L6564T

Application information

Doc ID 022671 Rev 1

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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

Section 6.2 for more

details). This line is typically used to allow the PWM controller of the cascaded DC-DC

converter to shut down the L6564T 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. The interface circuits 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 33.

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

L6564T

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

shutdown function that is activated when a condition of mains undervoltage 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 function in open loop and this

may 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 shut down the unit

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

VFF (5) during the voltage falling and an 80 mV threshold hysteresis prevents 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.