AN2042 datasheet(13/30 Pages) STMICROELECTRONICS

13 / 30 page

AN2042

Application description

13/30

The feedback to the primary side is achieved thanks to the diodes D9 and D10, which

decouple the two loops and drive the optocoupler OPT. The legs R23-C11 and R24-C12 are

connected for feedback stabilization.

The zener diode DZ2 is connected at the non-inverting input of the voltage control

operational amplifier in order to clamp the maximum voltage on the pin in any operative

condition.

The PWM control is realized using the first operational amplifier to generate a sawtooth

waveforms at 270 Hz (given by the leg R19-C13), which is compared with a variable voltage

(set by the potentiometer R21): the generated signal will drive the NPN transistor Q3. When

the transistor is "ON", the SMPS works in "current control" mode limiting the max output

current while, when the transistor is "OFF", it works in "voltage control" mode, regulating the

output voltage below the LEDs threshold and consequently switching them off.

During the dimming operation, the transistor Q3 is switched off and the voltage on pin 11 of

IC2 is pulled up and limited to VDZ1. Consequently, the VIPer stops switching and the output

current falls to zero, while the output voltage decrease down to VOUT = n · VF(OFF), where n

is the number of LEDs and VF(OFF) is the threshold voltage. Further decrease of the output

voltage is not possible because of the high output impedance. Doing so, the output voltage

never falls to zero, resulting in a big improvement in the dynamic behavior of the dimming

function, with a slight impact on the efficiency PDISS = (VOUT-VDZ2)/R8.

In open load condition, the maximum voltage is regulated by R8, R9 and DZ2 according to

the reference voltage given by (Equation 5).

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