DETAILED DESCRIPTION

SETTING FOR THE CONSTANT SINK CURRENT VALUE

R

( )=41

W

IREF

´

V

(V)

IREF

I

(mA)

OLC

(1)

GRAYSCALE (GS) CONTROL FUNCTION

TLC5947

Where:

value is shown in Figure 10. Table 1 describes the constant current output versus external resistor value.

Table 1. Constant-Current Output versus External Resistor Value

30

1640

25

1968

20

2460

15

3280

10

4920

5

9840

2

24600

Each constant current sink output OUT0–OUT23 (OUTn) turns on (starts to sink constant current) at the fifth

rising edge of the grayscale internal oscillator clock after the BLANK signal transitions from high to low if the

grayscale data latched into the grayscale data latch are not zero. After turn-on, the number of rising edges of the

internal oscillator is counted by the 12-bit grayscale counter. Each OUTn output is turned off once its

corresponding grayscale data values equal the grayscale counter or the counter reaches 4096d (FFFh). The

PWM control operation is repeated as long as BLANK is low. OUTn is not turned on when BLANK is high. The

timing is shown in Figure 18. All outputs are turned off at the XLAT rising edge. After that, each output is

controlled again from the first clock of the internal oscillator for the next display period, based on the latest

grayscale data.

When the IC is powered on, the data in the grayscale data shift register and latch are not set to default values.

Therefore, grayscale data must be written to the GS latch before turning on the constant current output. BLANK

should be at a high level when powered on to keep the outputs off until valid grayscale data are written to the

latch. This avoids the LED being randomly illuminated immediately after power-up. If having the outputs turn on

at power-up is not a problem for the application, then BLANK does not need to be held high. The grayscale

functions can be controlled directly by grayscale data writing, even though BLANK is connected to GND.

Copyright © 2008, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Link(s): TLC5947