Datasheet AP3917C (Diodes) - 6

AP3917C Operation Description Overall Introduction
The AP3917C is a universal AC input step-down power switcher. Peak current and switching frequency reduce as the load decreases, so the device can get excellent efficiency performance at light load and improve the overall system efficiency. Coordinating with an external single- winding inductor can achieve a low BOM cost solution.
VBP Waveform and ON/OFF Control
The AP3917C control circuit power supply voltage VBP is charged by the internal high-voltage regulator. When the BP voltage is charged to
T
VBP_HVOFF (8.5V), the IC starts up, and the internal high-voltage regulator is turned off. When the BP voltage drops below VBP_HVON (8.2V), the
C
internal high-voltage regulator turns on again to charge the external BP capacitor.
U D O R
When fault conditions happen, such as some protections like overload faults, short-circuit faults, over temperature faults, and open-loop faults,
P
the AP3917C stops switching. Afterwards an internal current source IBP_LATCH discharges the external BP capacitor. The internal high-voltage regulator will not turn on again until the voltage on BP capacitor drops below VBP_RESTART (4.5V). The restart time interval is proportional to the
W
capacitance of external BP capacitor—the larger capacitance of the external BP capacitor, the longer restart time.
E N
The restart time after a fault is about V  V V  V t  C  ( BP _ FAULT BP _ RESTART BP _ HVOFF BP _ RESTART  ) RESTART BP I I BP _ LATCH HV Where: V is actual voltage value of BP pin at the time of fault, which is between V and V BP _ FAULT BP _ HVON BP _ HVOFF Figure 1 shows the typical waveform of VBP. VBP_FAULT VBP_HVOFF=8.5V VBP VBP_HVON=8.2V V
BP
_
RESTART
=4.5V ON ON ON ON HV REGULATOR OFF OFF OFF Figure 1. VBP Waveform and HV Regulator ON/OFF Status
Auxiliary VBP Supply
If the output voltage is higher than the voltage of VBP_HVON, an auxiliary VBP supply can be implemented to reduce overall power consumption by connecting a resistor (R4) between C2 and C3. A standby power of less than 30mW can be achieved especially in a no-load condition. Figure 2 shows the low standby power circuit with the auxiliary VBP supply. R 4 R1 D2 L1 C3 C 2 R2 L2 FR BP S VO+ L FB S R C1 C5 NC D3 C4 R3 Drain NC AP3917C VO- BD1 Figure 2. Low Standby Power Circuit With Auxiliary VBP Supply AP3917C 6 of 11 May 2019 Document number: DS41542 Rev. 2 - 2
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