AP3790 datasheet(11/17 Pages) DIODES

11 / 17 page

AP3790

Document number: DS37221 Rev. 3 - 2

11 of 17

www.diodes.com

October 2014

AP3790

A Product Line of

Diodes Incorporated

Operation Principle Description (Cont.)

CCM Protection

The AP3790 is designed to operate in discontinuous conduction mode (DCM) in both CV and CC modes. To avoid operating in continuous

conduction mode (CCM), the AP3790 detects the falling edge of the FB input voltage on each cycle. If a 0V falling edge of FB is not detected,

the AP3790 stops working.

OVP & OCkP

The AP3790 includes output over-voltage protection (OVP) and open circuit protection (OCkP) circuitry. If the voltage at FB pin exceeds 7.5V,

90% above the normal detection voltage, or the 0V falling edge of the FB input can’t be monitored, the AP3790 immediately shuts down and

keeps the internal circuitry enabled to discharge the VCC capacitor to the UVLO turn-off threshold. After that, the device returns to the start

state and a start-up sequence ensues.

Short Circuit Protection (SCP)

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 AP3790 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 8 is the AP3790 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 9, V

OUT is short and the voltage of FB pin is lower than VFB(SCP) over than tSCP, the AP3790

triggers the SCP and enters the hiccup mode.

VTH_ST

VFB(SCP)

tSCP

VCC

VFB

VOUT

VOUT(SCP)

Figure 8. Normal Start-up

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