AP39811

Document number: DS39992 Rev. 5 - 2

12 of 18

www.diodes.com

June 2019

© Diodes Incorporated

AP39811

Operation Description (continued)

Figure 5: VCPR and VFB Versus IO

5.

Constant Current Control

In AP39811, Formula 8 shows the related parameters that determine the output current. To get a constant output current, the VCS and tONS/tSW is

fixed in AP39811 during CC mode. Meanwhile, reliable control logic is integrated within AP39811 to ensure the system swift smoothly between CC

mode and CV/SV mode.

(8)

6.

Multiple Segment Peak Current

In the original PFM PSR system, the switching frequency decreases with the decreasing output current, which will encounter audible noise issue

when switching frequency decreases below 20kHz.

In order to avoid audible noise issue, the AP39811 uses 3-segment primary peak current control method at CV and SV mode, the current sense

threshold voltage is piecewise defined. As shown in Figure 6, the low threshold VCS_L is set under 3% CC load, the high threshold VCS_H is set

above 33% CC load. Within the range from 3% to 33%, the threshold VCS_M increases basing on the load condition, the VCS_M is carefully

calculated inside the AP39811 to make the system operate at a constant switching frequency which is higher than 20kHz.

As for the special IV curve, the maximum switching frequency occurs at the slanted voltage mode. During this mode, the VCS reaches its

maximum threshold VCS_H, so the frequency varies only with the input power, carefully select the primary inductance to get a reasonable

frequency feature.

IO

VCPR

VFB

2.4V

Inflection point

Range A

Range B

0

CC

SW

ONS

SW

ONS

OUT

t

t

*

Rcs

Vcs

*

Ns

Np

*

2

1

t

t

*

Ipk

*

Ns

Np

*

2

1

I



