AP1684A

Document number: DS37095 Rev. 1 - 2

8 of 13

www.diodes.com

August 2014

© Diodes Incorporated

AP1684A

A Product Line of

Diodes Incorporated

Application Information (Cont.)

So, the current sense resistor R8 is determined:

mean

o

ref

cs

I

V

k

R

_

_

8









Transformer Selection (T1)

The non-isolated buck circuit in Figure 1 is usually selected, and the system is operating at boundary conduction mode.

The system’s operating

frequency does not keep constant, the minimum switching frequency at the crest is set as fmin, and then the buck inductance value L can be got:

m in

_

_

_

2

8

)

2

(

f

V

V

V

R

V

V

L

rms

in

ref

cs

o

o

rms

in

















Where,

VO is the output voltage.

Vin_rms is the RMS value of the input voltage.

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:

8

_

R

B

A

V

L

B

A

I

L

N

m

e

ref

cs

m

e

pk

L















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:

d

o

cc

L

aux

V

V

V

N

N







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 AP1684A adopts constant on-time control method, the AP1684A 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

s

R

t

initial

on

12

_

10

1

80









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











10

max

_

_

10

2

8

25

.

1

1

rms

in

U

R

L

R

The system operation mode is determined by R2, to guarantee the system working at BCM mode, resistance R2 is generally selected as R2

≤ R1.