DM133 datasheet(14/20 Pages) SITI

14 / 20 page

DM133

點晶科技股份有限公司

SILICON TOUCH TECHNOLOGY INC.

16-Bit Constant Current LED Drivers

-13-

Version

：A.024

LED Short Detection

Example:

Consider the following conditions: LED’s (R, G, B) Vf variations are between 1.7V to

4.0V (R:1.7~2.4V, G/B: 3.0~4.0V), VLED=5V, ILED =20mA (R), 15mA (G) and 10mA (B).

In order to detect the short LED, we need to add four components in the LED drive

system: a demultiplexer (SWnormal/open, Swshort) and three Rshorts . And assume the Vsw

is 0.1V in all current condition. The algorithm for detecting the short LED is based on the

forward voltage of the LED in different conditions (normal/short/open circuit).

First, we need to decide the value of Rshort. Two inequalities are follows:

VLED－Vsw－ILED×Rshort－Vf ﹤0.3V

(LED is OK or Open)

(1)

VLED－Vsw－ILED×Rshort

＞0.4V

(LED is Short)

(2)

Let’s take the B LED as an example. From (1), Rshort ＞160Ω (Vf=3.0V) and Rshort ＞

60Ω (Vf=4.0V). Then we choose Rshort ＞160Ω. From (2), we can calculate the Rshort ﹤

450Ω. Finally, we take a proper Rshort ＝ (160+450)/2 ＝ 305Ω. Follow the same steps,

we can get 203Ω for G LED. Note: as ILED is larger than 15mA (ex. ILED =20mA (R)), we

still put the 15mA in the above two equations. Then we’ll get the 247Ω for R LED.

Turn on the SWshort switches one after one. Then follow the LED open detection

procedure; turn on only one LED in the scan process. The Valarm will be “L” as LED is

normal or open. Once the LED is short, The Valarm will be changed to “H”.

Alarm

DM 1 3 3

Alarm

DM 1 3 3

Alarm

DM 1 3 3

Alarm

DM 1 3 3

Alarm

DM 1 3 3

Alarm

DM 1 3 3

R short1

ILED1

R short2

ILED2

R short3

ILED3

V LED

D em u ltip lex er

V sw

DM133 Datasheet(PDF) 14 Page - Silicon Touch Technology Inc.

DM133 Datasheet(HTML) 14 Page - Silicon Touch Technology Inc.

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL