AN2870 datasheet(11/41 Pages) STMICROELECTRONICS

11 / 41 page

AN2870

Device blocks description

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3.2.3

Current comparator and choke saturation detection

The current comparator senses the voltage across the current sense resistor (Rpfccs) and,

by comparing it with the programming signal delivered by the multiplier, determines the

exact time when the external MOSFET has to be switched off.

When PFC MOSFET is turned on, parasitic drain capacitances are discharged and an

intense current spike can be seen by PFCCS (Figure 5). In past solutions, an RC filter

between sense resistor and current sense input was commonly used to reject these spikes,

but it introduced a delay between the instant the current crosses the threshold and the

actual activation of internal comparator. This delay may cause the inductor saturation, then

an over dimensioned inductor had to be used. In L6585DE, an internal leading edge

blanking structure (LEB) masks the first 200 ns of the PFC gate at the time current spikes

occurs; the filter is no longer necessary and the inductor can be smaller and lighter. On the

other hand this LEB limits the maximum available “ON time”.

Moreover, the device is provided with a second comparator on the PFC current sense pin

that turns off immediately the PFC MOSFET if the voltage on the pin, normally limited within

1.0 V, exceeds 1.7 V. A current peak limiting control is therefore achieved avoiding MOSFET

overheating in case of boost inductor’s hard saturation. In this case the current up-slope

becomes so large (50-100 times steeper) that during the current sense propagation delay

the current may reach abnormally high values.

Figure 5.

PFCCS pin waveforms

AN2870 Datasheet(PDF) 11 Page - STMicroelectronics