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VIPER25HD Datasheet(PDF) 32 Page - STMicroelectronics

Part No. VIPER25HD
Description  Off-line high voltage converters
Download  40 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER25HD Datasheet(HTML) 32 Page - STMicroelectronics

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Operation description
VIPER25
32/40
Doc ID 15585 Rev 4
7.14
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 37 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 37.
Burst mode timing diagram, light load management
7.15
Brown-out protection
Brown-out protection is a not-latched shutdown function activated when a condition of mains
under voltage is detected. The Brown-out comparator is internally referenced to VBRth
threshold, see Table 8 on page 8, and disables the PWM if the voltage applied at the BR pin
is below this internal reference. Under this condition the power MOSFET is turned off. Until
the Brown out condition is present, the VDD voltage continuously oscillates between the
VDDon and the UVLO thresholds, as shown in the timing diagram of Figure 38 on page 33. A
voltage hysteresis is present to improve the noise immunity.
The switching operation is restarted as the voltage on the pin is above the reference plus the
before said voltage hysteresis. See Figure 5 on page 10.
The Brown-out comparator is provided also with a current hysteresis, IBRhyst. The designer
has to set the rectified input voltage above which the power MOSFET starts switching after
brown out event, VINon, and the rectified input voltage below which the power MOSFET is
switched off, VINoff. Thanks to the IBRhyst, see Table 8 on page 8, these two thresholds can
be set separately.
time
time
time
VCOMP
V
FBbm +VFBbmhys
V
FBbm
I
DD1
I
DD0
IDD
IDRAIN
I
D_BM
Burst Mode


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