AP1695 Datasheet, PDF (9/15 Pages) Diodes Incorporated – OFFLINE, HIGH PF, HIGH EFFICIENCY DIMMABLE LED DRIVER IC

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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}
  I_{o _ mean}
AP1695
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 V_{in_ rms} V_o )  R8 V_o
V_{cs _ ref}  2 V_{in _ rms}  f_min
Where,
V_o is the output voltage.
V_{in _ 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:
N_L

L i_pk
A_e  B_m

L V_{cs _ ref}
A_e  B_m  R8
Where,
A_e is the core effective area.
B_m is the maximum magnetic flux density.
The auxiliary winding is power supply for VCC, the winding turns number Naux is:
N _aux

N_L
 ^Vcc
V_o  V_d
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 adopts constant on time control method, the AP1695 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:
t_{on_initial}  80 R110^12 s
To guarantee the system with no overshoot phenomenon, the resistor R1 is selected:
AP1695
Document number: DS37312 Rev. 1 - 2
9 of 15
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June 2014
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