LT3782
8
3782fg
Operation
The LT3782 is a two phase constant frequency current mode
boost controller. Switching frequency can be programmed
up to 500kHz. During normal switching cycles, the two
channels are controlled by internal fl ip-fl ops and are 180
degrees out-of-phase.
Referring to the Block Diagram, the LT3782’s basic func-
tions include a transconductance amplifer (g
m
) to regulate
the output voltage and to control the current mode PWM
current loop. It also includes the necessary logic and fl ip-
fl op to control the PWM switching cycles, two high speed
gate drivers to drive high power N-Channel MOSFETs, and
2-phase control signals to drive external gate drivers for
optional synchronous operation.
In normal operation, each switching cycle starts with a
switch turn-on. The inductor current of each channel is
sampled through the current sense resistor and amplifi ed
then compared to the error amplifi er output V
C
to turn
the switch off. The phase delay of the second channel is
controlled by the divide-by-two D fl ip-fl op and is exactly
180 degrees out-of-phase of the fi rst channel. With a re-
sistor divider connected to the FB pin, the output voltage
is programmed to the desired value. The 10V gate drivers
are suffi cient to drive most high power N-channel MOSFET
in many industrial applications.
Additional important features include shutdown, cur-
rent limit, soft-start, synchronization and programmable
maximum duty cycle. Additional slope compensation can
be added also.
Output Voltage Programming
With a 2.44V feedback reference voltage V
REF
, the output
V
OUT
is programmed by a resistor divider as shown in
the Block Diagram.
V
OUT
= 2.44 1+
R
F1
R
F2
Soft-Start and Shutdown
During soft-start, the voltage on the SS pin (V
SS
) controls
the output voltage. The output voltage thus ramps up fol-
lowing V
SS
. The effective range of V
SS
is from 0V to 2.44V.
The typical time for the output to reach the programmed
level is
C is the capacitor connected from the SS pin to Gnd.
Undervoltage Lockout and Shutdown
Only when V
RUN
is higher than 2.45V V
GBIAS
will be ac-
tive and the switching enabled. The LT3782 goes into low
current shutdown when V
RUN
is below 0.3V. A resistor
divider can be used on RUN pin to set the desired V
CC
undervoltage lockout voltage. 80mV of hysteresis is built
in on RUN pin thresholds.
Oscillation Frequency Setting and Synchronization
The switching frequency of LT3782 can be set up to 500kHz
by a resistor R
FREQ
from pin R
SET
to ground.
For f
SET
= 250kHz, R
FREQ
= 80k
Once the switching frequency f
SET
is chosen, R
FREQ
can be
found from the Switching Frequency vs R
FREQ
graph found
under the Typical Performance Characteristics section.
Note that because of the 2-phase operation, the internal
oscillator is running at twice the switching frequency. To
synchronize the LT3782 to the system frequency f
SYSTEM
,
the synchronizing frequency f
SYNC
should be two times
f
SYSTEM
, and the LT3782 switching frequency f
SET
should
be set below 80% of f
SYSTEM
.
f
SYNC
= 2f
SYSTEM
and f
SET
< (f
SYSTEM
• 0.8)
For example, to synchronize the LT3782 to 200kHz system
frequency f
SYSTEM
, f
SYNC
needs to be set at 400kHz and f
SET
needs to be set at 160kHz. From the Switching Frequency
vs R
FREQ
graph found under the Typical Performance
Characteristics section, R
FREQ
= 130k.
APPLICATIONS INFORMATION
