UCC1839

UCC2839

UCC3839

04/99

FEATURES

• Practical Secondary Side Control of

Isolated Power Supplies

• Provides a Self Regulating Bias

Supply From a High Input Voltage

Using an External N-Channel

Depletion Mode FET

• Onboard Precision, Fixed Gain,

Differential Current Sense Amplifier

• Wide Bandwidth Current Error

Amplifier

• 5V Reference

• High Current, Programmable Gm

Amplifier Optimized to Drive

Opto-couplers

Secondary Side Average Current Mode Controller

BLOCK DIAGRAM

UDG-97011

DESCRIPTION

The UCC3839 provides the control functions for secondary side average

current mode control in isolated power supplies. Start up, pulse width

modulation and MOSFET drive must be accomplished independently on

the primary side. Communication from secondary to primary side is antici-

pated through an opto-isolator.

Accordingly, the UCC3839 contains a fixed gain current sense amplifier,

voltage and current error amplifiers, and a Gm type buffer/driver amplifier

for the opto-isolator. Additional housekeeping functions include a precision

5V reference and a bias supply regulator.

Power for the UCC3839 can be generated by peak rectifying the voltage of

the secondary winding of the isolation transformer. From this unregulated

voltage, the UCC3839’s bias supply regulator will generate its own 7.5V

bias supply using an external, N-channel, depletion mode FET.

The UCC3839 can be configured for traditional average current mode con-

trol where the output of the voltage error amplifier commands the current

error amplifier. It can also be configured for output voltage regulation with

average current mode short circuit current limiting, employing two parallel

control loops regulating the output voltage and output current independ-

ently.