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In this mode of operation, at light load, the device operates discontinuously based on the COMP pin voltage, in

order to keep the efficiency very high also in these conditions. While the device is not switching the load dis-

charges the output capacitor and the output voltage goes down. When the feedback voltage goes lower than

the internal reference, the COMP pin voltage increases and when an internal threshold is reached, the device

starts to switch. In these conditions the peak current limit is set approximately in the range of 200mA-400mA,

depending on the slope compensation (see related section).

Once the device starts to switch the output capacitor is recharged. The feedback pin increases and, when it

reaches a value slightly higher than the reference voltage, the output of the error amplifier goes down until a

clamp is activated. At this point, the device stops to switch. In this phase, most of the internal circuitries are off,

so reducing the device consumption down to a typical value of 25

µA.

4.1.2 Low Noise Mode

If for noise reasons, the very low frequencies of the low consumption mode are undesirable, the low noise mode

can be selected. In low noise mode, the efficiency is a little bit lower compared with the low consumption mode

in very light load conditions but for medium-high load currents the efficiency values are very similar.

Basically, the device switches with its internal free running frequency of 600KHz. Obviously, in very light load

conditions, the device could skip some cycles in order to keep the output voltage in regulation.

4.1.3 Synchronization

The device can also be synchronized with an external signal from 500KHz up to 1.4MHz.

In this case the low noise mode is automatically selected. The device will eventually skip some cycles in very

light load conditions.

The internal synchronization circuit is inhibited in shortcircuit and overvoltage conditions in order to keep the

protections effective (see relative sections).

4.2 Short Circuit Protection

During the device operation, the inductor current increases during the high side turn on phase and decrease

during the high side turn off phase based on the following equations:

and

Due to this reason a second protection that fixes the maximum inductor valley current has been introduced. This

protection doesn't allow the high side MOSFET to turn on if the current flowing through the inductor is higher

inductor current down to this threshold.

So, the maximum peak current in worst case conditions will be:

4.3 Slope Compensation

∆I

ON

V

IN

V

OUT

–

()

L

----------------------------------- T

ON

⋅

=

∆I

OFF

V

OUT

()

L

------------------- T

OFF

⋅

=

I

PEAK

I

VALLEY

V

IN

L

--------- T

ON_MIN

⋅

+

=