LT3782 Linear Technology, LT3782 Datasheet - Page 8

• Manufacturer Part Number: LT3782
• Description: 2-Phase Step-Up DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT3782
With a 200ns one-shot timer on chip, the LT3782 provides
flexibility 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
Diagram), the current limit is set for 60mV/R
be placed very close to the power switch with very short
traces. A low pass R
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 Rectifier 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 rectifier
operation. Note that SGATE drives the top switch and
BGATE drives the bottom switch. To avoid cross conduc-
tion between top and bottom switches, the BGATE turn-on
is delayed 100ns (when DELAY pin is tied to R
SGATE turn-off (see Figure 2). If a longer delay is needed
8
Figure 1. Synchronizing with External Clock
PULSE WIDTH > 200ns
SGATE1
Figure 2. Delay Timing
U
BGATE1
SET
C
filter is needed across R
5V TO 20V
DELAY
U
VN2222
5k
SYNC
W
3782 F02
LT3782
3782 F01
S
SET
S
S
. R
(see Block
to filter out
U
pin) from
S
should
to compensate for the propagation delay of external gate
driver, a resistor divider can be used from R
to program V
example, for a switching frequency of 250kHz and delay of
150ns, then R
should be 1V, with V
R
Duty Cycle Limit
When DCL pin is shorted to R
frequency is less than 250kHz (R
mum duty cycle of LT3782 will be at least 90%. The
maximum duty cycle can be clamped to 50% by grounding
the DCL pin or to 75% by forcing the V
with a resistor divider from R
DCL pin input current is 0.2µA.
Slope Compensation
The LT3782 is designed for high voltage and/or high
current applications, and very often these applications
generate noise spikes that can be picked up by the current
sensing amplifier and cause switching jitter. To avoid
switching jitter, careful layout is absolutely necessary to
minimize the current sensing noise pickup. Sometimes
increasing slope compensation to overcome the noise can
help to reduce jitter. The built-in slope compensation can
be increased by adding a resistor R
to ground. Note that smaller R
compensation and the minimum R
R
FREQ2
FREQ
/2.
= 32.5k (see Figure 3).
FREQ1
DELAY
Figure 3. Increase Delay Time
R
R
FREQ1
FREQ2
47.5k
32.5k
+ R
RSET
for the longer delay needed. For
FREQ2
= 2.3V then R
SET
DELAY
R
should be 80k and V
SET
pin to ground. The typical
LT3782
SET
SLOPE
FREQ
SLOPE
3782 F03
pin and switching
DCL
SLOPE
FREQ1
> 80k), the maxi-
increases slope
from SLOPE pin
voltage to 1.2V
SET
= 47.5k and
allowed is
to ground
DELAY
3782fa