VIPER53SP-E datasheet(24/36 Pages) STMICROELECTRONICS

24 / 36 page

Short-circuit and overload protection

VIPer53 - E

24/36

Short-circuit and overload protection

A VCOMPovl threshold of about 4.35V has been implemented on the COMP pin. When

VCOMP goes above this level, the capacitor connected on the TOVL pin begins to charge.

When reaching typically VOVLth (4V), the internal MOSFET driver is disabled and the device

stops switching. This state is latched because of to the regulation loop which maintains the

COMP pin voltage above the VCOMPovl threshold. Since the VDD pin does not receive any

more energy from the auxiliary winding, its voltage drops down until it reaches VDDoff and

the device is reset, recharging the VDD capacitor for a new restart cycle. Note: If VCOMP

drops below the VCOMPovl threshold for any reason during the VDD drop, the device

resumes switching immediately.

The device enters an endless restart sequence if the overload or short circuit condition is

maintained. The restart duty cycle DRST is defined as the time ratio for which the device tries

to restart, thus delivering its full power capability to the output. In order to keep the whole

converter in a safe state during this event, DRST must be kept as low as possible, without

compromising the real start-up of the converter. A typical value of about 10% is generally

sufficient. For this purpose, both VDD and TOVL capacitors can be used to satisfy the

following conditions:

Equation 6

Equation 7

Refer to the previous start-up section for the definition of tss, and CVDD must also be

checked against the limit given in this section. The maximum value of the two calculus will

be adopted.

All this behavior can be observed on Figure 8 on page 10. In Figure 10 on page 11 the value

of the drain current Id for VCOMP = VCOMPovl is shown. The corresponding parameter IDmax

is the drain current to take into account for design purposes. Since IDmax represents the

maximum value for which the overload protection is not triggered, it defines the power

capability of the power supply.

OVL

12.5

–

tss

⋅⋅

VDD

810

RST

--------------

–

⎝⎠

⎛⎞

OVL

DDch2

⋅

DDhyst

---------------------------------------

⋅⋅

⋅

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