VIPER17HD datasheet(24/33 Pages) STMICROELECTRONICS

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Operation descriptions

VIPER17

24/33

7.11

Burst-mode operation at no load or very light load

When the voltage on feedback pin falls down 50 mV below the burst mode threshold, VFBbm,

power MOSFET is not more allowed to be switched on. It can be switched on again if the

voltage on feedback pin exceeds VFBbm. The voltage on PWM comparator non inverting

internal input, connected to feedback pin through a resistive voltage divider, is lower

clamped to a certain value leading to a minimum value, of 90 mA (typ.) for the drain peak

current.

When the load decrease the feedback loop reacts lowering the feedback pin voltage. As the

voltage goes 50 mV below VFBbm MOSFET stops switching. After the MOSFET stops, as a

result of the feedback reaction to the energy delivery stop, the feedback pin voltage

increases and exceeding VFBbm threshold MOSFET the power device start switching again.

Figure 30

shows this behavior called burst mode. Systems alternates period of time where

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

Figure 30. Burst mode timing diagram, light load management

I DS

VFBbm

50 mV

hyster.

Burst-mode

Normal-mode

50 mV

hyster.

Burst-mode

Normal-mode

100

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