Output stage setting

AN4205

16/23

Doc ID 023929 Rev 1

4

Output stage setting

The goals of this output stage of the LED driver are the constant current on the LED

components and low power losses during the linear regulation. Next, the setting and the

project of the output stage will be shown.

The first step is to define the output stage specifications that the user must set for the

innovative LED driver.

The bipolar device (Q1), connected to the Power MOSFET gate (Figure 10), controls the

Equation 32

In order to perform the linear regulation, the bipolar Q1 operates in forward active mode

(common emitter), therefore for small base current variations, the emitter-collector voltage

negative feedback. If the base voltage of the bipolar implies a rise in the collector voltage,

the gate of the Power MOSFET decreases because the emitter node is fixed by the Zener

device, vice versa if the collector voltage decreases. The base current variations depend on

bipolar.

Table 3.

Output stage specifications data

Name

Symbol

Value

Light emitter diode

LED

6 LEDs of

DC output current

DC output voltage

MOSFET drain-source voltage

Output voltage

I

DS

KV

GS

V

th

–

()

2

•

1

λV

DS

+

()

•

=