Application information

L6599AT

16/31

Doc ID 15534 Rev 3

Figure 7.

Burst-mode implementation:

a) narrow input voltage range; b) wide input voltage range

Equation 3

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

The resonant converter's switching frequency, however, depends also on the input voltage.

Therefore, in cases where there is quite a large input voltage range with the circuit of

Figure 7

the arrangement shown in Figure 7b, where the information on the converter's 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

the effect on the LINE pin voltage (see Section 6.6: Line sensing function).

Whichever circuit is used, 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 an 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 to even a 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 range) from the STBY pin to ground, placed as

close to the IC as possible to reduce switching noise pick-up, helps get 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 L6599AT allows

the PFC pre-regulator to be turned off during burst-mode operation, thereby eliminating the

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