AP3776 datasheet(12/17 Pages) DIODES

12 / 17 page

AP3776

Document number: DS36707 Rev. 4 - 2

12 of 17

www.diodes.com

February 2015

AP3776

A Product Line of

Diodes Incorporated

Function Description (Cont.)

VAUX

RFB1

RFB2

Tonp

700k

VCS

OUT

RCS

RLINE

K=0.4

ILINE

Figure 5. Adjustable Line Compensation Circuit

Fixed Cable Compensation

As we know, CPC voltage reflects the power system loading percentage. By introducing a rising voltage, which is linear to CPC voltage, to the

FB reference voltage, AP3776 can realize the fixed cable compensation to match the voltage drop at output cable. The cable compensation is

7% (typ).

Protection

FB Over Voltage Protection (OVP)

When the voltage of FB pin exceeds VFB(OVP) at Tons period, AP3776 immediately shuts down for tOFF(MAX) and then detects the FB voltage

again to see if VFB(OVP) condition is removed. The VCC voltage will drop during the tOFF(MAX). When VCC is still higher than VOPR(Min) after

tOFF(MAX), and VFB(OVP) condition is removed, the IC will enter normal operating mode; but if VFB(OVP) condition is not removed, AP3776 will

remain shut down and wait for another tOFF(MAX). When VCC drops to VOPR(Min) during the tOFF(MAX), AP3776 will enter the restart mode, and

VCC voltage changes between VTH(ST) and VOPR(Min) until VFB(OVP) condition is removed.

Open Circuit Protection (OCKP)

If the down resistance of FB pin short or up resistance open, there will be no voltage on FB pin, so the sample signal

can’t be monitored, and

the OCKP protection will be triggered. The protection operating process is the same as over voltage protection process.

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 tOFF(MAX), the SCP is triggered. Then the AP3776 enters hiccup mode

that the IC immediately shuts down and then restarts, so that the VCC voltage changes between VTH(ST) and VOPR(Min) 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 tOFF(MAX) 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 shows the AP3776 normal start-up waveform. If the voltage of FB pin reaches above VFB(SCP) during tOFF(MAX) after VCC gets to the

VTH(ST), the IC doesn’t enter the SCP mode. Figure 7 shows that VOUT is short and the voltage of FB pin is lower than VFB(SCP) during

tOFF(MAX), then the AP3776 triggers the SCP and enters the hiccup mode.

AP3776 Datasheet(PDF) 12 Page - Diodes Incorporated

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