AP1684A

Document number: DS37095 Rev. 1 - 2

7 of 13

www.diodes.com

August 2014

© Diodes Incorporated

AP1684A

A Product Line of

Diodes Incorporated

Application Information

The AP1684A 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 AP1684A 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

AP1684A

5

7

4

6

3

2

8

Figure 1. Typical BUCK Application Circuit

Components Selection Guide

If the system’s output spec is changed, please refer to the design sheet of the AP1684A and select the compatible system parameter. When the

system needs to be adjusted slightly, please refer to the table below and adjust the value of the related component.

Item

Description

Related Components

IO

LED current

R8

Output Current Ripple

Small current ripple is good for LED life

C4

ton_initial

System initial on time, used to start up the system

R1

Output Open Voltage

Setting the output voltage when the LED is open

R5, R6

Line Compensation

To get a good line regulation

R7, R9

Startup Time

System startup time

R3, C3, T1

EMI

Pass EN 55022 class B with 6DB margin

L1, C1, C2

Design Parameters

Setting the Current Sense Resistor R8

As the AP1684A 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 AP1684A will sense the peak current of the inductance by sensing the voltage dropped on the current sense resistor

R8, and the constant current control is realized by controlling the peak current. In buck structure, when the VO is higher than VIN, no energy will be

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,

Vcs_ref is the reference of the current sense, and the typical value is 1V.

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