VIPER27 datasheet(17/31 Pages) STMICROELECTRONICS

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VIPER27

Operation descriptions

Doc ID 15133 Rev 5

17/31

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 (see Table 7 on page 6) the power MOSFET is switched OFF, the

energy transfers to the IC interrupted and consequently the VDD voltages decreases,

Figure 23 on page 16. Later, if the VIN is lower than VDRAIN_START (see Table 7 on page 6),

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

VDD(RESTART) threshold (see Table 7 on page 6). 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 0.6 mA (typ.) instead of

the 3 mA (typ.) of a normal start up converter phase. This feature together with the low

VDD(RESTART) threshold 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.

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