Experiment 2
PWM control circuitry
Datasheets
A few tips regarding the UC3525
- Unless you want to shut down the IC, tie the "shutdown" pin to ground.
- Leave the "sync" and "soft start" pins disconnected. The voltage on the soft start pin limits the maximum duty cycle; if a capacitor is connected to this pin, then when power is applied the capacitor will be slowly charged by the internal 50 uA current source, and the maximum duty will slowly increase from zero to maximum. The sync pin is used to synchronize multiple PWM chips.
- Output pins 11, 13, and 14: If Vc (pin 13) is tied to a power supply voltage, then the gate drive pulses will be steered alternately to outputs A and B (pins 11 and 14) in succeeding switching periods. Hence the switching frequency and duty cycle are reduced by a factor of one-half at a single output pin. This feature is commonly needed in full-bridge or push-pull circuits, and can also be used to interleave two parallel-connected converters. To disable this feature (for a single transistor converter), one can tie pins 11 and 14 to ground, and connect a pullup resistor from pin 13 to the power supply. This effectively "wire ORs" the outputs, and the gate drive signal will then appear at pin 13, with full duty cycle and switching frequency.
- The "reference regulator" is a small internal power supply that operates from +Vin (8-35 V) and produces a regulated 5V at pin 16. This regulator can supply up to 50 mA that can be used in external circuitry. If you connect other circuitry to pin 16, it is recommended that you place a bypass capacitor from pin 16 to pin 12.