LT3782 Linear Technology, LT3782 Datasheet - Page 7

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

APPLICATIO S I FOR ATIO
Operation
The LT3782 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 flip-flop and are
180 degrees out of phase.
Referring to the Block Diagram, the LT3782 basic func-
tions include a transconductance amplifer (gm) to regu-
late the output voltage and to control the current mode
PWM current loop. It also includes the necessary logic and
flip-flop 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 amplified
then compared to the error amplifier output V
switch off. The phase delay of the second channel is
controlled by the divide-by-two D flip-flop and is exactly
180 degrees out of phase of the first channel. With a
resistor divider connected to the FB pin, the output voltage
is programmed to the desired value. The 10V gate drivers
are sufficient to drive most high power N-Channel MOSFET
in many industrial applications.
Additional important features include shutdown, current
limit, soft-start, synchronization and programmable maxi-
mum duty cycle. Additional slope compensation can be
added also.
Output Voltage Programming
With a 2.44V feedback reference voltage V
V
Block Diagram.
OUT
V
OUT
is programmed by a resistor divider as shown in the
=
2 44 1
.
⎛
⎝ ⎜
+
U
R
R
F
F
1
2
⎞
⎠ ⎟
U
W
REF
C
, the output
to turn the
U
Soft-Start and Shutdown
During soft-start, the voltage on the SS pin (V
the output voltage. The output voltage thus ramps up
following V
2.44V. The typical time for the output to reach the pro-
grammed 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 LT3782 goes into low
current shutdown when V
divider can be used on RUN pin to set the desired V
undervoltage lockout voltage. A 120mV 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
Once the switching frequency f
found from the Switching Frequency vs R
found under the Typical Electrical 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
the synchronizing frequency f
f
be set below 80% of f
For example, to synchronize the LT3782 to 200kHz system
frequency f
f
Frequency vs R
cal Characteristics section, R
SYSTEM
SET
For f
f
t
SYNC
=
needs to be set at 160kHz. From the Switching
C
SET
, and the LT3782 switching frequency f
10
= 2f
• . 2 44
= 250kHz, R
µ
SYSTEM
SS
RUN
SYSTEM
A
. The effective range of V
V
FREQ
is higher than 2.45V V
, f
graph found under the Typical Electri-
and f
SYNC
SYSTEM
FREQ
FREQ
SET
needs to be set at 400kHz and
RUN
from pin R
= 80k
.
< (f
FREQ
SET
SYNC
is below 0.3V. A resistor
SYSTEM
is chosen, R
= 130k.
should be two times
GBIAS
SET
SS
• 0.8)
to ground.
LT3782
is from 0V to
will be active
SS
FREQ
FREQ
SET
) controls
SYSTEM
should
can be
graph
3782fa
7
CC
,