AP1685

Document number: DS37311 Rev. 1 - 2

8 of 14

www.diodes.com

September 2014

© Diodes Incorporated

AP1685

A Product Line of

Diodes Incorporated

Application Information

The AP1685 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 AP1685 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

Drain

CS

GND

U1 AP1685

C3

R7

R5

R6

R4

D1

C4

R2

AC

Input

F1

VR1

DB1

C2

L1

C1

R8

D2

R1

RI

S

R9

Figure 1. Typical Application Circuit

Design Parameters

Setting the Current Sense Resistor R8

As the AP1685 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 AP1685 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

m in

_

_

_

2

8

)

2

(

f

V

V

V

R

V

V

L

rms

in

ref

cs

o

o

rms

in

















Where,