LTC3722-1/LTC3722-2
25
372212fb
For more information www.linear.com/LTC3722
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 LI
2
, 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).
Figure 15. Ripple Current Cancellation vs Duty Cycle
1
0
0 0.25 0.5
DUTY CYCLE
NORMALIZED
OUTPUT RIPPLE
CURRENT
ATTENUATION
3722212 F15
NOTE: INDUCTOR(S) DUTY CYCLE
IS LIMITED TO 50% WITH CURRENT
DOUBLER PHASE SHIFT CONTROL.
Figure 17. High Side Gate Driver Circuitry
Figure 16. Isolated Drive Circuitry
IN1
OUT1
IN2
OUT2
2:1:1
OUTE
OUTF
LTC3722
GND1
GND2
LTC1693-1
372212 F16
IN
OUT
OUTA
OR
OUTC
GND
1/2
LTC1693-1
372212 F17
V
CC
0.1µF
2µF
CER
0.1µF
SIGNAL
TRANSFORMER
2k
BAT
54
BRIDGE
LEG
REGULATED
BIAS
POWER
MOSFET
V
IN
LTC3722
OPERATION
