AP3108LS

Document number: DS42105 Rev. 3 - 2

11 of 16

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February 2021

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AP3108LS

Operation Description (continued)

S

If Fault Occurs

GATE

Vcc

I_CHG

VST

VCC-UVLO

6V

I CHARGE-L

I CHARGE - H

I CHARGE-FAULT

I CHARGE-H

0mA

0mA

Figure 6

X-CAP Discharge Function

For the higher power application, to attenuate the differential mode noise, an X-CAP is usually used before the rectifier bridge, and there are

paralleled resistors to discharge the X-CAP for safety consideration when the AC line is off. The paralleled resistors have large power dissipation

and will increase the standby power. The AP3108LS integrates an X-CAP discharge function to replace discharge resistors and decreases the

standby power.

This function contains two processes; the first process detects the condition of the AC line through HV pin, this detected voltage is named as Vb.

When the system is plugged in, an inner timer of 40ms within the AP3108LS begins to work, meanwhile, a phase-drifted and filtered signal Vc is

generated based on Vb, compare Vb with Vc as shown in Figure 7.

Whenever signal Vc crosses over with signal Vb, the inner 40ms timer will be reset which represents the AC line is on. If the system is

disconnected from AC line, the cross-over signal of Vc and Vb will disappear and the 40ms timer will continue to count until it reaches 40ms, at this

moment, the second process, discharge process, will come into effect and a 1.7mA discharge current will flow through HV pin to GND lasting for

40ms. After the AC line is off, the first process and the second process will act alternately until the HV Pin voltage is discharged below 10V even

when the VCC voltage is lower than VCC-UVLO.

Vb

Vc

Timer

Reset

IDISCH-X

X-CAP Discharge Current

0mA

40ms

0mA

Figure 7

Built-In Slope Compensation

It is well known that a continuous current mode SMPS may become unstable when the duty cycle exceeds 50%. The built-in slope compensation

in the AP3108LS can keep the system stable.

Built-In VCC LDO

The AP3108LS integrates a VCC LDO circuitry, the LDO regulates the wide range VCC_IN which is rectified from auxiliary winding to an acceptable

value. It makes the AP3108LS a good choice in wide range output voltage application.