Datasheet

APPLICATIONS INFORMATION
Voltage Reference
The reference regulator of the UC1526A is based on a
precision band-gap reference, internally trimmed to ±1%
accuracy. The circuitry is fully active at supply voltages
above +7V, and provides up to 20mA of load current to
external circuitry at +5.0V. In systems where additional
current is required, an external PNP transistor can be
used to boost the available current. A rugged low fre-
quency audio-type transistor should be used, and lead
lengths between the PWM and transistor should be as
short as possible to minimize the risk of oscillations.
Even so, some types of transistors may require collec-
tor-base capacitance for stability. Up to 1 amp of load
current can be obtained with excellent regulation if the
device selected maintains high current gain.
Soft-Start Circuit
The soft-start circuit protects the power transistors and
rectifier diodes from high current surges during power
supply turn-on. When supply voltage is first applied to
the UC1526A, the under-voltage lockout circuit holds
RESET LOW with Q3. Q1 is turned on, which holds the
soft-start capacitor voltage at zero. The second collector
of Q
1 clamps the output of the error amplifier to ground,
guaranteeing zero duty cycle at the driver outputs.
When the supply voltage reaches normal operating
range,
RESET will go HIGH. Q1 turns off, allowing the
internal 100µA current source to charge C
S. Q2 clamps
the error amplifier output to 1V
BE above the voltage on
C
S. As the soft-start voltage ramps up to +5V, the duty
cycle of the PWM linearly increases to whatever value
the voltage regulation loop requires for an error null.
Under-Voltage Lockout
The under-voltage lockout circuit protects the UC1526A
and the power devices it controls from inadequate sup-
ply voltage, If +V
IN is too low, the circuit disables the
output drivers and holds the
RESET pin LOW. This pre-
vents spurious output pulses while the control circuitry is
stabilizing, and holds the soft-start timing capacitor in a
discharged state.
The circuit consists of a +1.2V bandgap reference and
comparator circuit which is active when the reference
voltage has risen to 3V
BE or +1.8V at 25°C. When the
reference voltage rises to approximately +4.4V, the cir-
cuit enables the output drivers and releases the
RESET
pin, allowing a normal soft-start. The comparator has
350mV of hysteresis to minimize oscillation at the trip
point. When +V
IN to the PWM is removed and the refer-
ence drops to +4.2V, the under-voltage circuit pulls
RE-
SET LOW again. The soft-start capacitor is immediately
discharged, and the PWM is ready for another soft-start
cycle.
The UC1526A can operate from a +5V supply by con-
necting the V
REF pin to the +VIN pin and maintaining the
supply between +4.8 and +5.2V.
Digital Control Ports
The three digital control ports of the UC1526A are bi-di-
rectional. Each pin can drive TTL and 5V CMOS logic di-
rectly, up to a fan-out of 10 low-power Schottky gates.
Each pin can also be directly driven by open-collector
TTL, open-drain CMOS, and open-collector voltage
comparators; fan-in is equivalent to 1 low-power Schot-
tky gate. Each port is normally HIGH; the pin is pulled
LOW to activate the particular function. Driving
SYNC
LOW initiates a discharge cycle in the oscillator. Pulling
SHUTDOWN LOW immediately inhibits all PWM output
pulses. Holding
RESET LOW discharges the soft-start
Figure 1. Extending Reference Output Current
Figure 2. Under-Voltage Lockout Schematic
Figure 3. Soft-Start Circuit Schematic
UC1526A
UC2526A
UC3526A
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