AN4149 datasheet(25/43 Pages) STMICROELECTRONICS

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AN4149

L4984D biasing circuitry

Equation 73

By selecting a commercial value of 56 k

Ω:

Equation 74

Three resistors 3.3 M

Ω in series provide the calculated value for the PFC_OK high resistor.

Notice that both feedback dividers connected to the L4984D pin #1 (INV) and pin #6

(PFC_OK) can be selected without any constraints. The unique criterion is that both dividers

have to sink a current from an output bus which needs to be significantly higher than the

current biasing the error amplifier and the PFC_OK comparator. The OVP function handles

"normal" overvoltage conditions, i.e. those resulting from an abrupt load/line change or

occurring at startup. In case 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) will saturate high and the voltage on its inverting input (INV) will drop from its

steady-sate value (2.5 V).

An additional comparator monitors the voltage on pin INV, comparing it against a reference

located at 1.66 V. When the voltage on pin PFC_OK exceeds 2.5 V and, simultaneously, the

voltage on pin INV 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 in order to restart the L4984D it is necessary to recycle the input

power, so that the Vcc voltage falls below 6 V (VCCrestart).

The pin PFC_OK doubles its function as a non-latched IC disable: a voltage below 0.23 V

shuts down the L4984D, reducing its consumption below 2.2 mA. To restart simply let the

voltage on the pin rise 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 will result in shutting down the L4984D and stopping the pre-regulator.

divider

PFC

REF













−

⋅

PFC

REF

MAX

OUT













−

⋅

623

430

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