L6563S datasheet(31/42 Pages) STMICROELECTRONICS

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L6563S

Application information

Doc ID 16116 Rev 3

31/42

6.7

Power management/housekeeping functions

A special feature of this IC is that it facilitates the implementation of the “housekeeping”

circuitry needed to co-ordinate the operation of the PFC stage to that of the cascaded DC-

DC converter. The functions realized by the housekeeping circuitry ensure that transient

conditions like power-up or power down sequencing or failures of either power stage be

properly handled.

This device provides some pins to do that. One communication line between the IC and the

PWM controller of the cascaded dc-dc converter is the pin PWM_LATCH (Figure 44b),

which is normally open (high impedance) when the PFC works properly, and goes high if it

loses control of the output voltage (because of a feedback loop disconnection) with the aim

of latching off the PWM controller of the cascaded dc-dc converter as well (see “Feedback

failure protection” section for more details).

A second communication line 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 drive in burst mode

operation the L6563S in case of light load and to minimize the no-load input consumption.

Interface circuits like those are shown in Figure 43.

Figure 43.

Interface circuits that let dc-dc converter's controller IC drive L6563S in

burst mode

The third communication line is the pin PWM_STOP (#9), which works in conjunction with

the pin RUN (#10). The purpose of the PWM_STOP pin is to inhibit the PWM activity of both

the PFC stage and the cascaded dc-dc converter. The pin is an open collector, normally

open, that goes low if the device is disabled by a voltage lower than 0.8 V on the RUN pin. It

is important to point out that this function works correctly in systems where the PFC stage is

the master and the cascaded dc-dc converter is the slave or, in other words, where the PFC

stage starts first, powers both controllers and enables/disables the operation of the dc-dc

stage. The pin RUN can be used to start and stop the main converter. In the simplest case,

to enable/disable the PWM controller the pin PWM_STOP can be connected to the output of

the error amplifier (Figure 44a).

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