Datasheet

LTC3878
7
3878fa
pin FuncTions
RUN/SS (Pin 1): Run Control and Soft-Start Input. A
capacitor to ground on this pin sets the ramp time to full
output current (approximately 3s/µF) when RUN/SS is
open. The switching outputs are disabled when below 1.5V.
The device is in micropower shutdown when under 0.7V.
If left open, there is an internal 1.2µA pull-up current on
RUN/SS. INTV
CC
is enabled when RUN/SS exceeds 0.7V.
PGOOD (Pin 2):
Power Good Output. This open-drain
logic output is pulled to ground when the output voltage is
outside of a ±7.5% window around the regulation point.
V
RNG
(Pin 3): V
DS
Sense Voltage Range Input. The maxi-
mum allowed bottom MOSFET V
DS
sense voltage between
SW and PGND is equal to (0.133)V
RNG
. The voltage applied
to V
RNG
can be any value between 0.2V and 2V. If V
RNG
is
tied to SGND, the device operates with a maximum valley
current sense threshold of 93mV typical. If V
RNG
is tied
to INTV
CC
, the device operates with a maximum valley
current sense threshold of 186mV typical.
FCB (Pin 4): Forced Continuous Input. Connect this pin
to INTV
CC
to enable discontinuous mode for light load
operation. Connect this pin to SGND to force continuous
mode operation in all conditions.
I
TH
(Pin 5): Current Control Threshold and Error Amplifier
Compensation Point. The current comparator threshold
increases with this control voltage. The voltage ranges
from 0V to 2.4V, with 0.8V corresponding to zero sense
voltage (zero current).
SGND (Pin 6): Signal Ground. All small-signal components
should be connected to SGND. Connect SGND to PGND
using a single PCB trace.
I
ON
(Pin 7): On-Time Current Input. Tie a resistor from
V
IN
to this pin to set the one-shot timer current and thus
the switching frequency.
V
FB
(Pin 8): Error Amplifier Feedback Input. This pin con-
nects the error amplifier to an external resistive divider
from V
OUT
.
NC (Pin 9): For factory use only. Can be connected to any
voltage equal to or less than INTV
CC
.
V
IN
(Pin 10): Main Input Supply. The supply voltage can
range from 4V to 38V. For increased noise immunity de-
couple
this pin to PGND with an RC filter.
INTV
CC
(Pin 11): Internal 5.3V Regulator Output. The
driver and control circuits are powered from this voltage.
Decouple this pin to PGND with a minimum of 1µF, 10V
X5R or X7R ceramic capacitor.
BG (Pin 12): Bottom Gate Drive. This pin drives the gate
of the bottom N-Channel power MOSFET between PGND
and INTV
CC
.
PGND (Pin 13): Power Ground. Connect this pin as close
as practical to the source of the bottom N-channel power
MOSFET, the (–) terminal of C
INTVCC
and the (–) terminal
of C
VIN
.
SW (Pin 14): Switch Node. The (–) terminal of the bootstrap
capacitor, C
B
, connects to this node. This pin swings from
a diode voltage below ground up to V
IN
.
TG (Pin 15): Top Gate Drive. This pin drives the gate of the
top N-channel power MOSFET between SW and BOOST.
BOOST (Pin 16): Boosted Floating Driver Supply. The (+)
terminal of the bootstrap capacitor, C
B
, connects to this
node. This node swings from (INTV
CC
– V
SCHOTTKY
) to
V
IN
+ (INT
VCC
– V
SCHOTTKY
).