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Precision 5V Reference (1%)
BLOCK DIAGRAM
UDG-93003-1
DESCRIPTION
The UC3848 family of PWM control ICs makes primary
side average current mode control practical for isolated
switching converters. Average current mode control in-
sures that both cycle by cycle peak switch current and
maximum average inductor current are well defined and
will not run away in a short circuit situation. The UC3848
can be used to control a wide variety of converter topolo-
gies.
In addition to the basic functions required for pulse width
modulation, the UC3848 implements a patented tech-
nique of sensing secondary current in an isolated buck
derived converter from the primary side. A current wave-
form synthesizer monitors switch current and simulates
the inductor current down slope so that the complete cur-
rent waveform can be constructed on the primary side
without actual secondary side measurement. This infor-
mation on the primary side allows for full DC control of
output current.
The UC3848 circuitry includes a precision reference, a
wide bandwidth error amplifier for average current con-
trol, an oscillator to generate the system clock, latching
PWM comparator and logic circuits, and a high current
4/96
output driver. The current error amplifier easily interfaces
with an optoisolator from a secondary side voltage sens-
ing circuit.
A full featured undervoltage lockout (UVLO) circuit is con-
tained in the UC3848. 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 supply current required by the chip is only 500碌A and
the output is actively held low.
Two on board protection features set controlled limits to
prevent transformer core saturation. Input voltage is mon-
itored and pulse width is constrained to limit the maxi-
mum 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.
Patents embodied in the UC3848 belong to Lambda
Electronics Incorporated and are licensed for use in ap-
plications employing these devices.
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