AP3303

Document number:DS41016 Rev. 5 - 2

13 of 16

www.diodes.com

May 2019

© Diodes Incorporated

AP3303

Operation Description (continued)

Externally Triggered Protection

The AP3303 reserves flexible protection mode for power design. The CTRL pin can achieve external programmable protection. A high threshold of

VTH-CTRL-H is set for any over voltage protection. If the CTRL pin voltage is higher than the threshold for 7 switching cycles, the CTRL-High

protection will be triggered. A low threshold of VTH-CTRL-L is usually used for external over temperature protection. To realize the external OTP, a

proper NTC should be connected from the CTRL pin to the ground. An inner current of 100µA flows through the NTC from the CTRL pin. If the

CTRL pin voltage is lower than the VTH-CTRL-L for 32ms duration at least, the CTRL-Low protection will be triggered. Whenever the protection is

triggered, the system will stop the output drive signal and will restart after the VCC voltage falling below the UVLO voltage.

System Protection

LOVP, FOCP, SSCP, VCC OVP, OTP

The AP3303 provides versatile protection to ensure the reliability of the power system. LOVP achieves line voltage overvoltage protection, if the

detected AC line voltage is higher than VLOVP for 7 switching cycles, the LOVP protection will be triggered. FOCP protection is an ultra-fast short-

current protection which is helpful to avoid catastrophic damage of the system when the secondary rectifier is short. The primary peak current will

be monitored by SENSE pin through a primary sense resistor, whenever the sampled voltage reaches the threshold of VTH-FOCP for 7 switching

cycles continuously, the FOCP protection will be activated to shut down the switching pulse. SSCP might be triggered at ultra-low DC bus voltage

condition or other failure condition that short the SENSE pin to ground. The SSCP module senses the voltage across the primary sense resistor

with a delay of 3µs after the rising edge of primary GATE signal, this sensed signal is compared with VTH-SSCP. If it is lower than VTH-SSCP for 7

switching cycles, the SSCP protection will be triggered and the drive signal will be disabled. All these protections described above will restart the

system when the VCC voltage falls below UVLO. Although the external OTP can be easily implemented through CTRL pin, the AP3303 still

reserves the inner OTP with a hysteresis for any necessary use.

VCC Maintain Mode

During light-load or transient-load condition, VFB will drop and be lower than VBURST, thus the PWM drive signal will be stopped, and there is no

energy for transferring to the output. Therefore, the IC VCC supply voltage may decrease to the UVLO threshold voltage and system may enter the

unexpected restart mode. To avoid this, the AP3303 holds a so-called VCC maintain mode which can supply energy to VCC.

When VCC decreases to a setting threshold as VM, the VCC maintain mode will be awaked and a charging current of ICHARGE-H will flow to the VCC

pin. With VCC maintain mode, the VCC is not easy to touch the shutdown threshold during the startup process and transient load condition. This will

also simplify the system design. The minimum VCC voltage is suggested to be designed a little higher than VCC maintain threshold thus can

achieve the best balance between the power loss and step load performance.

Leading-Edge Blanking Time

A narrow spike on the leading edge of the current waveform can usually be observed when the power MOSFET is turned on. A 250ns leading-

edge blank is built-in to prevent the false-trigger caused by the turn-on spike. During this period, the current limit comparator and the PWM

comparator are disabled and the gate driver cannot be switched off.

At the time of turning-off the MOSFET, a negative undershoot (maybe larger than -0.3V) can occur on the SENSE pin. So it is strongly

recommended to add a small RC filter or at least connect a resistor “R” on this pin to protect the IC (Shown as Figure 7).

SENSE

GATE

Large undershoot (more than

-0.3V) may damage the SENSE pin

R

C

Necessary

Figure 7