AP39811

Document number: DS39992 Rev. 5 - 2

13 of 18

www.diodes.com

June 2019

© Diodes Incorporated

AP39811

Operation Description (continued)

Figure 6: VCS and fS Versus Io

7.

Leading Edge Blanking

When the power switch is turned on, a turn-on spike will occur on the VCS sense resistance. To avoid false-termination of the switching pulse, a

300ns leading-edge blanking is built in. During this blanking period, the current sense comparator is disabled and the primary MOSFET cannot be

turned off.

8.

Valley Turn-on

When the off time (tOFF) is shorter than tVAL-ON, the AP39811 power system can work with valley turn-on. It can reduce the switching-on power

losses and achieve high overall efficiency. At the same time, because of valley turn-on the switching frequency has the random jitter feature,

which will be benefitial to conductive EMI performance. And valley turn-on can also reduce the power switch turn-on spike current and then

achieve the better radioactive EMI performance.

9.

VCS Jitter

Even though the valley turn-on function produces the random frequency jitter feature, an active frequency jitter function is added in the AP39811.

The active frequency dithering is realized by applying variation on VCS reference (VCS_REF). The VCS_REF is changed every 2 cycles and the period

of variation is 12 cycles, which is shown as Figure 7. The variation between VCS4 and VCS1 is +/-3% using the mean level as a reference.

Figure 7: VCS Jitter

10. Adjustable Line Compensation

In real system, there exists a delay time, from the VSOURCE reach the inner VCS threshold to the actual switch turn-off point. The delay time

contains the propagation time of the inner comparator and the driver delay, and it does not change with line voltage. The delay time leads to

different primary peak current under different line voltage, which results in different output current in CC mode.

VCS1

VCS2

VCS3

VCS4

VCS3

VCS2

VCS1

VCS2

VCS3

VCS4

VCS3

VCS2

VCS1

Mean Level

AM

FM

FM

kHz

100%

33%

3%

Vo

VCS_H

20

VCS_L

IO/IO_CC