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UC3526 Datenblatt(PDF) 4 Page - Texas Instruments |
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UC3526 Datenblatt(HTML) 4 Page - Texas Instruments |
4 / 10 page UC1526 UC2526 UC3526 APPLICATIONS INFORMATION Voltage Reference T he 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 +VIN 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 3VBE 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 +VIN 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 VREF 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 Q3. Q1 is turned on, which holds the soft-start capacitor voltage at zero. The second collector of Q1 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. Q1 turns off, allowing the internal 100mA current source to charge CS. Q2 clamps the error ampli- fier output to 1VBE 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 |
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Ähnliche Beschreibung - UC3526 |
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