LX1571 Microsemi Corporation, LX1571 Datasheet - Page 7

• Manufacturer Part Number: LX1571
• Description: PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
• Manufacturer: Microsemi Corporation

Copyright © 1997
Rev. 0.9.3
STEADY-STATE OPERATION
Steady-state operation is best described by referring to the main
block diagram and the typical application circuit shown in Fig-
ure 2. The output drive turns the external power MOSFET on
and current ramps up in the inductor. Inductor current is sensed
with an external resistor (or in the case of LX1571 with a current
transformer) and is compared to the threshold at the inverting
input of the current sense (C.S.) comparator. This threshold is
set by the voltage feedback loop, which is controlled by the
error amplifier. Exceeding this threshold resets the PWM latch
and turns the MOSFET off. The Output drive goes low, turning
the C
ing the C
1.5V, it sets the PWM latch and turns the output drive back on
prior to the next rising edge of the transformer voltage, and the
cycle repeats.
(Figure 4A) shows typical waveforms in the steady-state condi-
COMP
The Steady-State Operation Timing Diagram - Normal Mode
C.S.
S.S.
V
1/97
C
FB
T
T
charging current off and the discharging current on, caus-
1
2
3
4
8
T
voltage to ramp down. When this voltage goes below
2.5V
Error Amp
P
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
HASE
M
P
FIGURE 3 — STEADY-STATE OPERATION BLOCK DIAGRAM
ODULATED
R E L I M I N A R Y
Timing / Duty Cycle
1V
Control
AC S
I C D E S C R I P T I O N
YNCHRONOUS
R
C.S. Comp
D
tion. Notice that when the current sense signal turns the MOSFET
off, it also synchronizes the output drive to the transformer volt-
age (see discussion under heading Timing Section). In addition,
the energy transfer occurs only when both transformer voltage
and OUT DRV pin are "HI" at the same time, establishing the
effective on-time of the converter. This shows that the regula-
tion of this converter is achieved by modulating the trailing edge
of the output drive with respect to the leading edge of the AC
voltage, while maintaining a fixed output drive duty cycle. In
other words, the converter duty cycle seen by L1 is controlled by
varying the phase between the AC voltage and the output driver
signal (phase modulation). Maximum converter duty cycle is
achieved when both signals are in phase, as shown in Figure 4B.
The LX1570/71 output drive always maintains a fixed duty cycle
( 54%), since both charge and discharge currents are almost equal
as shown in Figures 4A and 4B.
S
CHG
CONTROL
DISCH
CONTROL
LATCH
SET CONTROL
A T A
ECONDARY
S
-S
H E E T
IDE
C
ONTROLLER
PWM Latch
R
S
Q
2.5V
5V
LX1570/1571
Internal
2.5V
Bias
REF
6
7
5
OUT DRV
V
GND
CC
7