AN4205 datasheet(19/23 Pages) STMICROELECTRONICS

19 / 23 page

AN4205

Output stage setting

Doc ID 023929 Rev 1

19/23

Figure 13.

Details of switching regulation

The VDS signal is processed by an operational amplifier (A.O.) and is then sent to the “non-

inverting input” of the “UC3842” driver, which decides the state of the Power MOSFET M1

put in a flyback topology Figure 13. If the VDS value is greater than the threshold (fixed by

specification), the "UC3842" driver stops the PWM regulation and the output voltage

decreases, vice versa the PWM regulation starts again.

The main advantages for processing the VDS signal rather than the VBUS as in other

standard linear regulation systems, are listed as follows:

To keep the ripple output voltage low

To fix the average value of the VDS voltage in order to minimize the power losses of the

power transistor

To guarantee the VDS value lower than the threshold even if one or more LED

components are short

Uc3

842

Vcc

VBus

Input Switching Control

MOSFET M2

Bipolar Q 1

LEDs

Output Switching Control

R19

DZ3

MOSFET M1

AM16379v1

AN4205 Datasheet(PDF) 19 Page - STMicroelectronics