AN2782 datasheet(8/39 Pages) STMICROELECTRONICS

8 / 39 page

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

AN2782

8/39

Doc ID 14763 Rev 2

characteristics. Consequently, all of the properties of the standard FOT control are

maintained.

Due to the highly non-linear nature of the TOFF modulation introduced by T1 and R0, its

effects are discussed only qualitatively and the quantitative aspects are provided graphically

for a specific case in [7].

As a practical rule, it is convenient to first select a capacitor and then to calculate the resistor

needed to achieve the desired TOFF (see Section 4.3.7).

As the gate voltage VGD goes high, the resistor Rs charges the timing capacitor C as quickly

as possible up to VZCDclamp, without exceeding clamp rating (IZCDx =10 mA). Then it must

fulfill the following inequalities:

where VGD (assume VGD = 10 V) is the voltage delivered by the gate driver, VGDx = 15 V its

maximum value, and VF the forward drop on D.

When working at high line/light load the on-time of the power switch becomes very short and

the resistor Rs alone is no longer able to charge C up to VZCDclamp. The speed-up capacitor

Cs is then used in parallel to Rs. This capacitor causes an almost instantaneous charge of C

up to a level, after that Rs completes the charge up to VZCDclamp. It is important that the

steep edge caused by Cs does not reach the clamp level, otherwise the internal clamp of the

L6562A undergoes uncontrolled current spikes (limited only by the dynamic resistance of

the 1N4148 and the ESR of Cs) that could overstress the IC. Cs must then be:

(6)

(7)

ZCDclamp

ZCDx

ZCDclamp

GDx

−

⋅

−

ZCDclamp

GDx

ZCDclamp

−

AN2782 Datasheet(PDF) 8 Page - STMicroelectronics