AN2115 datasheet(14/33 Pages) STMICROELECTRONICS

14 / 33 page

Operation description

AN2115

14/33

Doc ID 11165 Rev 5

4.4

Short circuit protection

Due to the peak current mode architecture, the peak current flowing through the high side

switch is accurately sensed.

When this current reaches the peak current limit threshold, the P-channel MOSFET is

turned off. In this way, the ON time of the high side switch, TON, is reduced and the output

voltage decreases. The TON can decrease down to its minimum value of around 200 nsec

(TMIN). In this condition, however, a strong overload or short circuit could result in a further

increase in peak current.

Equation 4

Equation 5

It can be observed in the equations above that in short circuit condition the output voltage is

zero and thus the negative slope will be zero. Since the positive slope increases with every

cycle, the inductor current will likewise increase cycle by cycle.

In order to determine at what point this phenomenon will cease, some real parameters

should be considered.

Figure 10.

Equivalent buck converter circuit (during ON time)

Figure 11.

Equivalent buck converter circuit (during OFF time)

Considering the figures above, particularly during OFF time, even though the output voltage

is equal to zero, the output current generates the voltage drop necessary to produce a

negative slope on the parasitic resistances.

ΔI

out

–

()

------------------------------

•

(Positive slope)

ΔI

OFF

out

-----------

OFF

•

(Negative slope)

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