APR349

Document number: DS43554 Rev. 2 - 2

7 of 10

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July 2021

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APR349

Operation Description (continued)

Smart-Blanking Control

Because of the resonance oscillation of the primary inductance and equivalent output capacitance of the switching device, the voltage on the

MOSFET drain pin has an under damped voltage ring in DCM operation mode. In some special occasions, such as output transition periods or

light-load conditions, the SR MOSFET would turn on falsely during the tOFF period when the ring of the VDRAIN meets the conduction requirement.

When the turn

–on of the primary MOSFET meets the false turn-on of the SR, the transformer would immediately be shoot through. To avoid such

faults, the device has a built-in smart blanking function to prevent continuous false turn-ons. The SR gate driver will blank for a period of 20

μs

when detecting a false turn-on twice in a period of 4

μs in tOFF period. Figure 3 shows a smart-blanking control method.

VDRAIN

Gate

tONP tONS

tOFF

tONP tONS

tOFF

< 4µs

tBLANK = 20µs

< 4µs

tBLANK = 20µs

Figure 3. Smart-Blanking Control Method

GATE Turn-Off Logic

When VDRAIN rises to the turn-off threshold (-7mV), the synchronous rectification MOSFET gate voltage will be pulled down quickly from a low

voltage to zero after a very short turn-off delay. For CCM mode, the secondary VDRAIN will rise up as the primary MOSFET turns on. Once drain

voltage rises to the turn-off threshold, the gate driving signal will immediately shut down.