LTC3722-2_15 Datasheet, PDF (25/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
secondary. The drive to the inductors is 180 degrees out-
of-phase which provides partial ripple current cancellation
in the output capacitor(s). Reduced capacitor ripple current
lowers output voltage ripple and enhances the capacitors’s
reliability. The amount of ripple cancellation is related to
duty cycle (see Figure 15). Although the current doubler
requires an additional inductor, the inductor core volume
is proportional to LI², thus the size penalty is small. The
transformer construction is simplified without a center-tap
winding and the turns ratio is reduced by one-half compared
to a conventional full wave rectifier configuration.
Synchronous rectification of the current doubler second-
ary requires two ground referenced N-channel MOSFETs.
The timing of the LTC3722-1/LTC3722-2 drive signals is
shown in the Timing Diagram.
Full Bridge Gate Drive
The full bridge converter requires high current MOSFET
gate driver circuitry for two ground referenced switches
and two high side referred switches. Providing drive to the
ground referenced switches is not too difficult as long as
the traces from the gate driver chip or buffer to the gate
and source leads are short and direct. Drive requirements
are further eased since all of the switches turn on with zero
VDS, eliminating the Miller effect. Low turn-off resistance
is critical, however, in order to prevent excessive turn-off
losses resulting from the same Miller effects that were not
an issue for turn-on. The LTC3722-1/LTC3722-2 does not
require the propagation delays of the high and low side
drive circuits to be precisely matched as the DirectSense
ZVS circuitry will adapt accordingly. As a result, LTC3722‑1/
LTC3722-2 can drive a simple NPN-PNP buffer or a gate
driver chip like the LTC1693-1 to provide the low side gate
drive. Providing drive to the high side presents additional
challenges since the MOSFET gate must be driven above
the input supply. A simple circuit (Figure 17) using a single
LTC1693-1, an inexpensive signal transformer and a few
discrete components provides both high side gate drives
(A and C) reliably.
The LTC4440 high side driver can also be applied. The
LTC4440 eliminates the signal transformer and is preferred
for applications where V_IN is less than 80V (max).
1
NORMALIZED
OUTPUT RIPPLE
CURRENT
ATTENUATION
NOTE: INDUCTOR(S) DUTY CYCLE
IS LIMITED TO 50% WITH CURRENT
DOUBLER PHASE SHIFT CONTROL.
0
0
0.25
0.5
DUTY CYCLE
3722212 F15
Figure 15. Ripple Current Cancellation vs Duty Cycle
LTC3722
OUTE
OUTF
2:1:1
LTC1693-1
IN1 OUT1
GND1
GND2
IN2 OUT2
Figure 16. Isolated Drive Circuitry
372212 F16
LTC3722
OUTA
OR
OUTC
REGULATED
V_IN
BIAS
0.1µF
0.1µF
2k
V_CC
IN OUT
BAT
1/2
54
LTC1693-1
2µF
CER
POWER
MOSFET
GND
SIGNAL
TRANSFORMER
Figure 17. High Side Gate Driver Circuitry
BRIDGE
LEG
372212 F17
For more information www.linear.com/LTC3722
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