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AP3700Z-E1 Datasheet(PDF) 7 Page - BCD Semiconductor Manufacturing Limited |
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AP3700Z-E1 Datasheet(HTML) 7 Page - BCD Semiconductor Manufacturing Limited |
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7 / 11 page  LOW POWER PWM CONTROLLER FOR OFF-LINE BATTERY CHARGER AP3700/A Data Sheet 7 BCD Semiconductor Manufacturing Limited Mar. 2008 Rev. 1. 4 Function Description 1. Startup Circuit Figure 3 is the functional block diagram of AP3700/A, and there are 3 external pins: the VCC pin, the OUT pin and GND pin. In typical application shown by Figure 10, the VCC pin is used for both bias supply and feedback control. The OUT pin directly drives external NPN transistor or MOSFET, and also provides initial bias power for UVLO comparator. When the IC works in PWM mode, the auxiliary winding will supply the VCC enough operating current. Figure 8 shows the start-up sequence of the VCC and the VOUT. 2. VCC/Feedback Control An opto-coupler and secondary constant voltage/ current controller consists of voltage feedback network. When load is heavy, the voltage on VCC will be lower to enlarge duty cycle; on the contrary, if load drops, the voltage on VCC will rise to reduce duty cycle. 3. Frequency Dither Frequency dither is performed by periodically spreading a single switching frequency into adjacent frequency band, so the peak energy is spread. This technique can improve EMI by reducing both quasi- peak and average EMI emissions. AP3700/A has reference switching frequency of 60 kHz, and its frequency deviation is ±2.5kHz in period of 2ms. Figure 9 shows the frequency dither influence to the waveform . 4. Current Limit Control The AP3700/A employs current mode control to improve transient response and voltage stability. In Figure 10, 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. CH1: VCC CH2: OUT Figure 8. Start-up Sequence of VCC and VOUT Figure 9. Frequency Dither Influences the Swithcing Cycle |
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