AP3108LS

Document number: DS42105 Rev. 3 - 2

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February 2021

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AP3108LS

Operation Description (continued)

Brown In/Out Protection

To avoid potential high-current stress at low line voltage, the AP3108LS introduces a reliable brownout protection. The AC line voltage is detected

through HV pin. A pair of high-voltage diodes are connected to the AC line which will rectify the AC input voltage to a double-frequency positive

voltage referring to GND, a ~20k

Ω resistor is recommended to be added to improve the surge immunity. When the voltage across HV pin is higher

than VBR-IN for about 100µs of tBR-IN and VCC reaches VST, the GATE pin will output drive signals and the system starts to work. If the HV pin

voltage falls below VBR-OUT and lasts for 50ms of tBR-OUT, the GATE pin will turn off and the system will shut down until the line voltage rises over

its brown-in voltage again.

SOVP/SUVP Protection

The AP3108LS provides output OVP and UVP protection function. The auxiliary winding voltage during secondary rectifier conducting period

reflects the output voltage. A voltage divide network is connected to the auxiliary winding and DEM pin, the DEM pin will detect the equivalent

output voltage with a delay of tSAMPLE from the falling edge of GATE driver signal, as shown in Figure 8. The detected voltage will be compared to

the SOVP and SUVP threshold voltage VTH-SOVP and VTH-SUVP. If the SOVP or SUVP threshold is reached continuously by 7 switching cycles, the

SOVP or SUVP protection will be triggered, the AP3108LS will shut down and the system will restart when the VCC voltage falls below the UVLO

voltage.

To prevent from false-trigger of SUVP during start up process, a blank time of tBLANK-SUVP is set during which the SUVP protection function is

ignored.

Externally Triggered Protection

The AP3108LS reserves flexible protection mode for power design. The CTRL pin can achieve external programmable protection. A high

threshold of VTH-CTRL-H is set for any over voltage protection, the CTRL pin voltage will be sampled with a delay of tSAMPLE from the falling edge of

GATE and compared to VTH-CTRL-H, if the sampled voltage is higher than the threshold for 7 switching cycles, the CTRL-High protection will be

triggered. A low threshold of VTH-CTRL-L is usually used for external over temperature protection. To realize the external OTP, a proper value NTC

should be connected from the CTRL pin to the ground. An inner current of 100µA flows through the NTC from the CTRL pin. The voltage of the

CTRL pin changes along with the resistance of NTC. The AP3108LS will detect the voltage of CTRL pin ahead of the falling edge of GATE with

about 100ns. If the detected voltage is lower than the VTH-CTRL-L for 32ms duration at least, the CTRL-Low protection will be triggered.

Whenever the protection is triggered, the system will stop the output drive signal and will restart after the VCC voltage falling below the UVLO

voltage. The CTRL-High and CTRL-Low protection sample time is illustrated as Figure 8.

GATE

DEM

CTRL

Sample signal for SOVP/SUVP

Sample signal for CTRL_H protection

Sample signal for CTRL_L protection

100ns

2ms

Figure 8