Datasheet AP3917C (Diodes) - 7

AP3917C Operation Description
(continued) The value of R4 can be determined by the following equation: V  V O BP _ R 4 HVON  I BP2
Constant Voltage Operation
The AP3917C can be used in a buck circuit as shown in the typical application circuit. In the beginning of each cycle, the internal integrated MOSFET turns ON when the FB voltages fall below the reference voltage VFB (2.5V). The FB voltage is derived from the sampling capacitor
T
voltage, which can reflect output voltage.
C U D
The ON period time is determined by the inductor current variable value IL, (IL is the gap of the peak-current limitation value IPK and the initial
O
inductor current value IINI), the inductance value, and the input voltage. The ON time calculation is as follows:
R P
I  I  I L PK INI t  L   L  ON V V
W
IN _ DC IN _ DC
E
Where I is zero in DCM status.
N
INI When the inductor current reaches the peak current limitation, the internal MOSFET will turn off. The inductor current charges the sampling capacitor (C3) and the output capacitor (C4) via the freewheeling diode D2 and D3 respectively. In this stage, the sampling capacitor voltage reflects the output voltage. The output voltage can be regulated by sampling the FB voltage. In the MOSFET OFF time, the inductor current decreases linearly from peak current. When the inductor current falls below the output current, the FB voltage begins to decrease with the sampling capacitor voltage. Once the FB voltage is detected below the reference voltage of primary MOSFET turn-on threshold, a new switching cycle starts. The regulated output voltage can be described as the following equation: R  R V  V  ( 1 2 ) O FB R2 Figures 3(a) and 3(b) show the operation diagram under DCM and CCM. Driver Driver I IPK LIMIT O I I INDUCTANCE INDUCTANCE IO IPK LIMIT V V O O V V FB_SAMPLE V FB_SAMPLE V FB=2.5V FB=2.5V Figure 3(a). DCM Figure 3(b).CCM
Startup Control
A three-stage control method is designed for soft start function in startup process. During these stages, the OFF time changes from 60µs to 30µs then from 30µs to 15µs (tMINOFF). Every stage has 128 switching cycles (see Figure 4), but once the output voltage reaches the rated value, the startup process ends no matter what stage it is then AP3917C enters normal operation mode. AP3917C 7 of 11 May 2019 Document number: DS41542 Rev. 2 - 2
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