AP3981D2

Operation Principle Description (cont.)

The final compensated VCS is:

(8)

In the above equations, VPP is 1.8V at CC mode, RLC is 55Ω, R2 is 60kΩ, R1 is two times of R2, VAUX is the value of the auxiliary winding voltage

during primary-on period, which proportional to bus voltage. Based on the equation, it can be concluded that a smaller RFB1 results in deeper line

compensation. If the delay time, tDELAY, is known—typically 150ns in AP3981D2, the RFB1 can be calculated as a reference for the system design.

11. Protection

AP3981D2 provides versatile protections to prevent the system from damage under various fault conditions. Most protections trigger auto-recovery

mode in which the system restarts as soon as the VCC drops to VOPR(MIN). When the fault conditions are removed, the system recovers 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 stops outputting drive signal

immediately, and the system enters auto-recovery mode.

Output Overvoltage Protection (SOVP)

As previously described, the FB pin voltage during tons reflects the output voltage proportionally. This voltage can be used to realize SOVP.

AP3981D2 sets a higher threshold, VFB(OVP), to shutdown the system if the sampled voltage reaches the threshold continuously for three switching

cycles. The SOVP is then triggered, and the system enters auto-recovery mode.

Output Undervoltage Protection (SUVP)

Like SOVP, the AP3981D2 also integrated the SUVP protection. If the detected voltage on FB pin is lower than VFB(SUVP) for 128ms, the SOVP is

triggered, and the system enters 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 is triggered, and the system enters 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 due to transformer saturation, AP3981D2 integrates a maximum VCS threshold VCS(MAX) to protect the system. If there are

three consecutive pulses where VCS exceeds the threshold, the controller shuts down and enters auto-recovery mode.

Overtemperature Protection (OTP)

If the IC junction temperature exceeds the threshold of TOTP, AP3981D2 shuts down immediately and enters auto-recovery mode. Note that even

when the VCC reaches VTH_ST, the IC does not output any drive pulse until the junction temperature falls at a hysteresis temperature of +30°C.

Brown-In/ Brown-Out Protection

AP3981D2 detects the bus voltage at each switching cycle through FB pin during tONP period. When the VCC reaches VTH_ST after power on,

AP3981D2 outputs one switching pulse to check if the detected bus voltage on FB pin is higher than VFB_NEG_H. In this case, the system starts up

normally; otherwise, AP3981D2 stops outputting following pulses. The VCC then drops below VOPR(MIN), and the system repeats 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, and if the

detected voltage is lower than VFB_NEG_L for three consecutive switching cycles, the IC shuts down and enters auto-recovery mode. This function

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

LC

PP

REF

_

CS

V

*

3

2

V

*

3

1

V

+

=

AP3981D2

Document number: DS41421 Rev. 2 - 2

13 of 16

www.diodes.com

May 2019

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