Application information

L6599AF

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DocID028175 Rev 3

Figure 7. Burst mode implementation: a) narrow input voltage range; b) wide input voltage range

Equation 3

transformer peak currents are low enough not to cause audible noise.

Resonant converter switching frequency, however, depends also on the input voltage;

therefore, in the case of quite a large input voltage range with the circuit of Figure 7a, the

arrangement shown in Figure 7b, where the information on the converter input voltage is

added to the voltage applied to the STBY pin. Due to the strongly non-linear relationship

between switching frequency and input voltage, it is more practical to find empirically the

LINE pin voltage (see Section 6.6: Line sensing function on page 23).

Whichever circuit is in use, its operation can be described as follows. As the load falls below

outputs low, so that both MOSFETs of the half bridge leg are in OFF-state. The voltage

response to the energy burst and stops the IC. In this way, the converter works in a burst

mode fashion with a nearly constant switching frequency. A further load decrease then

causes a frequency reduction, which can go down even to few hundred hertz. The timing

diagram of Figure 8 illustrates this kind of operation, showing the most significant signals.

A small capacitor (typically in the hundred pF) from the STBY pin to ground, placed as close

to the IC as possible to reduce switching noise pick-up, helps obtain clean operation.

To help the designer meet energy saving requirements even in power-factor-corrected

systems, where a PFC pre-regulator precedes the DC-DC converter, the L6599AF allows

that the PFC pre-regulator can be turned off during burst mode operation, therefore

1

f

f

RF

8

3

RF

min

max

min

max

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