Input stage of the AC/DC converter

AN4205

14/23

Doc ID 023929 Rev 1

The timing resistor can be calculated from the following formula:

Equation 30

The UC3842 current sensing input is done externally with ground-referenced resistor R3.

Under normal operation the peak voltage across R3 is controlled by the error amplifier

according to the following formula:

Equation 31

The next step is the error amplifier (E/A) configuration. The R6, R7 and C7 error amplifier

compensation circuit is chosen in order to ensure converter stability while providing good

dynamic response.

2.5

Overvoltage protection circuit

Figure 9.

Overvoltage protection circuit

The overvoltage protection circuit is required in order to not damage the LED lamps. It is

connected between the output and ground. The Zener diode voltage has been chosen to be

slightly above that of the output voltage. When the Zener diode voltage is reached, current

flows into the resistor and as soon as the voltage, through the diode, exceeds one of the

internal silicon diodes (1 V) of the UC3842 driver, the PWM is stopped. In this way the

voltage across the output capacitor decreases. When the input of the error amplifier of

UC3842 is lower than 2.5 V (internal reference voltage), the PWM restarts. So this

overvoltage protection circuit temporarily stops the PWM driver, but it ensures system

operation as soon as the condition of the error amplifier is reached.

f

()

1.72

R5 k

() C9 μF

()

•

------------------------------------------

=

I

P peak

,

V

C

1.4V

–

3R

S

--------------------------

=

3

6

5

4

8

1

2

7

DZ3

D6

R17

VBus

AM16375v1