Datasheet

LT3782
9
3782fg
With a 200ns one-shot timer on chip, the LT3782 provides
exibility on the external sync pulse width. The sync pulse
threshold is about 1.2V (Figure 1).
Current Limit
Current limit is set by the 60mV threshold across SEN1P,
SEN1N for channel one and SEN2P, SEN2N for channel
two. By connecting an external resistor R
S
(see Block
Diagram), the current limit is set for 60mV/R
S
. R
S
should
be placed very close to the power switch with very short
traces. A low pass R
C
lter is needed across R
S
to fi lter out
the switching spikes. Good Kelvin sensing is required for
accurate current limit. The input bypass capacitor ground
should be at the same ground point of the current sense
resistor to minimize the ground current path.
Synchronous Rectifi er Switches
For high output voltage applications, the power loss of
the catch diodes are relatively small because of high duty
cycle. If diodes power loss or heat is a concern, the LT3782
provides PWM signals through SGATE1 and SGATE2 pins
to drive external MOSFET drivers for synchronous recti-
er operation. Note that SGATE drives the top switch and
BGATE drives the bottom switch. To avoid cross conduction
between top and bottom switches, the BGATE turn-on is
delayed 100ns (when DELAY pin is tied to R
SET
pin) from
SGATE turn-off (see Figure 2). If a longer delay is needed
to compensate for the propagation delay of external gate
driver, a resistor divider can be used from R
SET
to ground to
program V
DELAY
for the longer delay needed. For example,
for a switching frequency of 250kHz and delay of 150ns,
Figure 1. Synchronizing with External Clock
Figure 2. Delay Timing
5V TO 20V
VN2222
PULSE WIDTH > 200ns
5k
SYNC
3782 F01
LT3782
3782 F02
DELAY
BGATE1
SGATE1
SET
APPLICATIONS INFORMATION