect this pin to INTV
CC
to force continuous conduction
mode of operation. Connect to GND to enable pulse-skip-
ping mode of operation. To select Burst Mode operation,
tie this pin to INTV
CC
through a resistor no less than 50k,
but no greater than 250k. A clock on the pin will force the
controller into forced continuous mode of operation and
synchronize the internal oscillator.
FREQ/PLLFLTR (Pin 2/Pin 16): The phase-locked loop’s
lowpass filter is tied to this pin. Alternatively, a resistor
can be connected between this pin and GND to vary the
frequency of the internal oscillator.
RUN (Pin 3/Pin 1): Run Control Input. A voltage above
1.22V on this pin turns on the IC. However, forcing this
pin below 1.1V causes the IC to shut down the IC. There
is a 2μA pull-up current on this pin.
TK/SS (Pin 4/Pin 2): Output Voltage Tracking and Soft-Start
Input. A capacitor to ground at this pin sets the ramp rate
for the output voltage. An internal soft-start current of of
1μA charges this capacitor.
I
TH
(Pin 5/Pin 3): Current Control Threshold and Error
Amplifier Compensation Point. The current comparator
tripping threshold increases with its I
TH
control voltage.
FB (Pin 6/Pin 4): Error Amplifier Feedback Input. This pin
receives the remotely sensed feedback voltage from an
external resistive divider across the output.
SENSE
–
(Pin 7/Pin 5): Current Sense Comparator Inverting
Input. The (–) input to the current comparator is connected
to the output.
SENSE
+
(Pin 8/Pin 6): Current Sense Comparator Non-
inverting Input. The (+) input to the current comparator
is normally connected to the DCR sensing network or
current sensing resistor.
PGOOD (Pin 9/Pin 7): Power Good Indicator Output. Open-
drain logic out that is pulled to ground when the output
voltage exceeds the ±10% regulation window, after the
internal 17µs power bad mask timer expires.
GND (Pin 10/Pin 8, Exposed Pad Pin 17): Ground. All
small-signal components and compensation components
should be Kelvin connected to this ground. The (–) terminal
of CV
CC
and the (–) terminal of C
IN
should be closely con-
nected to this pin. The exposed pad should be soldered
to ground for good thermal conductivity.
BG (Pin 11/Pin 9): Bottom Gate Driver Output. This pin
drives the gate of the bottom N-channel MOSFET between
GND and INTV
CC
.
INTV
CC
(Pin 12/Pin 10): Internal 5V Regulator Output. The
control circuit is powered from this voltage. Decouple this
pin to GND with a minimum 2.2μF low ESR tantalum or
ceramic capacitor.
V
IN
(Pin 13/Pin 11): Main Input Supply. Decouple this pin
to GND with a capacitor.
BOOST (Pin 14/Pin 12): Boosted Floating Driver Supply.
The (+) terminal of the boost-strap capacitor is connected
to this pin. This pin swings from a diode voltage drop
below INTV
CC
up to V
IN
+ INTV
CC
.
TG (Pin 15/Pin 13): Top Gate Driver Output. This is the
output of a floating driver with a voltage swing equal to
INTV
CC
superimposed on the switch node voltage.
SW (Pin 16/Pin 14): Switch Node Connection to the In-
ductor. Voltage swing at this pin is from a Schottky diode
(external) voltage drop below ground to V
IN
.
