BLOCK DESCRIPTION

BLOCK DIAGRAM

1

2

5

6

7

8

3

4

40 kW

5 pF

+

+

+

x

x

K

2.5 V

2.3 V

+

+

OVP

Enable

2.7/2.5 V

g

0.67/0.57 V

0.23/0.15 V

Current Sense Comparator

S

Q

Q

R

+

Restart Timer

+

V

Bias Reg

V

REF

Init. Bias

1.7/1.4 V

UVLO

V

REF

GOOD

OVP

VSENSE

EAOUT

VINS

ISENSE

TZE

GND

GDRV

VDD

REF

Zero Energy

Detect

UDG-08130

75 mV

Current

Reference

Generator

UVLO and Reference Circuit

Error Amplifier

UCC28810

UCC28811

This circuitry generates a precision reference voltage used to obtain a tightly controlled UVLO threshold. In

reference voltages for OVP, enable, zero energy detect and the current reference generator circuits. An internal

rail of 7.5 V is also generated to drive all the internal circuitry.

The voltage error amplifier in the UCC2881x is a transconductance amplifier with a typical transconductance

value of 90

µS. The advantage in using a transconductance amplifier is that the inverting input of the amplifier is

solely determined by the external resistive-divider from the output voltage and not the transient behavior of the

amplifier itself. This allows the VSENSE pin to be used for sensing over voltage conditions.

The sink and source capability of the error amplifier is approximately 10

µA during normal operation of the

slew-rate of the compensation capacitor results in a faster start-up and transient response. This prevents the

output voltage from drifting too high or too low, which can happen if the compensation capacitor were to be

driven by the normal drive current of 10-

µA. When VSENSE rises above the normal range, the enhanced sink

current capability increases to 1 mA, nominal. When VSENSE falls below the normal range, the UCC28810 can

source more than 1 mA and the UCC28811 sources approximately 300

µA. The limited source current in the

UCC28811 helps to gradually increase the error voltage on the EAOUT pin preventing a step increase in line

current. The actual rate of increase of the voltage on the EAOUT pin is dependent on the compensation network

externally connected to the EAOUT pin.

Copyright © 2008, Texas Instruments Incorporated

Submit Documentation Feedback

7

Product Folder Link(s): UCC28810 UCC28811