AP3987CT

Document number: DS41644 Rev. 2 - 2

11 of 15

www.diodes.com

June 2019

© Diodes Incorporated

AP3987CT

Operation Description (continued)

When FB open circuit protection, transformer saturation protection and current sense resistor fault (short or open) protection are triggered the

device enters the fast recovery mode. The only difference with auto recovery is that the VCC discharge current is kept operating current ICC_OPR

and the system can fast restart.

When internal or external over temperature protection is activated, the device enters the hold mode. Once the hold mode is triggered, the

AP3987CT doesn

’t signal any pulse until the fault condition is removed, and VCC Voltage is hold not lower than LVCC voltage.

11.2 Short Circuit Protection (SCP)

The Short Circuit Protection (SCP) detection principle is similar to the normal output voltage feedback detection by sensing FB pin voltage. When

the detected FB pin voltage is below VFB(SCP) for a duration of about tSCP, the SCP is triggered. Then the AP3987CT enters hiccup mode that the

IC immediately shuts down and then restarts, so that the VCC voltage changes between VTH_ST and UVLO threshold until VFB(SCP) condition is

removed.

As to the normal system startup, the time duration of FB pin voltage below VFB(SCP) should be less than tSCP to avoid entering SCP mode. But for

the output short condition or the output voltage below a certain level, the SCP mode should happen.

Figure 6 is the AP3987CT normal start-up waveform that the voltage of FB pin is above VFB(SCP) during tSCP after VCC gets to the VTH_ST, which

doesn

’t enter the SCP mode. As shown in Figure 7, VOUT is short and the voltage of FB pin is lower than VFB(SCP) during tSCP, the AP3987CT

triggers the SCP and enters hiccup mode.

VTH_ST

VFB(SCP)

tSCP

VCC

VFB

VOUT

VOUT(SCP)

5V

Figure 6. Normal Start-up

VTH_ST

VOPR(MIN)

VFB(SCP)

tSCP

VCC

VFB

VOUT

0V

Figure 7. Short Circuit Protection (SCP) and Hiccup Mode

11.3 Transformer Saturation Protection via Primary Peak Current Limitation

When the transformer saturation happens, the voltage of EM pin (internal pin) will increase promptly and be over the reference voltage VEM(MAX). If

two consecutive pulses exceed the value, the device shuts down and enters into auto-recovery mode.