L4984D datasheet(30/35 Pages) STMICROELECTRONICS

30 / 35 page

Power management and housekeeping functions

L4984D

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DocID024474 Rev 1

Power management and 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 7: Feedback

failure detection (FFD)

for more details). Typically this line is used to allow the PWM

controller of the cascaded DC-DC converter to shut down the L4984D 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. Interface circuits like those

are shown in Figure 37. 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 37. Interface circuits that let DC-DC converter controller IC disable the L4984D

Another available function is 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 work in open loop, which 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 to not

decay to zero monotonically. For these reasons it is usually preferable to shut down the unit

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

the VFF pin while the voltage is falling. An 80 mV hysteresis prevents rebounding at input

voltage turn-off.

The soft-start function is performed by PFC_OK enable threshold (VPFC_OK_E) only at

startup, but not on the following activations, to ensure a proper burst-mode operation (as

described in Figure 33). For this reason pin MULT is suggested to be used as remote

ON/OFF control.

In Table 6 it is possible to find a summary of all of the above mentioned working conditions

that cause the device to stop operating.

AM13252v1

L6566A

CC_PFC

L4984D

L6599A

PFC_STOP

L4984D

PFC_OK 6

L6591

PFC_STOP

L4984D

PFC_OK

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