Datasheet

7
LTC3778
3778f
UU
U
PI FU CTIO S
RUN/SS (Pin 1): Run Control and Soft-Start Input. A
capacitor to ground at this pin sets the ramp time to full
output current (approximately 3s/µF) and the time delay
for overcurrent latchoff (see Applications Information).
Forcing this pin below 0.8V shuts down the device.
V
ON
(Pin 2): On-Time Voltage Input. Voltage trip point for
the on-time comparator. Tying this pin to the output
voltage makes the on-time proportional to V
OUT
. The
comparator input defaults to 0.7V when the pin is grounded,
2.4V when the pin is tied to INTV
CC
.
PGOOD (Pin 3): Power Good Output. Open drain logic
output that is pulled to ground when the output voltage is
not within ±10% of the regulation point.
V
RNG
(Pin 4): Sense Voltage Range Input. The voltage at
this pin is ten times the nominal sense voltage at maximum
output current and can be set from 0.5V to 2V by a resistive
divider from INTV
CC
. The nominal sense voltage defaults
to 70mV when this pin is tied to ground, 140mV when tied
to INTV
CC
.
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).
FCB (Pin 6): Forced Continuous Input. Tie this pin to
ground to force continuous synchronous operation at low
load, to INTV
CC
to enable discontinuous mode operation at
low load or to a resistive divider from a secondary output
when using a secondary winding.
SGND (Pin 7): Signal Ground. All small-signal compo-
nents and compensation components should connect to
this ground, which in turn connects to PGND at one point.
I
ON
(Pin 8): On-Time Current Input. Tie a resistor from V
IN
to this pin to set the one-shot timer current and thereby set
the switching frequency.
V
FB
(Pin 9): Error Amplifier Feedback Input. This pin
connects the error amplifier input to an external resistive
divider from V
OUT
.
EXTV
CC
(Pin 10): External V
CC
Input. When EXTV
CC
ex-
ceeds 4.7V, an internal switch connects this pin to INTV
CC
and shuts down the internal regulator so that controller
power is drawn from EXTV
CC
. Do not exceed 7V at this pin
and ensure that EXTV
CC
< V
IN
.
V
IN
(Pin 11): Main Input Supply. Decouple this pin to
SGND with an RC filter (1Ω, 0.1µF).
INTV
CC
(Pin 12): Internal 5V Regulator Output. The inter-
nal control circuits are powered from this voltage. De-
couple this pin to power ground with a minimum of 1µF
low ESR tantalum or ceramic capacitor.
DRV
CC
(Pin 13): Voltage Supply to Bottom Gate Driver.
Normally connected to the INTV
CC
pin through a decoupling
RC filter (1Ω, 0.1µF). Decouple this pin to power ground
with a minimum of 4.7µF low ESR tantalum or ceramic
capacitor. Do not exceed 7V at this pin.
BG (Pin 14): Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET between ground and DRV
CC
.
PGND (Pin 15): Power Ground. Connect this pin closely to
the source of the bottom N-channel MOSFET or to the
bottom of the sense resistor when used, the (–) terminal
of C
VCC
and the (–) terminal of C
IN
.
SENSE
–
(Pin 16): Current Sense Comparator Input. The
(–) input to the current comparator is used to accurately
Kelvin sense the bottom side of the sense resistor or
MOSFET.
SENSE
+
(Pin 17): Current Sense Comparator Input. The
(+) input to the current comparator is normally connected
to the SW node unless using a sense resistor (See Appli-
cations Information).
SW (Pin 18): Switch Node. The (–) terminal of the boot-
strap capacitor C
B
connects here. This pin swings from a
diode voltage drop below ground up to V
IN
.
TG (Pin 19): Top Gate Drive. Drives the top N-channel
MOSFET with a voltage swing equal to DRV
CC
superim-
posed on the switch node voltage SW.
BOOST (Pin 20): Boosted Floating Driver Supply. The (+)
terminal of the bootstrap capacitor C
B
connects here. This
pin swings from a diode voltage drop below DRV
CC
up to
V
IN
+ DRV
CC
.