LTC3851A
8
3851afa
pin FuncTions
(GN and MSE/UD)
MODE/PLLIN (Pin 1/Pin 15): Forced Continuous Mode,
Burst Mode or Pulse-Skipping Mode Selection Pin and Ex-
ternal 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-skipping
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 cause
the controller to operate in 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.
I
LIM
(Pin 9/Pin 7): Current Comparator Sense Voltage
Range Input. Tying this pin to GND, FLOAT or INTV
CC
selects the maximum current sense threshold from three
dif ferent levels.
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
.
