AP3301 datasheet(11/16 Pages) DIODES

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AP3301

Document number: DS38018 Rev. 4 - 2

11 of 16

www.diodes.com

June 2016

AP3301

Operation Description (Cont.)

Figure 2

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

can improve the stability, so there is no need for design engineer to spend much time on that.

FB Pin and Short Circuit Protection

This pin is normally connected to the opto-coupler and always paralleled with a capacitor for loop compensation. When the voltage at this pin is

greater than VFB-OLP and lasts for about tDELAY-OLP, the IC will enter the protection mode. For AP3301, the system will enter hiccup mode to wait

the VCC decreasing to low UVLO level, then the IC will try to restart until the failure removed. And when this voltage is less than 1.55V, the IC will

stop the drive pulse immediately. Therefore, this feature can be used for short circuit protection, which makes the system immune from damage.

Normally, output short makes the VFB value to the maximum because the opto-coupler is cut off.

When VCC decreases to a setting threshold, the VCC maintain comparator will output some drive signal to make the system switch and provide a

proper energy to VCC pin. The VCC maintain function will cooperate with the PWM and burst mode loop which can make the output voltage

variation be within the regulation. This mode is very useful for reducing startup resistor loss and achieving a better standby performance with a

low value VCC capacitor. The VCC is not easy to touch the shutdown threshold during the startup process and step load. 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 standby and step load performance.

Brown-in/Brown-out

During turning-on interval of primary power MOSFET, the DEM pin is clamped to GND, the current flows through RDEM and pin DEM reflecting

the rectified value of AC line voltage as showed as Figure 3, this current multiplies with RBNO represents the sample voltage of AC line voltage,

take the peak value of the sample voltage to compare with the internal fixed threshold.

When the AC source is on, the IC will send several drive signals to confirm that the DEM pin current is higher than IBNI during primary MOSFET

turning on time, then AP3301 starts to provide drive signal, and if the current decreases below IBNO for about 50ms, AP3301 enters brown out

protection and no drive pulse output, a new startup will not begin until the sampled signal is lower than IBNI again and VCC is higher than VUVLO

threshold.

SENSE

GATE

GND

DEM

AP3301

VCC

Large undershoot (more than

-0.3V) may damage the SENSE pin

Necessary

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