DM133

點晶科技股份有限公司

SILICON TOUCH TECHNOLOGY INC.

16-Bit Constant Current LED Drivers

-11-

Version

：A.024

5)LED Open Detection

The DM133 monitors the whole system, but its alarm mechanism won’t burden the

control system until some fault happens. The ALARM pin is used for both signaling the

thermal alarm and LED disconnection. When ALARM is low, either overheating or LED

disconnection occurs. And when ALARM returns to high, either the LED is re-connected

or the temperature is lowered down. Therefore, ENABLE is used to tell which situation

occurs when ALARM goes low.

Assume ENABLE is low and ALARM is low now. By turning ENABLE high, all the

current outputs are off and hence, LED open detection is disabled. Then, if ALARM is

changed to high, LED disconnection must have occurred. On the other hand, if the

ALARM remains low, then, the IC is obviously overheated. The constant current output

can be turned on sequentially to identify which output’s LED is disconnected. Table 1

shows the detectimg operation and the corresponding occurance.

ENABLE

ALARM

OCCURANCE

Low

High

LED Disconnection

Low

High

Low

Low

Thermal Alarm

Table 1. ALARM pin Output Example

The open-drain ALARM pin will go low when the current output pins are turned on

and below 0.3V. Hence, to prevent the ALARM from going low when LED is in the

normal condition, the supply voltage for LED should be set so that the driver output

voltage goes above 0.4V. Note that it takes 0.2us for the detection function to work

LED Number

1

2

3

4

LED Status

Good

Not-Good

Good

Not-Good

OUTn

On

On

On

On

Detection Result

Good

Not-Good

Good

Not-Good

ALARM

Low (case 2, 4)

LED Number

1

2

3

4

LED Status

Good

Not-Good

Good

Not-Good

OUTn

On

On

Off

Off

Detection Result

Good

Not-Good

Good

Good

ALARM

Low (case 2)

LED Number

1

2

3

4

LED Status

Good

Not-Good

Good

Not-Good

OUTn

Off

Off

Off

Off

Detection Result

Good

Good

Good

Good

ALARM

High