VIPER35 datasheet(23/44 Pages) STMICROELECTRONICS

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DocID026980 Rev 4

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VIPer35

Operation description

8.4

Power-down description

At converter power-down, the system loses its ability to regulate as soon as the decreasing

input voltage is so low to reach the peak current limitation. VDD voltage drops and when it

falls below VDDoff threshold (see Table 7) the power MOSFET switches off, the energy is

interrupted, VDD voltage decreases, the start-up sequence is inhibited and the power-down

is completed. This feature prevents any restart attempt and ensures a monotonic output

voltage decay during the system power-down.

8.5

Auto-restart description

Every time a protection is tripped, the IC automatically restarts after a duration depending

on the discharge and recharge of CVDD capacitor. As shown in Figure 31, after a fault, the IC

stops and VDD voltage decreases because of IC consumption. As soon as VDD voltage falls

below VDD(RESTART) threshold and if the DC input voltage is higher than VDRAIN_START

threshold, the internal HV current source turns on and it starts to charge CVDD capacitor with

the current IDDch2 (0.6 mA, typ.). As soon as VDD voltage reaches VDD(ON) threshold, the IC

restarts.

Figure 31. Timing diagram: behavior after short-circuit

8.6

Quasi-resonant operation (QR)

The control core of the VIPER35 is a current mode PWM controller with a zero-current

detect circuit designed for quasi-resonant (QR) operation, a technique whose benefits are:

minimum turn-on losses, low EMI emission and safe behavior in case of short-circuit. At

heavy load the converter operates in quasi-resonant mode; operation synchronizes

MOSFET turn-on to the transformer demagnetization by detecting the resulting negative-

going edge of the voltage across any winding of the transformer. The system works close to

the boundary between discontinuous (DCM) and continuous conduction (CCM) of the

transformer and as a result, the switching frequency is different according to different

line/load conditions. See the hyperbolic-like portion reported in Figure 32.

VIPER35 数据表(PDF) 23 Page - STMicroelectronics