AP1684

Document number: DS36547 Rev. 3 - 2

8 of 13

www.diodes.com

February 2014

© Diodes Incorporated

AP1684

A Product Line of

Diodes Incorporated

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

8

_

R

B

A

V

L

B

A

I

L

N

m

e

ref

cs

m

e

pk

L















Where,

d

o

cc

L

aux

V

V

V

N

N







Where,

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.

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.

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.

0.1V

1µs

Valley

FB

Figure 2. Valley on Control