3700EZ-E1 データシートの表示（PDF） - BCD Semiconductor
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3700EZ-E1 Datasheet PDF : 12 Pages
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Data Sheet
LOW POWER PWM CONTROLLER FOR OFF-LINE BATTERY CHARGER
Function Description
1. Startup Circuit
Figure 3 is the functional block diagram of AP3700E,
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.
AP3700E
Figure 9 shows the start-up sequence of the VCC and
the VOUT.
Figure 10. Frequency Dithering Influences
the Swithcing Cycle
CH1: VCC
CH2: OUT
Figure 9. Start-up Sequence of VCC and 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.
AP3700E has reference switching frequency of 60
kHz, and its frequency deviation is ±2.5kHz in period
of 2ms. Figure 10 shows the frequency dithering influ-
ence to the waveform .
4. Current Limit Control
The AP3700E employs current mode control to
improve transient response and voltage stability. In
Figure 11, 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. 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 by reducing both quasi-
peak and average EMI emissions.
May 2008 Rev. 1. 1
BCD Semiconductor Manufacturing Limited
8
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