AP3981C

Document number: DS41619 Rev. 2 - 2

13 of 16

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June 2019

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AP3981C

Operation Description (continued)

The final compensated VCS is:

(8)

In the above formulas, VPP is 1.8V at CC mode, RLC is 55, R2 is 60k, R1 is two times of R2, VAUX is the auxiliary winding voltage during

primary-on period, which is proportional to bus voltage. Based on the formula, we can make a conclusion that a smaller RFB1 results in deeper

line compensation. If we know the delay time tDELAY, typically 150ns in AP3981C, we can calculate the RFB1 as a reference for the system design.

11. Protection

The AP3981C 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.

Output Over Voltage Protection (SOVP)

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

The AP3981C set a higher threshold, VFB(OVP), to shut down the system if the sampled voltage reached the threshold continuously for 3 switching

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

Output Under Voltage Protection (SUVP)

Like SOVP, the AP3981C 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 AP3981C integrates 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, AP3981C 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 AP3981C detects the bus voltage at each switching cycle through FB pin during tONP period. When the VCC reaches VTH_ST after power on,

the AP3981C 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 the AP3981C 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 suddenly released after power off.

LC

PP

REF

_

CS

V

*

3

2

V

*

3

1

V

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