AP3981B

Document number: DS41422 Rev. 2 - 2

13 of 17

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May 2019

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AP3981B

Operation Principle Description (continued)

11. Protection

The AP3981B provides versatile protections to prevent the system from damage under various fault conditions. Most protections will trigger

auto-recovery mode in which the system will restart as soon as the VCC drops to VOPR(MIN), when the fault conditions are removed, the system

will recover to normal operation automatically.

VCC OVP

A VCC OVP threshold is set to protect the IC from damage. When the VCC OVP protection is triggered, the IC will stop outputting drive signal

immediately and the system will enter auto-recovery mode.

Secondary-Side Over Voltage Protection (SOVP)

As it described above that the FB pin voltage during tONS reflects the output voltage proportionally, this voltage can be used to realize SOVP.

The AP3981B set a higher threshold, VFB(OVP), to shut down the system if the sampled voltage reaches the threshold continuously for 3

switching cycles, the SOVP will be triggered and the system will enter auto-recovery mode.

Secondary-Side Under Voltage Protection (SUVP)

Like SOVP, the AP3981B also integrates the SUVP protection. If the detected voltage on FB pin is lower than VFB(SUVP) for 128ms, the SOVP

will be triggered and the system will enter auto-recovery mode.

Output Short Protection (SCP)

A much lower threshold is set on FB pin to protect the system when output short condition occurs. If the detected FB voltage is lower than 0.78V

for 64ms, the SCP will be triggered and the system will enter auto-recovery mode.

Transformer Anti-Saturation Protection

Under some fault conditions or bad system design, the transformer may approach saturation and the current increases dramatically. To avoid

power device damage since of transformer saturation, the AP3981B integrated a maximum VCS threshold VCS(MAX) to protect the system. If

there are 3 consecutive pulses where VCS exceeds the threshold, the controller will shut down and enter auto-recovery mode.

Over Temperature Protection (OTP)

If the IC junction temperature exceeds the threshold of TOTP, the AP3981B will shut down immediately and enter auto-recovery mode. Note that

even when the VCC reaches VTH_ST, the IC will not output any drive pulse until the junction temperature falls of a hysteresis temperature of

+30°C.

Brown In / Brown Out Protection

The AP3981B detects the bus voltage at each switching cycle through FB pin during tONP period. When the VCC reaches VTH_ST after power-on,

the AP3981B will output one switching pulse to check if the detected bus voltage on FB pin is higher than VFB_NEG_H. If so, the system will start

up normally, otherwise AP3981B will stop outputting following pulses, then the VCC will drop below VOPR(MIN) and the system will repeat the

process described above until the detected FB voltage is higher than VFB_NEG_H. When the power is off or there is a ditch in bus voltage, if the

detected voltage is lower than VFB_NEG_L for 3 consecutive switching cycles, the IC will shut down and enter auto-recovery mode. This function

is very useful when the bulk capacitor is open, or when the heavy load is suddenly released after power off.