LTC3722-2 Linear Technology, LTC3722-2 Datasheet - Page 20

• Manufacturer Part Number: LTC3722-2
• Description: Synchronous Dual Mode Phase Modulated Full Bridge Controllers
• Manufacturer: Linear Technology

LTC3722-1/LTC3722-2
OPERATIO
4. Using two CS transformers summed together.
5. Choose a CS transformer optimized for high frequency
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 Fig-
ure 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
tion. The FB pin is connected to GND, forcing COMP high.
The collector of the optoisolator is connected to COMP
20
applications.
RAMP
CS
Figure 11. Current Transformer Sense Circuitry
FILTERING
OPTIONAL
R
SLOPE
U
COMP
R
LEB
SS
CS
FB
12 A
1.2V
V
REF
SOURCE
R
S
–
+
+
–
MB
ERROR
AMPLIFIER
SOFT-START
AMPLIFIER
BLANKING
Figure 12. Phase Modulation Circuitry (LTC3722-1)
N:1
®
1431) combina-
CURRENT
TRANSFORMER
50k
14.9k
MD
SOURCE
1922 F10
–
+
COMPARATOR
MODULATION
PHASE
650mV
Q
+
–
S
R
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 be-
tween GND and the two bottom bridge transistors, a
voltage proportional to the output inductor current will be
seen across R
connected to CS, usually through a small resistor (R
When the voltage on CS exceeds either (COMP/5.2)
– 650mV, or 270mV, 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 270mV
CS threshold will terminate the cycle, as COMP will be
driven high, such that the attenuated version exceeds the
270mV threshold. In extreme conditions, the 650mV
threshold on CS will be exceeded, invoking a soft-start/
restart cycle.
CLK
FROM
CURRENT
LIMIT
COMPARATOR
CLK
CLK
SENSE
MODULATION
TOGGLE
PHASE
LOGIC
S Q
R
F/F
Q
Q
. The high side of R
3722 F11
www.DataSheet4U.com
A
B
C
D
SENSE
is also
SLOPE
372212i
).