Microsemi

Linfinity Microelectronics Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 4

Copyright

© 2000

Rev. 1.1b,2005-03-09

LX1681/1682

Voltage-Mode PWM Controllers

TM

®

TH EORY OF O PERAT ION

GENERAL DESCRIPTION

The LX1681/82 are voltage-mode pulse-width modulation

controller integrated circuits. The internal oscillator and ramp

generator frequency is fixed at 200kHz. The devices have internal

compensation, so that no external compensation is required.

POWER UP and INITIALIZATION

At power up, the LX1681/82 monitors the supply voltage to

both the +5V and the +12V pins (there is no special requirement

for the sequence of the two supplies). Before both supplies reach

their under-voltage lock-out (UVLO) thresholds, the soft-start (SS)

pin is held low to prevent soft-start from beginning; the oscillator

control is disabled and the top MOSFET is kept OFF.

SOFT-START

Once the supplies are above the UVLO threshold, the soft-start

capacitor begins to be charged up by the reference through a 20k

internal resistor. The capacitor voltage at the SS pin rises as a

simple RC circuit. The SS pin is connected to the amplifier's non-

inverting input that controls the output voltage. The output voltage

will follow the SS pin voltage if sufficient charging current is

provided to the output capacitor. The simple RC soft-start allows

the output to rise faster at the beginning and slower at the end of

the soft-start interval. Thus, the required charging current into the

output capacitor is less at the end of the soft-start interval so

decreasing the possibility of an over-current. A comparator

monitors the SS pin voltage and indicates the end of soft-start

OVER-CURRENT PROTECTION (OCP) and HICCUP

limit point. The comparator senses the current 1µs after the top

MOSFET is switched on. Experiments have shown that the

MOSFET drain voltage will ring for 200-500ns after the gate is

turned on. In order to reduce inaccuracies due to ringing, a 1µs

delay after gate turn-on is built into the current sense comparator.

The set resistor is selected to set the current limit for the

set resistor exceeds the 400mV V TRIP threshold, the OCP

comparator outputs a signal to reset the PWM latch and to start

voltage on the SS/ENABLE pin reaches a 0.3V threshold, hiccup

finishes and the circuit soft-starts again. During hiccup, the top

MOSFET is OFF and the bottom MOSFET remains ON. Hiccup is

disabled during the soft-start interval, allowing the circuit to start

up with the maximum current. If the rise speed of the output

voltage is too fast, the required charging current to the output

capacitor may be higher than the limit-current. In this case, the

peak MOSFET current is regulated to the limit-current by the

current-sense comparator. If the MOSFET current still reaches its

limit after the soft-start finishes, the hiccup is triggered again. The

hiccup ensures the average heat generation on both MOSFET’s and

the average current to be much less than that in normal operation,

if the output has a short circuit. Over-current protection can also be

the upper MOSFET, for greater set-point accuracy. See

Application Information section.

OSCILLATOR FREQUENCY

An internal oscillator sets the switching frequency at 200 kHz.