AN2782 datasheet(6/39 Pages) STMICROELECTRONICS

6 / 39 page

The circuit implementing the line-modulated fixed off-time with the new L6562A

AN2782

6/39

Doc ID 14763 Rev 2

The circuit implementing the line-modulated fixed

off-time with the new L6562A

The circuit that implements LM-FOT control with the L6562A is shown in Figure 6. During

the ON-time of the MOSFET the gate voltage VGD = 15 V is high, the diode D is forward-

biased and the voltage at the ZCD pin is internally clamped at VZCDclamp ≈ 5.7 V. During the

off-time of M VGD = 10 V is low, the diode D is reverse-biased and the voltage at the pin

decays with an exponential law until it reaches the triggering threshold (VZCDtrigger ≈ 0.7 V)

that causes the switch to turn on. The time needed for the ZCD voltage to go from VZCDclamp

to VZCDtrigger defines the duration of the off-time TOFF.

Figure 6.

Circuit implementing FOT control with the L6562A

The circuit of Figure 6 makes TOFF a function of the RMS line voltage thanks to the peak-

holding effect of T1 (which acts as a buffer) along with R and C whose time constant is

significantly longer than a line half-cycle. With the addition of R0 and T, as long as the

voltage on the ZCD pin during TOFF is above Vmult+VBE, C is discharged through R and R0,

following the law:

As V’ZCD(t) falls below Vmult+VBE, T1 is cut off and C is discharged through R only, so that

its evolution from that point on is described by:

V'ZCD(t) decreases from VZCDclamp = 5.7 V to Vmult+VBE in the following time period t':

(1)

(2)

(3)

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