L4984D datasheet(22/35 Pages) STMICROELECTRONICS

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Feedback failure detection (FFD)

L4984D

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DocID024474 Rev 1

Feedback failure detection (FFD)

The OVP function handles “normal” overvoltage conditions, i.e. those resulting from an

abrupt load/line change or occurring at startup. If the overvoltage is generated by a

feedback failure, for instance when the upper resistor of the output divider (R1) fails open,

eventually the error amplifier output (COMP) saturates high and the voltage on its inverting

input (INV) drops from its steady-sate value (2.5 V). An additional comparator monitors the

voltage on the INV pin, comparing it against a reference located at 1.66 V. When the voltage

on pin PFC_OK exceeds 2.5 V and, simultaneously, that on the INV pin falls below 1.66 V,

the FFD function is triggered: the gate drive activity is immediately stopped, the device is

shut down and its quiescent consumption reduced. This condition is latched and to restart

the IC it is necessary to recycle the input power, so that the VCC voltage goes below 6 V.

The pin PFC_OK doubles its function as a not-latched IC disable: a voltage below 0.23 V

shuts down the IC, reducing its consumption below 2 mA. To restart, simply let the voltage

on the pin go above 0.27 V. Note that these functions offer complete protection against not

only feedback loop failures or erroneous settings, but also against a failure of the protection

itself. Either resistor of the PFC_OK divider failing short or open or a pin PFC_OK floating

results in shutting down the IC and stopping the pre-regulator.

L4984D датащи(PDF) 22 Page - STMicroelectronics