May 1997
ML4818*
Phase Modulation/Soft Switching Controller
GENERAL DESCRIPTION
The ML4818 is a complete phase modulation control IC
suitable for full bridge soft switching converters. Unlike
conventional PWM circuits, the phase modulation
technique allows for zero-voltage switching transitions
and square wave drive across the transformer. The IC
modulates the phases of the two sides of the bridge to
control output power.
The ML4818 can be operated in current mode. The delay
times for the outputs are externally programmable to
allow the zero-voltage switching transitions to take place.
Pulse-by-pulse current limit, integrating fault detection,
and soft start reset are provided. The under-voltage
lockout circuit features a 6V hysteresis with a low starting
current to allow off-line start up with a low power bleed
resistor. A shutdown function powers down the IC, putting
it into a low quiescent state.
FEATURES
s
Full bridge phase modulation zero voltage switching
circuit with programmable ZV transition times
Constant frequency operation to 500kHz
Current mode operation
Cycle-by-cycle current limiting with integrating fault
detection and restart delay
Precision buffered 5V reference (+1%)
Four 1.5A peak current totem-pole output drivers
Under-voltage lockout circuit with 6V hysteresis
s
s
s
s
s
s
s
Power DIP package allows higher dissipation
* Some Packages Are Obsolete)
BLOCK DIAGRAM
10
SHUTDOWN
REFERENCE
AND
UNDER-VOLTAGE
LOCKOUT
INHIBIT
OUTPUTS
R
Q
3
5
RAMP
E/A OUT
0.7V
S
ΦMOD
V
CC
V
REF
24
13
11
2
CLOCK
R
T
C
T
OSC
20
+
+
Q
DELAY
A2 OUT
16
–
T FLIP
FLOP
V+
I
1
T
Q
DELAY
V
CC
A1 OUT
8
INV
+5V
ERROR
AMP
–
17
+
9
SOFT START
3V
V
CC
B1 OUT
–
+
R
DELAY
22
V
CC
R
12
RC
RESET
I
LIM
1V
Q
I
2
S
S
Q
B2 OUT
DELAY
21
R
DELAY
GND
*PINS 1, 6, 7, 15, 18, 19 AND 23 ARE GND
4
+
–
14
*
1