LTC3722-2_15 Datasheet, PDF (21/28 Pages) Linear Technology – Synchronous Dual Mode Phase Modulated Full Bridge Controllers

LTC3722-1/LTC3722-2
OPERATION
Phase Modulator (LTC3722-1)
The LTC3722-1 phase modulation control circuitry is
comprised of the phase modulation comparator and
logic, the error amplifier, and the soft-start amplifier (see
Figure 12). Together, these elements develop the required
phase overlap (duty cycle) required to keep the output
voltage in regulation. In isolated applications, the sensed
output voltage error signal is fed back to COMP across the
input to output isolation boundary by an optical coupler
and shunt reference/error amplifier (LT^®1431) combina-
tion. The FB pin is connected to GND, forcing COMP high.
The collector of the optoisolator is connected to COMP
directly. The voltage COMP is internally attenuated by the
LTC3722-1. The attenuated COMP voltage provides one
input to the phase modulation comparator. This is the
current command. The other input to the phase modula-
tion comparator is the RAMP voltage, level shifted by
approximately 650mV. This is the current loop feedback.
During every switching cycle, alternate diagonal switches
(MA-MD or MB-MC) conduct and cause current in an output
inductor to increase. This current is seen on the primary
of the power transformer divided by the turns ratio. Since
the current sense resistor is connected between GND and
the two bottom bridge transistors, a voltage proportional
to the output inductor current will be seen across R_SENSE.
The high side of R_SENSE is also connected to CS, usually
through a small resistor (R_SLOPE). When the voltage on
CS exceeds either (COMP/4.3) –650mV, or 300mV, the
overlap conduction period will terminate. During normal
operation, the attenuated COMP voltage will determine
the CS trip point. During start-up, or slewing conditions
following a large load step, the 300mV CS threshold will
terminate the cycle, as COMP will be driven high, such
that the attenuated version exceeds the 300mV threshold.
In extreme conditions, the 650mV threshold on CS will be
exceeded, invoking a soft-start/restart cycle.
Selecting the Power Stage Components
Perhaps the most critical part of the overall design of the
converter is selecting the power MOSFETs, transformer,
inductors and filter capacitors. Tremendous gains in ef-
ficiency, transient performance and overall operation can
be obtained as long as a few simple guidelines are followed
with the phase-shifted full bridge topology.
FB
COMP
SS
R_LEB
CS
ERROR
AMPLIFIER
–
1.2V
+
50k
PHASE
MODULATION
COMPARATOR
–
V_REF
12µA _SOFT-START
AMPLIFIER
+
+
+
650mV
–
–
IDEAL
14.9k
TOGGLE
F/F
Q
CLK
Q
PHASE
MODULATION
LOGIC
SQ
CLK R
FROM
CURRENT
LIMIT
COMPARATOR
BLANKING
QS
R CLK
Figure 12. Phase Modulation Circuitry (LTC3722-1)
A
B
C
D
372212 F12
For more information www.linear.com/LTC3722
372212fb
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