PRODUCT DATABOOK 1996/1997
LX1681/1682
V
O LTA G E
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O D E
P W M C
O N T R O L L E R S
P
R O D U C T I O N
D
A T A
S
H E E T
T H E O R Y O F O P E R AT I O N
GENERAL DESCRIPTION
The LX1681/82 are voltage-mode pulse-width modulation con-
troller 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
20kΩ 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
REF
.
OVER-CURRENT PROTECTION (OCP) and HICCUP
The LX1681/1682 family uses the R
DS(ON)
of the upper MOSFET,
together with a resistor (R
SET
) to set the actual current 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 compara-
tor draws a current (I
SET
), whose magnitude is 45µA. The set
resistor is selected to set the current limit for the application.
When the sensed voltage across the R
DS(ON)
plus the 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
SS
) is discharged slowly (10 times
slower than when being charged up by R
SS
). When the 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
DS(ON)
of the upper MOSFET,
for greater set-point accuracy. See Application Information
section.
OSCILLATOR FREQUENCY
An internal oscillator sets the switching frequency at 200 kHz.
Copyright © 1999
Rev. 1.0 5/99
5
MICROSEMI [ MICROSEMI CORPORATION ]
