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AP3710P-G1

LOW POWER PWM CONTROLLER FOR OFF-LINE ADAPTER

BCD Semiconductor 

Data Sheet

LOW POWER PWM CONTROLLER FOR OFF-LINE ADAPTER

AP3710

Function Description (Continued)

2. VCC/Feedback Control

In Figure 14, an opto-coupler and secondary constant

voltage consists of voltage feedback network. Voltage

feedback loop consists of shunt regulator AZ431 and

FB/VCC terminal of AP3710. When load is heavy, the

voltage on VCC terminal will decrease to enlarge duty

cycle; on the contrary, if load drops, the voltage on

VCC will increase to reduce duty cycle.

5. Current Limit Control

The AP3710 employs current mode control to

improve transient response and voltage stability. In

Figure 3, the external inductor current through the

OUT pin is converted to a voltage by an internal

resistor, and this voltage will participate to control

duty cycle and peak inductor current.

3. Skip Cycle Mode Operation

The AP3710 enters skip cycle mode when load power

drops below a given level, and this is performed by

sensing the VCC voltage level, i.e., the heavier load

power, the lower VCC voltage level. In normal

operation, the VCC terminal indicates a peak

inductance current under certain load power. If the

load power decreases, VCC voltage level increases to

ask for less peak current. When it reaches a

determined value, the IC prevents the current from

decreasing further down and starts to blank the output

pulses, the IC then enters the skip cycle mode

operation. Figure 16 is the sketch for the two operation

mode.

Normal

PWM

Waveform

Cycle Skip

Waveform

0

t

Figure 16. Skip Cycle and Normal PWM Waveform

4. Slope Compensation

The AP3710 is current mode PWM controller, and it

regulates peak inductance current by its current

control loop. It is known that a continuous conduction

mode SMPS may induce noise and harmonic

oscillation on current sense or feedback signal, and

this is especially serious when duty cycle exceeds

50%. The internal slope compensation of AP3710 can

improve power supply stability by increasing the

current slope.

6. Frequency Dithering

Frequency dithering is performed by periodically

spreading a single switching frequency into adjacent

frequency band, so the peak energy is spread. This

technique can improve EMI performance by reducing

both quasi peak and average EMI emissions.

Figure 17. Frequency Dither Influences

the Switching Cycle

Aug. 2008 Rev. 1. 3

BCD Semiconductor Manufacturing Limited

10

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