lx1682 Microsemi Corporation, lx1682 Datasheet - Page 4

• Manufacturer Part Number: lx1682
• Description: Voltage - Mode Pwm Controllers
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.1b,2005-03-09
GENERAL DESCRIPTION
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
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
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
when SS pin voltage reaches 95% of V
The LX1681/82 are voltage-mode pulse-width modulation
At power up, the LX1681/82 monitors the supply voltage to
Once the supplies are above the UVLO threshold, the soft-start
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REF
.
T H E O R Y O F O P E R A T I O N
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OVER-CURRENT PROTECTION (OCP) and HICCUP
MOSFET, together with a resistor (R
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 comparator draws a current (I
The set resistor is selected to set the current limit for the
application. When the sensed voltage across the R
set resistor exceeds the 400mV V TRIP threshold, the OCP
comparator outputs a signal to reset the PWM latch and to start
hiccup mode. The soft-start capacitor (C
(10 times slower than when being charged up by R
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
implemented using a sense resistor, instead of using the R
the upper MOSFET, for greater set-point accuracy. See
Application Information section.
OSCILLATOR FREQUENCY
The LX1681/1682 family uses the R
An internal oscillator sets the switching frequency at 200 kHz.
Voltage-Mode PWM Controllers
P
RODUCTION
D
ATA
SET
), whose magnitude is 45µA.
SET
S
) to set the actual current
HEET
SS
) is discharged slowly
DS(ON)
LX1681/1682
DS(ON)
of the upper
SS
). When the
DS(ON)
plus the
of
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