TLC5921

LED DRIVER

SLLS390 – SEPTEMBER 1999

9

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

LOD function (LED open detection)

If any terminal voltage of constant current output (OUT0 TO 15) to be turned on is approximately below 0.3 V,

XDOWN output goes low during output on by knowing LED disconnection. This function is operational for

sixteen OUTn individually. To know which constant current output is disconnected, the level of XDOWN is

repeatedly checked 16 times from OUT0 to OUT15 turning one constant current output on. The power supply

voltage for LED should be set to that the constant current output is applied to above 0.4 V to prevent from

XDOWN low when LED is lighting on normally. Note that on-time should be minimum1

µs after the constant

current output is turned on since XDOWN output is required approximately 1

µs.

As discussed earlier, XDOWN is used for both TSD and LOD function. Therefore, BLANK is used to know which

one of TSD or LOD worked when XDOWN went low at LED disconnection, that is, in this condition, when set

BLANK to high, all the constant current outputs are turned off and LOD disconnection detection is disabled, then,

if XDOWN was changed to high, LED disconnection must be occurred.

Table 1 is an example for XDOWN output status using four LEDs.

Table 1. XDOWN 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

XDOWN

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

XDOWN

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 : output slope

When output current is 80 mA, the time to change constant current output to turn-on and turn-off is approximately

150 ns and 250 ns respectively. This allows to reduce concurrent switching noise occurred when multiple

outputs turn or off at the same time.

thermal pad

The thermal pad should be connected to GND to eliminate the noise influence since it is connected to the bottom

side of IC chip. Also, desired thermal effect will be obtained by connecting this pad to the PCB pattern with better

thermal conductivity.