L6699 datasheet(22/38 Pages) STMICROELECTRONICS

22 / 38 page

Operation at no load or very light load

L6699

22/38

Doc ID 022835 Rev 2

Equation 6

Note that, unlike the fmax considered in the Section 6.1: Oscillator, here fmax is associated to

some load PoutB greater than the minimum one. PoutB is normally such that the

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;

so, in case there is quite a large input voltage range, with the circuit of

Figure 14 the value of

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

shown in

Figure 15, 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 in PoutB. Take care in choosing

the total value RA + RB much greater than RL to minimize the effect on the LINE pin voltage

(see

Section 10: Line sensing function).

Figure 16.

Load-dependent operating modes: timing diagram

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

the value PoutB the frequency tries to exceed the maximum programmed value fmax and the

voltage on the STBY pin (VSTBY) goes below 1.26 V. The IC then stops with both gate-drive

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

VSTBY now increases as a result of the feedback reaction to the energy delivery stop and, as

it exceeds 1.29 V, the IC restarts switching. After a while, VSTBY goes down again in

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

increases the time between consecutive bursts and/or reduces the duration of each burst.

This reduces the average switching frequency, which can go down even to few hundred

hertz. The timing diagram of

Figure 16 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.

min

max

min

max

L6699 Datasheet(PDF) 22 Page - STMicroelectronics