VIPER17_0810 STMICROELECTRONICS [STMicroelectronics], VIPER17_0810 Datasheet - Page 24

• Manufacturer Part Number: VIPER17_0810
• Description: Off-line high voltage converters
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Operation descriptions
7.11
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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, V
power MOSFET is not more allowed to be switched on. It can be switched on again if the
voltage on feedback pin exceeds V
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 V
result of the feedback reaction to the energy delivery stop, the feedback pin voltage
increases and exceeding V
Figure 30
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
V
FB
I
FBbm
DS
shows this behavior called burst mode. Systems alternates period of time where
Normal - mode
Normal - mode
Normal - mode
FBbm
FBbm
threshold MOSFET the power device start switching again.
MOSFET stops switching. After the MOSFET stops, as a
FBbm
. The voltage on PWM comparator non inverting
Burst-mode
Burst-mode
Burst-mode
Normal - mode
Normal - mode
Normal - mode
100
hyster.
hyster.
50 mV
50 mV
VIPER17
FBbm
t
t
t
t
,