AP3108LS

Document number: DS42105 Rev. 3 - 2

10 of 16

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February 2021

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AP3108LS

Operation Description (continued)

tD

Compensated Value

GATE

VGS

IPK_PR

Detected VCSM

Actual VCSM

Figure 4

VDD

HV

VCSM

Sample

Block

Output

Current

Calculation

VCSM

SENSE

DEM

GND

VAUX

Rc

m:1

R1

R2

Rcs

RF

CF

Figure 5

HV Start-Up Circuit

A built-in HV Start-Up circuit in AP3108LS can help to simplify the power system design for ultra low standby application. For AP3108LS, there are

two HV Start-Up charging current: the ICHARGE-L when VCC is lower than 6V and the ICHARGE-H when the VCC voltage rises above 6V, which can

prevent the IC from overheat when VCC short-to-GND fault happens. The HV Start-Up circuit will stop working and has no additional power

dissipation when VCC voltage reaches the VST, at which the AP3108LS starts working and will supply energy to VCC from auxiliary winding.

However, the charging process described above is only for the normal system startup condition. Once some system faults occur and the

protection process is triggered, AP3108LS will shut down and VCC voltage will begin to decrease. The HV Start-Up circuit starts working again

when VCC voltage decreases below VCC-UVLO, and charges the VCC capacitor with current of ICHARGE-FAULT. This special design can reduce hugely

the input power dissipation when system fault happens, especially for output short condition. The HV Start-Up circuit working process is illustrated

in Figure 6.