Test results and significant waveforms

AN3118

22/31

Doc ID 16847 Rev 2

4.6

Power management and housekeeping functions

A special feature of the L6563H 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 L6563H provides some pins to do that.

As already mentioned, one communication line between the L6563H and the PWM

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

normally open when the PFC works properly. It goes high if the L6563H loses control of the

output voltage (because of a failure of the control loop) 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 L6563H in 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.

L6563H on/off control by a cascaded converter controller via PFC_OK or

RUN pin

The third communication line is the PWM_STOP pin (#11), which works in conjunction with

the RUN pin (#12). 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

(#12). 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 hi-end flat-TVs, or monitors), where the auxiliary converter also powers the

controllers of the main converter, the RUN pin (#12) can be used to start and stop the main

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

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

with it, to its soft-start pin. The EVL6563H-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 (#11) which is an open collector type, if it needs a pull-up

resistor, please connect it close to the cascaded PWM for better noise immunity.