STMicroelectronics Double output SMPS for power line application using ALTAIR04-900 primary controller EVLALTAIR900-M1 EVLALTAIR900-M1 Fiche De Données

18/29

Doc ID 18211 Rev 2

The R

ZCD

 resistor can be calculated as follows:

In this case the peak drain current does not depend on input voltage anymore.

One more consideration concerns the R

ZCD

 value: during MOSFET’s ON-time, the current 

sourced by the ZCD/FB pin, I

ZCD

, is compared with an internal reference current I

ZCDON

 (-50 

µA typical).

If I

ZCD

 < I

ZCDON

, the brownout function is activated and the IC is shut-down.

This feature is especially important when the auxiliary winding is accidentally disconnected 
and considerably increases the end-product’s safety and reliability.

When the voltage at the COMP pin falls 65 mV below a threshold fixed internally at a value, 
V

COMPBM

, the IC is disabled with the MOSFET kept in OFF state and its consumption 

reduced at a lower value to minimize Vcc capacitor discharge.

In this condition the converter operates in burst-mode (one pulse train every T

START

=500 

µs), with minimum energy transfer.

As a result of the energy delivery stop, the output voltage decreases: after 500 µs the 
controller switches-on the MOSFET again and the sampled voltage on the ZCD pin is 
compared with the internal reference. If the voltage on the EA output, as a result of the 
comparison, exceeds the V

COMPL

 threshold, the device restarts switching, otherwise it stays 

OFF for another 500 µs period.

In this way the converter works in burst-mode with a nearly constant peak current defined by 
the internal disable level. Then a load decrease causes a frequency reduction, which can go 
down even to few hundred hertz, thus minimizing all frequency-related losses and making it 
easier to comply with energy saving regulations. This kind of operation, shown in the timing 

 

.

CC  

Block

Aux

R zcd

R f b

I FF

Rsense

R f f

+

-

C C

F eedf orward

Logic

PWM

LOGI C

ZCD/FB

DRAIN

SOURCE

         

(4)

AUX

P

FF

ZCD

PRI

d

SENSE

N

L R

R

N

T R

⋅

=

⋅

⋅