MPZ2012S221A datasheet(33/56 Pages) STMICROELECTRONICS

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9.2

Switching frequency

A resistor connected to the FSW pin features the selection of the switching frequency (refer to

Section 4.1 Frequency selection table).

Connecting the resistor between the pins FSW and VCC, the internal dithering circuit is turned on (see Figure 3).

The selection of the switching frequency must take into account the minimum on-time of the device (TON MIN, as

indicated in the electrical characteristics table), as described by the following equation:

fSW_max≤ VOUT

TONMIN∙VIN∙η

(26)

The switching frequency affects the selection of the primary inductance as well as the transformer construction.

The efficiency also varies with the switching frequency (see Figure 24 and Figure 26).

9.3

Programmable power-up threshold

The Enable rising threshold is equal to 1.2 V typical (refer to Table 6. Electrical characteristics). The power-up

threshold is adjusted according to the following equation:

VPowerUP=1.2∙ 1+REN1REN2

(27)

Figure 40. Programming power-up threshold

VIN

EN/CLKIN

AGND

VIN

GND

L6983I

REN1

REN2

The Enable falling threshold is equal to 1.0 V typical (refer to Table 6. Electrical characteristics). The turn

threshold is obtained according to the following equation:

VPowerDown=1.0∙ 1+REN1REN2

(28)

L6983I

Switching frequency

DS14040 - Rev 1

page 33/56

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