Application information

L6563 - L6563A

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boost inductor the inrush current coming from the bridge rectifier adds up to the switched

current and, furthermore, there is little or no voltage available for demagnetization.

To cope with a saturated inductor, the L6563 is provided with a second comparator on the

current sense pin (CS, pin 4) that stops and latches off the IC if the voltage, normally limited

within 1.1V, exceeds 1.7V. Also the cascaded DC-DC converter can be stopped via the

PWM_LATCH pin that is asserted high. In this way the entire system is stopped and enabled

to restart only after recycling the input power, that is when the Vcc voltages of the L6563

and the PWM controller go below their respective UVLO thresholds. System safety will be

considerably increased.

To better suit the applications where a certain level of saturation of the boost inductor needs

to be tolerated, the L6563A does not support this protection function.

6.7

Power management/housekeeping functions

A special feature of this IC 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.

This device provides some pins to do that. As already mentioned, one communication line

between the IC and the PWM controller of the cascaded DC-DC converter is the

PWM_LATCH pin, which is normally open when the PFC works properly and goes high if it

loses control of the output voltage (because of a failure of the control loop) or if the boost

inductor saturates, with the aim of latching off the PWM controller of the cascaded DC-DC

converter as well (Section 6.2: Feedback Failure Protection (FFP) on page 18 for more

details).

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

PFC_OK pin (Section 6.2 on page 18 for more details ). Typically this line is used to allow

the PWM controller of the cascaded DC-DC converter to shut down the L6563/A in case of

light load, 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 shown in Figure 43, where the L6563/A works along with the L5991, PWM controller

with standby function, can be used. 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 42. Effect of boost inductor saturation on the MOSFET current and detection method