TLC5921

LED DRIVER

SLLS390 – SEPTEMBER 1999

8

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

setting for constant output current value

output current characteristics shown on Figure 5 for this external resistor value.

Note that more current flows if connect IREF to GND directly.

constant output current operation

When BLANK is low, the corresponding output is turned on if data latch value is 1, and turned off if data latch

value is 0. When BLANK is high, all outputs are forced to turn off. If there is constant current output terminal

left unconnected (includes LED disconnection), it should be lighted on after writing zero to corresponding data

latch to its output. If this operation is not done, supply current through constant current driver will increase.

shift register latch

The shift register latch is configured with 16

× 1 bits. The 1 bit for constant current output data represents ON

for constant current output if data is 1, or OFF if data is 0. The configuration of shift register latch is shown in

below.

(1 bits)

OUT15

Data

XLATCH

(1 bits)

OUT14

Data

(1 bits)

OUT1

Data

(1 bits)

OUT0

Data

Data Latch

16

SOUT

Shift Register

15

2

1

SIN

SCLK

Figure 3. Relationship Between Shift Register and Latch

SOUT output timing selection

By setting level of SOMODE, the SOUT output timing can be changed. When SOMODE is set to low, data is

clocked out to SOUT synchronized on the rising edge of SCLK, and when SOMODE is set to high, data is

clocked out to SOUT synchronized on the falling edge of SCLK. When SOMODE is set to high and shift

operation is done, the data shift error can be prevented even though SCLK signal is externally buffered in serial.

Note that the maximum data transfer rate in cascade operation is slower than that when SMODE is set to low.

TSD (thermal shutdown)

When the junction temperature exceeds the limit, TSD starts to function and turn constant current output off and

XDOWN goes low. Since XDOWN is configured with open-collector output, the outputs of multiple ICs can be

concatenated. To recover from constant current output off-state to normal operation, power supply should be

turned off and then turned on after several seconds.