TLC5911

LED DRIVER

SLLS402 – DECEMBER 1999

21

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

LOD function (LED open detection)

When LEDCHK is low, the LED disconnection detection function is disabled and XDOWN2 goes to a

high-impedance state. When LEDCHK is high, the LED disconnection detection function is enabled, and

XDOWN2 goes low if any LED is disconnected while monitoring the OUTn terminals to be turned on. This

function is operational for sixteen OUTn terminals individually. To determine which constant current output is

disconnected, the level of XDOWN2 is checked 16 times from OUT0 to OUT15 turning one constant current

output on. The power supply voltage should be set so the constant current output applied is above 0.4 V when

the LED is turned on normally. Also, since approximately 1000 ns is required from turning the constant current

output on to XDOWN2 output, the gray scale data to be turned on during that period should be applied.

Table 8 is an example of XDOWN2 output status using four LEDs .

Table 8. XDOWN2 Output Example

LED NUMBER

1

2

3

4

LED STATUS

GOOD

NG

GOOD

NG

OUTn

ON

ON

ON

ON

DETECTION RESULT

GOOD

NG

GOOD

NG

XDOWN2

LOW (by case 2, 4)

LED NUMBER

1

2

3

4

LED STATUS

GOOD

NG

GOOD

NG

OUTn

ON

ON

OFF

OFF

DETECTION RESULT

GOOD

NG

GOOD

GOOD

XDOWN2

LOW (by case 2)

LED NUMBER

1

2

3

4

LED STATUS

GOOD

NG

GOOD

NG

OUTn

OFF

OFF

OFF

OFF

DETECTION RESULT

GOOD

GOOD

GOOD

GOOD

XDOWN2

HIGH-IMPEDANCE

noise reduction

concurrent switching noise reduction

Concurrent switching noise has a potential to occur when multiple outputs turn on or off at the same time. To

prevent this noise, the device has delay output terminals such as XGSOUT, BOUT for GSCLK (gray scale clock),

and BLANK (blanking signal) respectively. By connecting these outputs to the GSCLK and BLANK terminals

of next stage IC, it allows differences in the switching time between ICs. When GSCLK is output to GSOUT

through the device, duty will be changed between input and output. The number of stages to be connected will

be limited depending on the frequency.

delay between constant current output

The constant current output has a delay time of approximately 20 ns between outputs. It means approximately

300 ns delay time exists between OUT0 and OUT15. This time difference by delay is effective for the reduction

of concurrent switching noise.