L6564 datasheet(26/34 Pages) STMICROELECTRONICS

26 / 34 page

Application information

L6564

26/34

Doc ID 16202 Rev 1

6.6

Power management/housekeeping functions

A communication line with the control IC of the cascaded dc-dc converter can be

established via the disable function included in the PFC_OK pin (see “Feedback failure

protection” section for more details). Typically this line is used to allow the PWM controller of

the cascaded dc-dc converter to shut down the L6564 in case of light load and to minimize

the no-load input consumption. Should the residual consumption of the chip be an issue, it is

also possible to cut down the supply voltage. Interface circuits like those are shown in figure

32. Needless to say, this operation assumes that the cascaded dc-dc converter stage works

as the master and the PFC stage as the slave or, in other words, that the dc-dc stage starts

first, it powers both controllers and enables/disables the operation of the PFC stage.

Figure 40.

Interface circuits that let dc-dc converter's controller IC disable the L6564

Another function available is the brownout protection which is basically a not-latched

shutdown function that is activated when a condition of mains under voltage is detected.

This condition may cause overheating of the primary power section due to an excess of

RMS current. Brownout can also cause the PFC pre-regulator to work open loop and this

could be dangerous to the PFC stage itself and the downstream converter, should the input

voltage return abruptly to its rated value. Another problem is the spurious restarts that may

occur during converter power down and that cause the output voltage of the converter not to

decay to zero monotonically. For these reasons it is usually preferable to shutdown the unit

in case of brownout. Brownout threshold is internally fixed at 0.8V and is sensed on pin VFF

(5) during the voltage falling and 80 mV threshold hysteresis prevents from rebounding at

input voltage turn off. In Table 5 it is possible to find a summary of all of the above

mentioned working conditions that cause the device to stop operating.

L6564 Datasheet(PDF) 26 Page - STMicroelectronics