VER.A5 2019.07 Page

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VFB voltage falls below 4.55V before the total time reaches TD_PL, then the VFB overvoltage protection will not be triggered. After the duration of the VFB

overvoltage protection sleep mode TSLEEP (i.e. 216TOSC) is reached, the chip will deactivate the VFB overvoltage protection, decrease the VFB voltage with

the help of the soft starter and, successively entering a soft start sequence assuming there is no other protection mode detected.

The VFB overvoltage protection function can also be used to detect protection from overpower (OPP), output short circuit (OSP) and open loop (OLP)

situations because if the VFB voltage increases to a level of 4.55V or higher due to overpower, output short circuit or open loop, the protection is being

activated.

The figure below shows the sequence of VFB and GATE signal in case of an output short circuit condition. T1 indicates the soft start process, VFB

changes with the capacitor charging voltage VSOFT during soft start sequence. T2 denotes VFB overvoltage protection delay time, with GATE signal not yet

forced to be stoped. T3 shows VFB overvoltage protection sleep time duration, after which the chip resets and re-initiates a soft start sequence.T4 is the

initial blanking time, it contains 4 periods of internal clock.

If an output fault condition remains, the above process will repeat itself periodically.

Fig.16 VFB overvoltage protection sequence

Overtemperature Protection

The BOS pin is connected to a NTC thermistor which realizes the overtemperature protection.

The overtemperature protection is enabled when the voltage at the BOS pin is below 2.4V and disabled when the BOS pin voltage is above 2.4V.

RI

V

I

BOS

2



(15)

Current from BOS pin with a voltage greater than 2.4V:

RI

V

I

BOS

2

4

3 



(16)

Current from the BOS pin with a voltage lower than 2.4V: