VIPER25HDTR datasheet(19/40 Pages) STMICROELECTRONICS

19 / 40 page

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

DDon

time

Short circuit occurs here

DDoff

DD(RESTART)

DDch (0.6mA)

time

FBolp

FBlin

REPETITION

0.3 x T

REPETITION

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