AP1695-20C 데이터 시트보기 (PDF) - Diodes Incorporated.
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AP1695-20C Datasheet PDF : 15 Pages
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AP1695-20C
Application Information Based on Buck Structure (Cont.)
Design Parameters
Setting the Current Sense Resistor R8
According to the equation of the output current, the current sense resistor R8 is determined:
R8 k Vcs _ ref
Io _ mean
Transformer Selection
The typical non-isolated buck circuit in Figure 1 is usually selected, and the system is operating at boundary conduction mode. The switching
frequency at the crest is set as fmin, the inductance can be calculated as below:
(
L
2 Vin_ rms Vo ) R8 Vo
Vcs _ ref 2 Vin _ rms fmin
Where,
Vo is the output voltage.
Vin _ rms is the RMS value of the input voltage.
According to Ferrari's law of electromagnetic induction, the winding turns number of the buck inductance NL is:
NL
L ipk
Ae Bm
L Vcs _ ref
Ae Bm R8
Where,
Ae is the core effective area.
Bm is the maximum magnetic flux density.
The auxiliary winding is power supply for VCC, the winding turns number Naux is:
N aux
NL
Vcc
Vo Vd
Where,
VCC is the power supply voltage for IC from auxiliary winding.
Vd is the voltage drop of the freewheel diode.
Setting the Initial On Time
As the AP1695-20C adopts constant on time control method, the AP1695-20C will generate an initial on time to start a working cycle. If the initial
on time is longer than the rated on time, overshoot will happen. The initial on time is determined by resister R1 shown in Figure 1.
According to initial on time generation mechanism, the ton_initial is:
ton _initial 80 R11012 s
To guarantee the system with no overshoot phenomenon, the resistor R1 is selected:
AP1695-20C
Document number: DS37538 Rev. 1 - 2
9 of 15
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March 2015
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