MPZ2012S221A datasheet(24/56 Pages) STMICROELECTRONICS

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The current limit intervention might affect the duty cycle as well as the frequency. Reaching the peak current

limit implies a duty cycle reduction (until the TON_MIN is reached). When the low-side switch is on, as long as the

current is above the IVY, any new clock cycle is ignored. The switching frequency is hence reduced. These effects

of both duty cycle and switching frequency are summarized in Figure 28.

A short-circuit at the primary output is a particular case of overcurrent. Figure 29 shows the behavior of the device

in case of a short-circuit present before the soft-start.

Figure 29. Soft-start procedure with short-circuit at the primary output

Figure 30 shows a persistent short-circuit condition (frequency reduced from 400 kHz to 100 kHz).

Figure 30. Overcurrent procedure in case of persistent short-circuit at the primary output

Secondary side

The increase of the secondary output current affects the current shape at primary side. In particular, the peak

currents in the high-side and low-side MOSFETs rise (in absolute value in the low-side, that is, the current

becomes more negative), in accordance with equations (Eq. (3)) and (Eq. (4)).

IPEAKHS=IOUT_pri+N∙IOUT_sec+∆IL2

(3)

IPEAKLS=−N∙IOUT_sec∙ 2D1−D −∆IL2+IOUT_prim

(4)

L6983I

Overcurrent protection

DS14040 - Rev 1

page 24/56

MPZ2012S221A Datasheet(PDF) 24 Page - STMicroelectronics