TLC5904

LED DRIVER

SLLS391 – NOVEMBER 1999

19

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

noise reduction

concurrent switching noise reduction

The 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 and 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, and the number of stages to be

connected will be limited depending on frequency.

output slope

When the output current is 80 mA, the time to change constant current output to turnon and turnoff is

approximately 150 ns and 250 ns respectively. This allows reduced concurrent switching noise when multiple

outputs turn on or off at the same time.

delay between constant current output

The constant current output has a delay time of approximately 30 ns between outputs. This means

approximately 450 ns delay time exists between OUT0 and OUT15. This time differences by delay allows

reduced concurrent switching noise as well as the output slope previously described. This delay time has the

same value at the 8 bits or 16 bits operation mode.

power supply

The followings should be taken into consideration:

1) VCCLOG, VCCANA, and VCCLED should be supplied by a single power supply to minimize voltage

differences between these terminals.

2) The bypass capacitor should be located between the power supply and GND to eliminate the variation of

power supply voltage.

GND

Although GNDLOG, GNDANA, and GNDLED are internally tied together, these terminals should be externally

connected to reduce noise influence.

thermal pad

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

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

better thermal conductivity.