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UDG-95037
DESCRIPTION
Pin numbers refer to 16-pin DIL and SOIC packages only.
The UC3548 family of PWM control ICs uses voltage feed-
forward control to regulate the output voltage of isolated
power supplies. The UC3548 resides on the primary side
and has the necessary features to accurately control sec-
ondary side short circuit current with average current
mode control techniques. The UC3548 can be used to
control a wide variety of converter topologies.
In addition to the basic functions required for pulse width
modulation, the UC3548 implements a patented technique
of sensing secondary current from the primary side in an
isolated buck derived converter. A current waveform syn-
thesizer monitors switch current and simulates the inductor
current downslope so that the complete current waveform
can be constructed on the primary side without actual sec-
ondary side measurement. This information on the primary
side is used by an average current mode control circuit to
accurately limit maximum output current.
The UC3548 circuitry includes a precision reference, a
wide bandwidth error amplifier for average current control,
an oscillator to generate the system clock, latching PWM
comparator and logic circuits, and a high current output
4/97
driver. The current error amplifier easily interfaces with an
optoisolator from a secondary side voltage sensing circuit.
A full featured undervoltage lockout (UVLO) circuit is con-
tained in the UC3548. UVLO monitors the supply voltage
to the controller (VCC), the reference voltage (VREF), and
the input line voltage (VIN). All three must be good before
soft start commences. If either VCC or VIN is low, the sup-
ply current required by the chip is only 500碌A(chǔ) and the
output is actively held low.
Two on board protection features set controlled limits to
prevent transformer core saturation. Input voltage is moni-
tored and pulse width is constrained to limit the maximum
volt-second product applied to the transformer. A unique
patented technique limits maximum duty cycle within 3%
of a user programmed value.
These two features allow for more optimal use of trans-
formers and switches, resulting in reduced system size
and cost.
Both patents embodied in the UC3548 belong to Lambda
Electronics Incorporated and are licensed for use in appli-
cations employing these devices.
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