AP1684 Datasheet, PDF (8/13 Pages) Diodes Incorporated – AC/DC, HIGH PF, HIGH EFFICIENCY LED DRIVER CONTROLLER

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AP1684
Application Information (Cont.)
The next step is determining the transformer’s winding turns number, the worst case operation condition of transformer is at the peak voltage area
of sine waveform input voltage where the current of across the inductance is the maximum value. The transformer design should be based on the
worst case operation condition to guarantee that the transformer is not saturated. 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,
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

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 AP1684 adopts constant on-time control method, the AP1684 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 which could damage the LED. And a good system performance does not permit overshoot, so
the appropriate initial on time should be guaranteed. And 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 is selected
R1 
1.25 L
10¹⁰ 
R8  2U _{in _ rms _ max}
The system operation mode is determined by R2, to guarantee the system working at BCM mode, resistance R2 is generally selected as R2≤R1.
Valley on Control Method
The valley on function can provide low turn-on switching losses for buck converter. The voltage across the collector and emitter of the BJT is
reflected by the auxiliary winding of the buck transformer. The voltage is sensed by the FB pin.
FB
AP1684
Document number: DS36547 Rev. 3 - 2
0.1V
1µs
Valley
Figure 2. Valley on Control
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February 2014
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