This document introduces UCC3884, a high-performance current mode PWM controller designed for single-ended switch mode power supplies. The chip implements a frequency foldback scheme that decreases the oscillator frequency as the output voltage falls below a programmed value. This technique decreases the average output current sourced into a low impedance load, which can occur during an output short circuit or overload condition. Excessive short circuit current is more prevalent in high frequency converters where the propagation delay and switch turn-off time forces a minimum attainable duty cycle. An accurate volt-second clamp limits the duty cycle during line or load transient conditions which could otherwise saturate the transformer. The volt-second clamp may also be used with an external overvoltage protection circuit to handle fault conditions such as current sense disconnect or current transformer saturation. The frequency foldback, volt-second clamp, cycle-by-cycle current limit, and overcurrent shutdown functions ensure that the power supply is always in a safe state under various fault conditions.
The document details three models of frequency foldback current mode PWM controllers designed for single-ended switch-mode power supplies. Key features include an accurate programmable volt-second clamp, a programmable maximum duty cycle clamp, oscillator synchronization, and overcurrent protection with full soft start.
The datasheet provides absolute maximum ratings, electrical characteristics tables covering the 5V reference, oscillator, error amplifier, PWM, current sense, and output stages. It also includes pin descriptions explaining functions such as CLKSYNC, COMP, CS, CSS, CT, FB, GND, GT, IOFF, ION, OUT, PGND, VDD, and VOUT.
| Reference Voltage (VREF) | 4.86V to 5.14V |
|---|---|
| Oscillator Accuracy | 360 kHz to 440 kHz |
| Maximum Duty Cycle (UCC1884) | 75% |
| Current Sense Shutdown Threshold | 1.235V to 1.365V |
| Soft Start Charge Current | -10uA to -30uA |
The specifications apply for TA = -55°C to +125°C.
The VOUT pin accomplishes frequency foldback by controlling the discharge current for the oscillator CT capacitor.