L6981 datasheet(11/49 Pages) STMICROELECTRONICS

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Figure 7. Light-load operation

A new switching cycle takes place once the voltage on the FB pins becomes lower than VFB,LCM.

The HS switch is kept on until the inductor current reaches ISKIP.

Once the current on the HS reaches the defined value, the device turns the HS off and turns the LS on. The LS is

kept enabled until one of the following conditions occurs:

The inductor current sensed by the LS becomes equal to zero.

The switching period ends.

If, at the end of the switching cycle, the voltage on the FB pin rises over the VFB,LCM threshold, the LS is kept

enabled until the inductor current becomes equal to zero. Otherwise, the device turns on the HS again and starts

a new switching pulse.

During the burst pulse, if the energy transferred to COUT increases the VFB level over the threshold defined in

Equation 1, the device interrupts the switching activities. The new cycle takes place only when VFB becomes

lower than the defined threshold. Otherwise, as soon as the LS is turned off, the HS is turned on.

Given the energy stored in the inductor during a burst, the voltage ripple depends on the capacitor value:

VOUT RIPPLE= ∆QILCOUT=∫0TBURSTILt dt

COUT

(2)

Figure 8. LCM operation with ISKIP = 350 mA typ. at zero load. L = 33 μH; COUT = 32 μF

L6981

Light-load operation

DS13554 - Rev 1

page 11/49

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