AN3119

Test results and significant waveforms

Doc ID 16849 Rev 2

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4.7

Power management and housekeeping functions

A special feature of the L6563S is that it facilitates the implementation of the “housekeeping”

circuitry needed to coordinate 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. The L6563S provides some pins to do that.

As already mentioned, one communication line between the L6563S and the PWM

controller of the cascaded DC-DC converter is the PWM_LATCH (pin #8), which is normally

open when the PFC works properly. It goes high if the L6563S loses control of the output

voltage (because of a failure of the control loop) or if the boost inductor saturates, with the

aim of also latching off the PWM controller of the cascaded DC-DC converter.

A second communication line can be established via the disable function included in the

RUN pin. Typically, this line is used to allow the PWM controller of the cascaded DC-DC

converter to shut down the L6563S in the case of light load, to minimize the no-load input

consumption of the power supply.

Examples of intefacing some ST half-bridges controllers are shown in Figure 34.

Figure 34.

L6563S on/off control by a cascaded converter controller via the PFC_OK

or RUN pin

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

the RUN pin (#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

(#10). 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. This function is quite flexible and can be used in different ways. In

systems comprising an auxiliary converter and a main converter (e.g. desktop PC's, silver

box or high end flat-TVs, and monitors), where the auxiliary converter also powers the

controllers of the main converter, the RUN pin (#10) can be used to start and stop the main

converter. In the simplest case, to enable/disable the PWM controller the PWM_STOP pin

(#9) can be connected to either the output of the error amplifier or, if provided the chip, to the

soft-start pin.

The EVL6563S-250W offers the possibility to test these function by connecting it to the

cascaded converter via the R30, R31, R32 series resistors. Regarding the PWM_STOP pin