AP1684

Document number: DS36547 Rev. 3 - 2

7 of 13

www.diodes.com

February 2014

© Diodes Incorporated

AP1684

A Product Line of

Diodes Incorporated

Application Information

The AP1684 is designed for single voltage application, and it features high power factor correction (PFC), low total harmonic distortion (THD), low

BOM cost and good EMI performance. The device can be widely used in non-dimmable LED application such as GU10, bulb lamps, down lamp,

etc. The AP1684 adopts constant on time control method within one AC cycle to achieve the high power factor and low THD. The control scheme

is very simple, the power factor correction effectiveness is obvious, and the constant current control is also good enough.

T1

OUT

+

VCC FB

OUT

CS

GND

C3

R7

R5

R6

R4

D1

C4

R3

Q1

AC

Input

F1

VR1

DB1

C2

L1

C1

R8

D2

R1

R2

RI

RM

R9

5

7

4

6

3

2

8

Figure 1. Typical Application Circuit

Design Parameters

Setting the Current Sense Resistor R8

As the AP1684 adopts constant on time control method, the current of the inductance will follow the input voltage to get a sinusoidal wave. The

current sense pin CS of the AP1684 will sense the peak current of the inductance by sensing the voltage dropped on the current sense resistor R8,

transferred from input to output which is called dead zone, and considering the dead zone of buck structure, the output current can be calculated

as below:

8

1

_

_

R

V

k

I

ref

cs

mean

o









Where,

K is the current modification coefficient, and the value of k is approximate to be 0.7.

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, and considering the limit of the BJT’s operating frequency, the minimum switching frequency at the crest is set

m in

_

_

_

2

8

)

2

(

f

V

V

V

R

V

V

L

rms

in

ref

cs

o

o

rms

in

















Where,