Circuit description

AN4599

6/26

DocID026993 Rev 1

With reference to the schematic in Figure 13:

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CF = C22 = 560 pF

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2.3

Burst mode operation at no load or very light load

To reduce the average switching frequency, the L6699 can operate in burst mode with a

series of a few switching cycles in between relatively long idle periods with both MOSFETs

in the off state.

The resulting average value of the residual magnetizing current and corresponding loss is

reduced considerably, thus facilitating converter compliance with energy saving

specifications.

L6699 can be operated in burst mode via pin 5 (STBY): if the voltage applied to this pin falls

below 1.26 V, the IC enters the low-consumption idle state, where both gate drive outputs

are low and the oscillator is stopped; the IC resumes normal operation when the voltage on

pin exceeds 1.26 V + 30 mV.

To implement burst mode operation, the voltage applied to the STBY pin needs to be

associated with the feedback loop.

The resonant converter switching frequency and hence burst mode activation strongly

depends on the variation of the input voltage.

Use the circuit in Figure 3 when the input voltage range is quite large.

Due to the high non-linear relationship between the switching frequency and input voltage, it

Figure 3. Wide input voltage range schematic