AP3303

Document number:DS41016 Rev. 5 - 2

11 of 16

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May 2019

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AP3303

Operation Description (continued)

Constant Over Current Protection

Cycle-by-cycle current limit is a popular method to achieve output over current protection. Actually, the turn-off delay of the MOSFET and the

higher switching frequency always result in the higher OCP current at high line voltage. To obtain a constant OCP current value with universal

input voltage, AP3303 adopts an effective line compensation circuitry. The function block is illustrated in Figure 4. The current IDEM which reflects

line voltage is scaled down and inversed to IL_OPP within AP3303, this IL_OPP flows through the inner compensation resistor ROPP and an

external filtering resistor RF, and then the final line compensation voltage is formed as:

Where VS is the sense voltage of RS

As above formula indicates, changing the compensation voltage at different line voltage is a good way to balance the OCP current. In a real

system, usually keep the

value fixed (220

kΩ is recommended). To change the line compensation voltage, a good solution is to change

.

Whenever the RF is changed, adjust the CF at the same time to offer an enough RC time to filter the spike on SENSE pin.

DEM

Line Voltage

Detector

Auxiliary

RDEM

RDOWN

ROPP

OCP

VREF1

21

HV

Gate

Rs

RF

CF

SENSE

SOVP

2.6V

SUVP

0.475V

IDEM

Vs

Figure 4

HV Start-Up Circuit

A built-in HV Start-Up circuit in AP3303 can help to simplify the power system design for ultra low standby application. For AP3303, there are two

HV Start-Up charging currents: the ICHARGE-L when VCC is lower than 3V and the ICHARGE-H when the VCC voltage rises above 3V, which can

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

dissipation when VCC voltage reaches the VST, then the AP3303 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, AP3303 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 the input

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

5.