FEATURES

• Precision Secondary Side Post

Regulation for Multiple Output Power

Supplies

• Useful for Both Single Ended and

Center Tapped Secondary Circuits

• Ideal Replacement for Complex

Magnetic Amplifier Regulated Circuits

• Leading Edge Modulation

• Does Not Require Gate Drive

Transformer

• High Frequency (>500kHz) Operation

• Applicable for Wide Range of Output

Voltages

• High Current Gate Driver (0.5A

Sink/1.5A Source)

• Average Current Limiting Loop

DESCRIPTION

The UCC3583 is a synchronizable secondary side post regulator for precision

regulation of the auxiliary outputs of multiple output power supplies. It

contains a leading edge pulse width modulator, which generates the gate

drive signal for a FET power switch connected in series with the rectifying

diode. The turn-on of the power switch is delayed from the leading edge of

the secondary power pulse to regulate the output voltage. The UCC3583

contains a ramp generator slaved to the secondary power pulse, a voltage

error amplifier, a current error amplifier, a PWM comparator and associated

logic, a gate driver, a precision reference, and protection circuitry.

The ramp discharge and termination of the gate drive signal are triggered

by the synchronization pulse, typically derived from the falling edge of the

transformer secondary voltage. The ramp starts charging again once its

low threshold is reached. The gate drive signal is turned on when the ramp

voltage exceeds the control voltage. This leading edge modulation technique

prevents instability when the UCC3583 is used in peak current mode

primary controlled systems.

The controller operates from a floating power supply referenced to the output

voltage being controlled. It features an undervoltage lockout (UVLO)

circuit, a soft start circuit, and an averaging current limit amplifier. The current

limit can be programmed to be proportional to the output voltage, thus

achieving foldback operation to minimize the dissipation under short circuit

conditions.