VIPER28LN datasheet(23/32 Pages) STMICROELECTRONICS

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VIPER28

Operation descriptions

Doc ID 15028 Rev 5

23/32

7.11

Burst-mode operation at no load or very light load

When the load decrease the feedback loop reacts lowering the feedback pin voltage. If it

falls down the burst mode threshold, VFBbm, the power MOSFET is not more allowed to be

switched on. After the MOSFET stops, as a result of the feedback reaction to the energy

delivery stop, the feedback pin voltage increases and exceeding the level, VFBbm +

VFBbmhys, the power MOSFET starts switching again. The burst mode thresholds are

reported on

Table 8 and Figure 26 shows this behavior. Systems alternates period of time

where power MOSFET is switching to period of time where power MOSFET is not switching;

this device working mode is the burst mode. The power delivered to output during switching

periods exceeds the load power demands; the excess of power is balanced from not

switching period where no power is processed. The advantage of burst mode operation is

an average switching frequency much lower then the normal operation working frequency,

up to some hundred of hertz, minimizing all frequency related losses. During the burst-mode

the drain current peak is clamped to the level, ID_BM, reported on Table 8.

Figure 26.

Burst mode timing diagram, light load management

time

VCOMP

VFBbm +VFBbmhys

VFBbm

IDD1

IDD0

IDD

IDRAIN

ID_BM

Burst Mode

VIPER28LN Datasheet(PDF) 23 Page - STMicroelectronics