LTC3851
8
3851fb
PIN FUNCTIONS
(GN and 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-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 fi lter 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
1.25V 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 2A 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
1A charges this capacitor.
I
TH
(Pin 5/Pin 3): Current Control Threshold and Error
Amplifi er Compensation Point. The current comparator
tripping threshold increases with its I
TH
control voltage.
FB (Pin 6/Pin 4): Error Amplifi er 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): 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 connected to this pin.
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 2.2F 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 fl oating 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
Inductor. Voltage swing at this pin is from a Schottky diode
(external) voltage drop below ground to V
IN
.
Exposed Pad (Pin 17, UD and MSE Packages Only):
Ground. Must be soldered to PCB, providing a local
ground for the IC.
