L6983 datasheet(14/63 Pages) STMICROELECTRONICS

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6.4

Light-load operation

The L6983 implements two different light load strategies:

Low consumption mode (LCM)

Low noise mode (LNM)

Please refer to Table 12. Order codes to select the part number with the preferred light load strategy.

6.4.1

Low consumption mode (LCM)

The LCM maximizes the efficiency at light load.

When the switch peak current request is lower than the ISKIP threshold (see Table 6. Electrical characteristics),

the device regulates VOUT by the skip threshold. The minimum voltage is given by:

VOUT, LCM= VFB, LCM∙RPH+RPL

RPL

(1)

Where VFB, LCM is 1.8% (typ.) higher than VFB.

The device interrupts the switching activities when two conditions happen together:

The peak inductor current required is lower than ISKIP

The voltage on the FB pin is higher than VFB, LCM

Figure 7. Light load operation

VCOMP

VCOMP,MIN

VSW

VFB

VFB, LCM

ISKIP

A new switching cycle takes place once the voltage on the FB pin 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:

1) The inductor current sensed by the LS becomes equal to zero

2) The switching period ends up

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 again the HS and starts

a new switching pulse.

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

Eq. (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=∫0TBURSTILtdt

COUT

(2)

L6983

Light-load operation

DS13116 - Rev 1

page 14/63

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