LX1570/1571

PRODUCT DA T ABOOK 1996/1997

Copyright © 1997

Rev. 0.9.3

1/97

10

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.

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 "3V

keep output drive off without requiring the error amplifier output

to swing to ground level. The transfer function between error

amp output (V

by:

V

where:

I

P

= inductor peak current,

G = resistor divider gain,

(-15 for LX1570, 3 for LX1571)

V

(0.65V typ)

IC DESCRIPTION

FIGURE 7 — MINIMUM CURRENT COMPARATOR EFFECT

DURING LOAD TRANSIENT

Transformer

Voltage

100V / Div.

LX157x

OUT DRV

20V / Div.

LX157x

COMP PIN

2V / Div.

Output Current &

Inductor Current

2A / Div.