L6564T datasheet(24/33 Pages) STMICROELECTRONICS

24 / 33 page

Application information

L6564T

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Doc ID 022671 Rev 1

However, in some applications such as AC-DC adapters, where the PFC pre-regulator is

turned off at light load for energy saving reasons, even a well-designed boost inductor may

occasionally slightly saturate when the PFC stage is restarted because of a larger load

demand. This happens when the restart occurs at an unfavorable line voltage phase, i.e.

when the output voltage is significantly below the rectified peak voltage. As a result, in the

boost inductor the inrush current coming from the bridge rectifier adds up to the switched

current and, furthermore, there is little or no voltage available for demagnetization.

To cope with a saturated inductor, the L6564T is provided with a second comparator on the

current sense pin (CS, pin 4) that stops the IC if the voltage, normally limited within 1.1 V,

exceeds 1.7 V. After that, the IC is attempted to restart by the internal starter circuitry; the

starter repetition time is twice the nominal value to guarantee lower stress for the inductor

and boost diode. Hence, the system safety is considerably increased.

Figure 32.

Effect of boost inductor saturation on the MOSFET current and detection method

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