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UC3526ADWTRG4 数据表(PDF) 5 Page - Texas Instruments |
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UC3526ADWTRG4 数据表(HTML) 5 Page - Texas Instruments |
5 / 11 page 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 Q1 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 100 µA current source to charge CS. Q2 clamps the error amplifier 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 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 +VIN 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 3VBE 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 +VIN 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 VREF 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 5 |
类似零件编号 - UC3526ADWTRG4 |
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类似说明 - UC3526ADWTRG4 |
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