VIPER53ESP-E datasheet(15/31 Pages) STMICROELECTRONICS

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VIPer53EDIP - E / VIPer53ESP - E

Short-Circuit and Overload Protection

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Short-Circuit and Overload Protection

A VCOMPovl threshold of about 4.4V 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:

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 2 on page 7. In Figure 7 on page 20 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 10

–

tss

⋅⋅

VDD

810

RST

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

–

⎝⎠

⎛⎞ COVL IDDch2

⋅

DDh yst

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

⋅⋅

⋅

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