AP39811

Document number: DS39992 Rev. 5 - 2

14 of 18

www.diodes.com

June 2019

© Diodes Incorporated

AP39811

Operation Description (continued)

RFB1

RFB2

FB

PFM

VAUX

VPP

VCS_ref

R1

R2

RS

RLC

VLC

Inside

AP39811

Figure 8: Line Compensation Control Circuit

In order to alleviate the difference under the universal input voltage, the AP39811 integrates the line compensation control circuit shown as

Figure 8. During the primary on stage, an inner switcher RS switches on, the RLC is much smaller compared with RFB2, so the RFB2’s effect on

line compensation can be neglected, the proportional line voltage is detected through RFB1 and RLC, and is added to the VCS threshold (VCS_REF).

The VLC can be derived from the Formula 9:

(9)

The final compensated VCS is :

(10)

In the above formulas, VPP is 1.8V at CC mode, RLC is 60, R2 is 60k, R1 is two times of R2, VAUX is the auxiliary winding voltage during

primary-on period, which is proportional to bus voltage. Based on the formula, we can make a conclusion that a smaller RFB1 results in deeper

line compensation. If we know the delay time, tDELAY, typically 150ns in AP39811, we can calculate the RFB1 as a reference for the system

design.

11. Protection

The AP39811 provides versatile protections to prevent the system from damage under various fault conditions. Most protections will trigger auto-

recovery mode in which the system will restart as soon as the VCC drops to VOPR(MIN), when the fault conditions are removed, the system will

recover 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 will stop output drive signal

immediately and the system will enter auto-recovery mode.

Output Over Voltage Protection (SOVP)

As it described above that the FB pin voltage during tONS reflects the output voltage proportionally, this voltage can be used to realize SOVP. The

AP39811 set a higher threshold, VFB(OVP), to shut down the system if the sampled voltage reached the threshold continuously for 3 switching

cycles, the SOVP will be triggered and the system will enter auto-recovery mode.

LC

1

FB

1

FB

AUX

PP

1

FB

LC

R

R

2

R

*

3

R

1

V

V

*

2

R

*

3

R

V









LC

PP

REF

_

CS

V

*

3

2

V

*

3

1

V



