TLC5902

LED DRIVER

SLLS382 – DECEMBER 1999

14

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

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

By using the upper 3 bits of the brightness control latch, GSCLK can be divided into 1/1 to 1/8. If GSCLK is set

to 8 times the speed (256

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

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

at once and applied to the brightness of day or night. When BC_ENA is pulled low, GSCLK is not divided. When

BC_ENA is pulled high, the brightness can be adjusted as shown in the following table.

CODE

FREQUENCY DIVISION RATIO

RELATIVE BRIGHTNESS RATIO

(%)

MSB 000 LSB†

1/1

12.5

001

1/2

25.0

010

1/3

37.5

011

1/4

50.0

100

1/5

62.5

101

1/6

75.0

110

1/7

87.5

111

1/8

100

† BC_ENA is low.

constant output current operation

The constant current output turns on (sink constant current) if all the gray-scale data latched into the gray-scale

latch is not zero on the falling edge of GCLK, after the next rising edge of GCLK when BLANK goes from high

to low. After that, the number of the falling edge is counted by the 8-bit gray scale counter. Output counted that

corresponds to gray-scale data is turned off (stop to sink constant current). If the shift register for gray scale is

updated during LATCH high, data on the gray-scale data latch is also updated, affecting the number of gray

scale of constant current output. Accordingly, during the on state of constant current output, LATCH is kept to

low and the gray-scale data latch is held. When unconnected constant current output terminals exist, operation

is complete after writing zero (data for LED turn off) to the corresponding gray-scale data latch. If this action is

protection

This device incorporates WDT and TSD functions. In WDT or TSD functions, the current output is stopped (Logic

portion is still operating). By monitoring the DOWN terminal, these failures are detected immediately. Since

DOWN output is configured as an open collector, outputs of multiple ICs are brought together.

WDT

When the scan signal is stopped during a fixed period in dynamic scanning operation, the constant current

output is turned off, preventing the LED from burning damage. The time detected can be set using the external

capacitor (C1). The typical value is approximately 3 ms without a capacitor, 33 ms with a 1000 pF capacitor,

and 300 ms with a 0.01

µF capacitor. Once the scan signal is applied again, the abnormal status is released

and normal operation is resumed. During static operation, the WDT function is disabled, connecting WD_CAP

to GND. The scan signal should be applied to the WDT terminal even though the WDT function is not used.