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LX157XCDMT View Datasheet(PDF) - Microsemi Corporation

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LX157XCDMT Datasheet PDF : 12 Pages
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LX1570/1571
PRODUCT DATABOOK 1996/1997
PHASE MODULATED AC SYNCHRONOUS SECONDARY-SIDE CONTROLLER
PR E L I M I N A RY D ATA S H E E T
IC DESCRIPTION
MINIMUM CURRENT COMPARATOR
One of the main advantages of replacing a Magnetic Amplifier
with a MOSFET, is the MOSFET's ability to respond quickly to
large changes in load requirements. Because the LX1570/71 re-
lies on the C.S. signal for synchronization, special circuitry had to
be added to keep the output drive synchronized to the trans-
former voltage during such load transient conditions. This con-
dition is best explained by referring to Figure 7. In Figure 7, it
can be seen that the load current is stepped from 0.4A to 4A,
causing the COMP pin to slew faster than the inductor current,
starting with the second switching cycle after the load transient
has occured. This condition eliminates the normal means of
resetting the PWM latch through the C.S. comparator path. To
compensate for this condition, a second comparator is ORed
with the C.S. comparator, which resets the latch on the falling
edge of the C.S. signal caused by the falling edge of the trans-
former voltage.
In other words, the function of the minimum C.S. comparator
is to turn OUT DRV off on the falling edge of the C.S. signal, if it
is not already off. This assures that the output drive is on before
the start of the next AC input cycle (Look-Ahead Switching™),
allowing maximum converter duty cycle.
Transformer
Voltage
100V / Div.
LX157x
OUT DRV
20V / Div.
LX157x
COMP PIN
2V / Div.
Output Current &
Inductor Current
2A / Div.
FIGURE 7 — MINIMUM CURRENT COMPARATOR EFFECT
DURING LOAD TRANSIENT
ERROR AMPLIFIER
The function of the error amplifier is to set a threshold voltage
for inductor peak current and to control the converter duty cycle,
such that power supply output voltage is closely regulated.
Regulation is done by sensing the output voltage and comparing
it to the internal 2.5V reference. A compensation network based
on the application is placed from the output of the amplifier to
GND for closed loop stability purposes as well as providing high
DC gain for tight regulation. The function of "3VBE" offset is to
keep output drive off without requiring the error amplifier output
to swing to ground level. The transfer function between error
amp output (VCOMP) and peak inductor current is therefore given
by:
V
COMP
-
3V
BE
=
I
P
*
G
where:
I = inductor peak current,
P
G = resistor divider gain,
(-15 for LX1570, 3 for LX1571)
V = diode forward voltage
BE
(0.65V typ)
10
Copyright © 1997
Rev. 0.9.3 1/97
 

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