L6917B

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imposed by the control loop at the next available clock cycle and the device works in the usual way until another

OCP event is detected.

The device limits the bottom of the inductor current triangular waveform. So the average current delivered can

slightly increase also in Over Current condition since the current ripple increases. In fact, the ON time increases

due to the OFF time rise because of the current has to reach the 140% bottom. The worst-case condition is

when the duty cycle reaches its maximum value (d=75% internally limited). When this happens, the device

works in Constant Current and the output voltage decrease as the load increase. Crossing the UVP threshold

causes the device to latch (FAULT pin is driven high).

Figure 5 shows this working condition

Figure 5. Constant Current operation

It can be observed that the peak current (Ipeak) is greater than the 140% but it can be determined as follow:

The device works in Constant-Current, and the output voltage decreases as the load increase, until the output

off, the FAULT pin is driven high and the device stops working. Cycle the power supply to restart operation.

The maximum average current during the Constant-Current behavior results:

In this particular situation, the switching frequency results reduced. The ON time is the maximum allowed (Ton-

MAX) while the OFF time depends on the application:

the load value, it can be simply considered that, for example, to have on OCP threshold of 170%, this will cor-

TonMAX

TonMAX

140%

Ipeak

Vout

Iout

Inom

UVP

Droop

effect

Ipea k

1.4 I

NOM

V

IN

Vout

MIN

–

L

--------------------------------------- To n

MAX

⋅

+

⋅

=

I

MAX

1.4 I

NOM

2

Ip eak

1.4 I

NOM

⋅

–

2

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

⋅

+

⋅

=

T

OFF

L

Ipe ak

1.4 I

NOM

⋅

–

Vout

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

⋅

=

f

1

To n

MAX

T

OFF

+

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

=