lt3782aeufd Linear Technology Corporation, lt3782aeufd Datasheet - Page 8

• Manufacturer Part Number: lt3782aeufd
• Description: 2-phase Step-up Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LT3782A
APPLICATIONS INFORMATION
8
Operation
The LT3782A is a two phase constant frequency current
mode boost controller. Switching frequency can be pro-
grammed 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 LT3782A’s basic func-
tions include a transconductance amplifi er (g
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
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
V
the Block Diagram.
OUT
V
OUT
is programmed by a resistor divider as shown in
= 2.44 1+
R
R
F2
F1
REF
m
) to regulate
, the output
C
to turn
Soft-Start and Shutdown
During soft-start, the voltage on the SS pin (V
the output voltage. The output voltage thus ramps up fol-
lowing V
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
and the switching enabled. The LT3782A goes into low
current shutdown when V
divider can be used on RUN pin to set the desired V
undervoltage lockout voltage. 80mV of hysteresis is built
in on RUN pin thresholds.
Oscillation Frequency Setting and Synchronization
The switching frequency of LT3782A can be set up to
500kHz by a resistor R
Once the switching frequency f
found from the Switching Frequency vs R
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 LT3782A to the system frequency f
the synchronizing frequency f
f
be set below 80% of f
For example, to synchronize the LT3782A to 200kHz sys-
tem frequency f
and f
Frequency vs R
formance Characteristics section, R
SYSTEM
For f
f
t =
SYNC
SET
C • 2.44V
SET
, and the LT3782A switching frequency f
10μA
SS
= 2f
needs to be set at 160kHz. From the Switching
. The effective range of V
= 250kHz, R
RUN
SYSTEM
FREQ
SYSTEM
is higher than 2.45V V
and f
graph found under the Typical Per-
SYSTEM
FREQ
, f
FREQ
SYNC
SET
RUN
from pin R
= 80k
.
< (f
SET
SYNC
needs to be set at 400kHz
is below 0.3V. A resistor
SYSTEM
is chosen, R
SS
should be two times
FREQ
is from 0V to 2.44V.
GBIAS
SET
FREQ
• 0.8)
= 130k.
to ground.
will be active
graph found
SS
FREQ
SET
) controls
SYSTEM
should
can be
3782af
CC
,