Datasheet

UC1526
UC2526
UC3526
APPLICATIONS INFORMATION
Voltage Reference
The reference regulator of the UC1526 is based on a tem-
perature compensated zener diode. The circuitry is fully
active at supply voltages above +8V, and provides up to
20mA of load current to external circuitry at +5.0V. In sys-
tems where additional current is required, an external
PNP transistor can be used to boost the available current.
A rugged low frequency 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 os-
cillations. Even so, some types of transistors may require
collector-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.
Under-Voltage Lockout
The under-voltage lockout circuit protects the UC1526
and the power devices it controls from inadequate supply
voltage, If +V
IN is too low, the circuit disables the output
drivers and holds the RESET
_______
pin LOW. This prevents
spurious output pulses while the control circuitry is stabi-
lizing, and holds the soft-start timing capacitor in a dis-
charged 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 ref-
erence voltage rises to approximately +4.4V, the circuit
enables the output drivers and releases the RESET
_______
pin,
allowing a normal soft-start. The comparator has 200mV
of hysteresis to minimize oscillation at the trip point.
When +V
IN to the PWM is removed and the reference
drops to +4.2V, the under-voltage circuit pulls RESET
_______
LOW again. The soft-start capacitor is immediately dis-
charged, and the PWM is ready for another soft-start cy-
cle.
The UC1526 can operate from a +5V supply by connect-
ing the V
REF pin to the +VIN pin and maintaining the sup-
ply between +4.8 and +5.2V.
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
UC1526, the under-voltage lockout circuit holds RESET
_______
LOW with Q
3. 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, guaran-
teeing zero duty cycle at the driver outputs. When the
supply voltage reaches normal operating range, RESET
_______
will go HIGH. Q
1 turns off, allowing the internal 100mA
current source to charge C
S. Q2 clamps the error ampli-
fier output to 1V
BE above the voltage on CS. As the soft-
start voltage ramps up to +5V, the duty cycle of the PWM
linearly increases to whatever value the voltage regula-
tion loop requires for an error null.
Digital Control Ports
The three digital control ports of the UC1526 are bi-direc-
tional. 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
Figure 2. Under-Voltage Lockout Schematic
Figure 1. Extending Reference Output Current
Figure 3. Soft-Start Circuit Schematic
4