MV1012SC

21

5.1.2

[B] Off-time modulation region

Operation waveforms in the off-time modulation region are those in discontinuous conduction mode shown in Figure 15.

off-time includes toff1, the period during which a current flows to D1. As shown by the formula in Figure 5, toff1 depends

on Vo. Therefore, if Vo changes, Io in the [B] region will also change.

Thus, the circuit is more likely to be affected by noise in the [B] region. The MOSFET is more likely to switch off at a timing

that varies from the time of peak current detection. The switching-on timing does not fall on a valley of the resonant

turned on, there will be a leading edge blanking (LEB) period in which noise is rejected. During LEB period, the MOSFET

cannot be turned off. (Figure 16(b)) Even during the LEB period, when switching-on noise or noise from an external circuit

exceeds the reference voltage, such noise will lead to false detection, and the MOSFET is turned off. (Figure 17(b)) As a

result, Io drops below the optimal level. Depending on the timing of the noise, the LED light may flicker.

If Coss of MOSFET, Cr of the resonant capacitor or Cj of the fly-wheel diode is large, the switching-on noise will also be

large. If switching-on noise causes false detection, adjust the CS filter (page 12), adjust the gate resistance (page 11), or

reselect the components above.

recommend measuring just the gate pin waveform.

ton

toff1

Ip

Io

t

t

toff2

Vi-Vo

Forced off-time

(a) Switched on

(b) Switched on

Figure 15 Operation waveform

in the [B] off-time modulation region

Figure 16 Example of switching-on

CAT.No.1A0602-1E