VIPER17LDTR datasheet(18/33 Pages) STMICROELECTRONICS

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Operation descriptions

VIPER17

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7.4

Power down operation

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 (8 V typical) the power MOSFET is switched OFF, the energy transfers

to the IC interrupted and consequently the VDD voltages decreases, Figure 23 on page 17.

Later, if the VIN is lower than VDRAIN_START (80 V typical), 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 operation

If after a converter power down, the VIN is higher than VDRAIN_START, the start up sequence

is not inhibited and will be activated only when the VDD voltage drops down the VDDrestart

threshold (4.5 V typical). This means that the HV start up current generator restarts the VDD

capacitor charging only when the VDD voltage drops below VDDrestart. The scenario above

described is for instance a power down because of a fault condition. After a fault condition,

the charging current is 0.6 mA (typ.) instead of the 3 mA (typ.) of a normal start up converter

phase. This feature together with the low VDDrestart threshold (4.5 V) 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 25 shows the IC behavioral after a

short circuit event.

Figure 25. Timing diagram: behavior after short circuit

7.6

Oscillator

The switching frequency is internally fixed to 60 kHz or 115 kHz. In both case the switching

frequency is modulated by approximately ±4 kHz (60 kHz version) or ±8 kHz

(115 kHz version) at 250 Hz (typical) rate, so that the resulting spread-spectrum action

distributes the energy of each harmonic of the switching frequency over a number of side-

band harmonics having the same energy on the whole but smaller amplitudes.

VDD

VDS

DDON

DDOFF

DDrest

IDD_CH

0.6 mA

Short circuit occurs here

Trep

< 0.03Trep

FB Pin

4.8 V

3.3 V

VDD

VDS

DDON

DDOFF

DDrest

IDD_CH

0.6 mA

Short circuit occurs here

Trep

< 0.03Trep

FB Pin

4.8 V

3.3 V

IDD_CH

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