STCMB1 datasheet(21/59 Pages) STMICROELECTRONICS

21 / 59 page

DocID028958 Rev 4

21/59

STCMB1

Application information

It is possible to prove that, when choosing:

Equation 6

the average value of the inductor current in a switching cycle will be:

Equation 7

(z)

The block diagram and the relevant waveforms of the ECOT-controlled TM PFC are shown

in Figure 9 and Figure 10.

On the one hand, a negative threshold is placed on the current sensing pin ISEN_PFC

(VISEN_PFC_Z = -25 mV, typ.): the on-time generator (ramp at pin TON) starts as the

(negative) voltage on the pin ISEN_PFC goes below VISEN_PFC_Z. This is equivalent to

having ILth = VISEN_PFC_Z/Rs, where Rs is the current sensing resistor.

On the other hand, when the threshold VISEN_PFC_Z is crossed by the rising signal on the

pin ISEN_PFC at the end of the off-time, an internal generator (Ios = 50

A typ.) is turned

on, which sources the current out of the pin ISEN_PFC. This becomes a positive offset

voltage on the pin itself, because of the external series resistor Ros (part of the RC filter of

the pin). In this way, the crossing of the threshold VISEN_PFC_Z by the negative-going signal

can be further delayed and the equivalent current threshold ILth set at:

Equation 8

The value of the series resistor Ros can be adjusted to find the optimum ILth. As a starting

point, one can select Ros based on the theoretical compensation condition:

Equation 9

Note in Figure 10 the effect of this offset in the region VISEN_PFC > VISENPFC_Z (green

portion of the inductor current waveform), compared to the unaffected waveform that would

occur with Ros = 0 (black portion of the same waveform in the previous cycle).

drain

out

Lth 

























sin

drain

Lpk

Lvy

PFC

ISEN

Lth





PFC

ISEN

drain

out





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