TLC5905

LED DRIVER

SLLS401 – NOVEMBER 1999

15

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

brightness control function

By writing data into the brightness control latch, current on all constant current outputs can be adjusted to control

the variation of brightness between ICs and the division ratio for the gray scale clock can be set to control the

variation of brightness for the total panel system.

output current adjustment on all constant current outputs – brightness adjustment between ICs

By using the lower 5 bits of the brightness control latch, output current can be adjusted to 32 steps as 1 step

of 1.6% current ratio between 100% and 51.6% when the output current is set to 100% of an external resistor.

By using this function, the brightness control between modules (ICs) can be adjusted, sending the desired data

externally even if ICs are mounted on a print-circuit board. When BCENA is pulled low, the output current is set

to 100%.

Table 3. Relative Current Ratio For Total Constant Current Output

CODE

CURRENT RATIO%

20 (mA)

80 (mA)

VIREF (TYP)

MSB 00000 LSB

51.6

10.3

41.3

0.63

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

11110

98.4

19.3

78.7

1.22

11111†

100

20.0

80.0

1.24

† BCENA is low.

frequency division ratio setting for gray scale clock – panel brightness adjustment

By using the upper 3 bits of the brightness control latch, the gray scale clock can be divided into 1/1 to 1/8. If

the gray scale clock is set to 8 times the speed (256

× 8 = 2048) of frequency during horizontal scanning time,

the brightness can be adjusted to 8 steps by selecting the frequency division ratio. Therefore, the total panel

brightness can be adjusted at once, and applied to the brightness of day or night. When BCENA is pulled low,

the gray scale clock is not divided. When BCENA is pulled high, the brightness can be adjusted as shown in

Table 4.

Table 4. Relative Brightness Ratio For Total Constant Current Output

CODE

FREQUENCY

DIVISION RATIO

RELATIVE BRIGHTNESS RATIO

(%)

MSB 000 LSB†

1/1

12.5

.

.

.

.

.

.

.

.

.

.

.

.

110

1/7

87.5

111

1/8

100

† BCENA is low.

OVM (output voltage monitor) function

By writing data into the OVM latch, the comparison voltage for voltage comparator of OUT0 to OUT15 can be

set and the output signal for XDOWN1 and XDOWN2 can be checked.