AP1685MTR-G1 데이터 시트보기 (PDF) - Diodes Incorporated.

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AP1685MTR-G1

OFFLINE, HIGH PF, HIGH EFFICIENCY LED DRIVER IC

Diodes Incorporated. 

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Diodes Incorporated

AP1685

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:

NL



L  I pk

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 AP1685 adopts constant on-time control method, the AP1685 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

ton_initial  80 R11012 s

To guarantee the system with no overshoot phenomenon, the resistor is selected

R1 

1.25 L

1010 

R8  2Uin _ rms_ max

Valley on Control Method

The valley on function can provide low turn-on switching losses for buck converter. The voltage across the drain and source of the power

MOSFET is reflected by the auxiliary winding of the buck transformer. The voltage is sensed by the FB pin.

FB

AP1685

Document number: DS37311 Rev. 1 - 2

0.1V

1µs

Valley

Figure 2. Valley on Control

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September 2014

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