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VIPER25HD Datasheet(PDF) 19 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) 19 Page - STMicroelectronics

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VIPER25
Operation description
Doc ID 15585 Rev 4
19/40
7.4
Power-down description
At converter power-down, the system loses regulation as soon as the input voltage is so low
that the peak current limitation is reached. The VDD voltage drops and when it falls below
the VDDoff threshold the power MOSFET is switched OFF, the energy transfers to the IC is
interrupted and consequently the VDD voltages decreases, see Figure 23 on page 17. Later,
if the VIN is lower than the threshold VDRAIN_START, the start-up sequence is inhibited and
the power-down completed. This feature is useful to prevent converter’s restart attempts and
ensures monotonic output voltage decay during the system power-down.
7.5
Auto-restart description
If after a converter power-down, the VIN is higher than VDRAIN_START, the power-up
sequence is not inhibited and will be activated only when the VDD voltage drops down the
VDD(RESTART) threshold (reported on Table 7 on page 7). This means that the HV start-up
current generator restarts the VDD capacitor charging only when the VDD voltage drops
below VDD(RESTART). The scenario above described is for instance a power-down because
of a fault condition. After a fault condition, the charging current, IDDch, is reduced to 0.6 mA
instead of 3 mA of the normal power-up converter phase. This feature together with the low
VDD(RESTART) threshold (reported on Table 7 on page 7) ensures that, after a fault, the
restart attempts of the IC has a very long repetition rate and the converter works safely with
extremely low power throughput. The Figure 26 shows the IC behavioral after a short circuit
event.
Figure 26.
Timing diagram: behavior after short circuit
I
DD
V
DD
V
DS
V
DDon
time
Short circuit occurs here
V
DDoff
V
DD(RESTART)
I
DDch (0.6mA)
time
time
time
V
FB
V
FBolp
V
FBlin
T
REPETITION
0.3 x T
REPETITION


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