L6699 datasheet(21/38 Pages) STMICROELECTRONICS

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L6699

Operation at no load or very light load

Doc ID 022835 Rev 2

21/38

Operation at no load or very light load

When the resonant half bridge is lightly loaded or totally unloaded, its switching frequency

reaches its maximum value. To keep the output voltage under control and avoid losing soft-

switching in these conditions, there must be some current flowing through the transformer's

magnetizing inductance. This current can be kept relatively low because of the adaptive

deadtime function; however, it produces power losses that prevent the converter's no load

consumption from achieving very low values anyhow.

To overcome this issue, the L6699 enables the user to make the converter operate

intermittently (burst-mode operation), with a series of a few switching cycles spaced out by

long idle periods where both MOSFETs are in the OFF-state, so that the average switching

frequency can be substantially reduced. As a result, the average value of the residual

magnetizing current and the associated losses is considerably cut down, therefore

facilitating the converter to comply with energy saving specifications.

The L6699 can be operated in burst-mode by using pin 5 (STBY): if the voltage applied to

this pin falls below 1.26 V, the IC enters an idle state where both gate-drive outputs are low,

the oscillator is stopped, the soft-start capacitor CSS keeps its charge and only the 2V

reference at the RFmin pin stays alive to minimize IC consumption and VCC capacitor

discharge. The IC resumes normal operation as the voltage on the pin exceeds 1.26 V by 30

mV.

In the L6699, the half bridge stops and restarts during burst-mode are more accurately

controlled with respect to its predecessors: the last cycle before stopping and the first cycle

after restarting are such that the DC voltage across the resonant capacitor Cr always stays

close to its steady-state value Vin/2. In this way, the anomalous current peaks due to V·s

unbalance in the transformer are minimized.

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

to the feedback loop.

Figure 14 shows the simplest implementation, suitable to a narrow

input voltage range (e.g. when there is a PFC front-end).

Essentially, RFmax defines the switching frequency fmax above which the L6699 enters burst-

mode operation. Once fmax is fixed, RFmax is found from the relationship:

Figure 14.

Narrow input voltage range

Figure 15.

Wide input voltage range

L6699 Datasheet(PDF) 21 Page - STMicroelectronics