L6599 datasheet(19/36 Pages) STMICROELECTRONICS

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L6599

Application information

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Essentially, RFmax will define the switching frequency fmax above which the L6599 will enter

burst-mode operation. Once fixed fmax, RFmax will be found from the relationship:

Note that, unlike the fmax considered in the previous section ("Chapter 7.1: Oscillator"), here

fmax is associated to some load PoutB greater than the minimum one. PoutB will be such that

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

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

hence, in case there is quite a large input voltage range with the circuit of Figure 23 the

value of PoutB would change considerably. In this case it is recommended to use the

arrangement shown in Figure 24 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

right amount of correction RA / (RA + RB) needed to minimize the change of PoutB. Just be

careful in choosing the total value RA + RB much greater than RC to minimize the effect on

the LINE pin voltage (see Chapter 7.6: Line sensing function).

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

the value PoutB the frequency will try to exceed the maximum programmed value fmax and

the voltage on the STBY pin (VSTBY) will go below 1.25V. The IC will then stop with both

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

voltage VSTBY will now increase as a result of the feedback reaction to the energy delivery

stop and, as it exceeds 1.3V, the IC will restart switching. After a while, VSTBY will go down

again in response to the energy burst and stop the IC. In this way the converter will work in a

burst-mode fashion with a nearly constant switching frequency. A further load decrease will

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

diagram of Figure 25 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, will help 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 device allows that

the PFC pre-regulator can be turned off during burst-mode operation, hence eliminating the

no-load consumption of this stage (0.5

÷ 1W). There is no compliance issue in that because

EMC regulations on low-frequency harmonic emissions refer to nominal load, no limit is

envisaged when the converter operates with light or no load.

To do so, the device provides pin 9 (PFC_STOP): it is an open collector output, normally

open, that is asserted low when the IC is idle during burst-mode operation. This signal will

be externally used for switching off the PFC controller and the pre-regulator as shown in

Figure 26 When the L6599 is in UVLO the pin is kept open, to let the PFC controller start

first.

max

---

min

max

min

-----------

–

---------------------

⋅

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