3

4

LDO

Control and

Reference

8

VDD

DRAIN

5

HVIN

FB

UVLO

+

PWM Controller

and Output Short

Circuit Protection

LEB

Leading Edge

Blanking Time

S

Q

Q

R

Current

Limit

1, 2

GND

Gate

High Voltage

Current Source

Thermal

Shutdown

UCC28881

www.ti.com

SLUSC36A – NOVEMBER 2015 – REVISED DECEMBER 2015

8 Detailed Description

8.1 Overview

The UCC28881 integrates a controller and a 700-V power MOSFET into one monolithic device. The device also

integrates a high-voltage current source, enabling start up and operation directly from the rectified mains voltage.

UCC28881 is the same family device as UCC28880 and it provides higher power handling capability.

The low-quiescent current of the device enables excellent efficiency. The device is suitable for non-isolated AC-

to-DC low-side buck and buck-boost configurations with level-shifted direct feedback, but also more traditional

high-side buck, buck boost and low-power flyback converters with low standby power can be built using a

minimum number of external components.

The device generates its own internal low-voltage supply (5 V referenced to the device’s ground, GND) from the

integrated high-voltage current source. The PWM signal generation is based on a maximum constant on-time,

minimum off-time concept, with the triggering of the on-pulse depending on the feedback voltage level. Each on-

pulse is followed by a minimum off-time to ensure that the power MOSFET is not continuously driven in an on-

state. The PWM signal is AND-gated with the signal from a current limit circuit. No internal clock is required, as

the switching of the power MOSFET is load dependent. A special protection mechanism is included to avoid

runaway of the inductor current when the converter operates with the output shorted or in other abnormal

conditions that can lead to an uncontrolled increase of the inductor current. This special protection feature keeps

the MOSFET current at a safe operating level. The device is also protected from other fault conditions with

thermal shutdown, under-voltage lockout and soft-start features.

8.2 Functional Block Diagram

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