Tsw=2LpVi+Vor

PiVi+nVo

VinVo + CpℎVi2−n2Vo2

Lp

VinVo

+π LpCpℎ2

(3)

8.2.3

Clamp capacitor selection

The clamp capacitor value determines the resonance period between the capacitor and the transformer leakage

inductance during phase 5 of the switching cycle (see Section 8.5.1 ).

It is suggested to choose the clamp capacitor value so that the phase 5 lasts approximately 15% of the switching

cycle in high line, high output voltage conditions, so

Cclamp≈ 0.15Tsw2

π2Lk

(4)

8.2.4

Secondary side drain sensing

The SRVDS pin is used for sensing the synchronous rectifier drain voltage to control turn-on and turn-off of the

synchronous rectifier MOSFET and to sense the drain oscillations to operate in valley switching. The first function

is performed by comparing SRVDS voltage with a 1 V and a 0 V internal threshold, while the second function is

implemented by comparing the SRVDS voltage with SVCC.

A proper external clamp circuit is required to limit the voltage on the pin below its operative range while being

able to correctly sense the MOSFET drain voltage from 0 V up to SVCC. A signal mosfet with gate tied to VDRV

as shown the typical configuration schematic (see Figure 4) is the suggested solution. The resistors RZCD and

RZCDPU in the schematic can be added to offset the signal and anticipate the IZCD comparator triggering.

8.2.4.1

SRVDS comparators

The SRVDS pin has three comparators connected to it which are used to control the switching cycle.

The following figure shows the waveforms associated to the SRVDS comparators when the system is operating in

valley skipping mode. The SR drain voltage is drawn as a light blue line, the clamped voltage on the SRVDS pin is

shown in dark blue.

Figure 5. SRVDS sensing waveforms

Vlsg

Vhsg

Vsrg

Ipri

Isec

Vlsd

Vsrd

Vsrvds

VTHSR1V

VDYNZCD

1

2

3

4

VZCD: This comparator is used to count the switching node oscillations in valley skipping mode and to obtain

valley turn-on. The comparator has a dynamic threshold, detecting when the voltage on SRVDS drops more than

VDYNZCD below the last peak reached. Referring to Figure 5, the comparator is triggered in instant 3 and 4

ST-ONEMP

External component selection

DS13989 - Rev 2

page 17/39