LT8631
9
8631fa
For more information www.linear.com/LT8631
PIN FUNCTIONS
V
IN
(Pin 1): The V
IN
pin powers the internal control circuitry
and is monitored by an undervoltage lockout comparator.
The V
IN
pin is also connected to the drain of the on chip
power switch. The V
IN
pin has high dI/dt edges and must
be decoupled to the GND pin of the device. The input
decouple capacitor should be placed as close as possible
to the V
IN
and GND pins.
EN/UV (Pin 3): The EN/UV pin is used to enable the LT8631
or to program the undervoltage lockout threshold with ex
-
ternal resistors. The LT8631 is in shutdown mode( I
Q
< 5µA)
when the EN/UV pin voltage is below 1.19V and active
mode
when the voltage exceeds 1.19V. Tie EN/UV to the
V
IN
pin if the EN/UV feature isn’t required.
PG (Pin 5): The PG pin is an open drain output that sinks
current when the feedback voltage deviates from the regula
-
tion point by ±7.5%. The PG pin has 1.9% of hysteresis.
NC6 (Pin 6): No Internal Connection. Leave this pin open
or connect to GND.
SYNC/MODE (Pin 7): The voltage present at the SYNC/
MODE pin determines the LT8631
operating mode.
Ground
this pin for low ripple Burst Mode operation at low output
loads. Apply a DC voltage greater than the SYNC/MODE
threshold for pulse-skipping operation at low output loads.
The LT8631 will synchronize it’s switching frequency to an
external clock applied to the SYNC/MODE pin. The external
clock signal must have a duty cycle between 20% and 80%
and be within the specified frequency range. The LT8631
will operate in pulse-skipping mode when the SYNC/MODE
pin is driven with an external clock.
RT (Pin 8): A resistor with a value between 8.66k and
187k must be connected between the RT pin and the GND
pin. The R
T
resistor sets the switching frequency. Do not
leave this pin floating.
NC9 (Pin 9): No Internal Connection. Leave this pin open
or connect to GND.
TR/SS (Pin 10): A capacitor with a minimum value of 100pF
must be connected between the TR/SS pin and the GND
pin. The voltage ramp rate on the TR/SS pin determines
the output voltage ramp rate. This pin can also be used
for voltage tracking. Do not leave this pin floating.
FB (Pin 11): The FB pin is the
negative input to the er-
ror
amplifier. The output switches to regulate this pin to
0.808V with respect to the GND pin.
V
OUT
(Pin 12): The V
OUT
pin is the output to the internal
sense resistor that measures current flowing in the in-
ductor. Connect
the output capacitor from the V
OUT
pin
to the GND pin.
NC13 (Pin 13): No Internal Connection. Leave this pin
open or connect to GND.
IND (Pin 14): The IND pin is the input to the internal sense
resistor that measures current flowing in the inductor.
NC15 (Pin 15): No Internal Connection. Leave this pin
open or connect to GND.
INTV
CC
(Pin 16): The INTV
CC
pin is the bypass pin for the
internal 3V regulator. Connect a 2.2µF bypass capacitor
from the INTV
CC
pin to the GND pin. Do not load the INTV
CC
pin with external circuitry.
BST (Pin 18): The BST pin is used to provide a drive
voltage, higher than the V
IN
voltage, to the topside power
switch. Place a 0.1µF capacitor between the BST and SW
pins as close as possible to the device.
SW (Pin 20): The SW pin is the output of the internal power
switches. Place the inductor and
BST capacitor as close
as possible to keep the SW PCB trace short.
GND
(Exposed Pad Pin 21): The exposed pad GND pin
is the ONLY GROUND CONNECTION for the device. The
exposed pad should be soldered to a large copper area
to reduce thermal resistance. The GND pin also serves as
small signal ground. For ideal operation all small signal
ground paths should connect to the GND pin at a single
point avoiding any high current ground returns.
