TLC5947 Datasheet, PDF (13/27 Pages) Texas Instruments – 24-Channel, 12-Bit PWM LED Driver with

TLC5947
www.ti.com ................................................................................................................................................. SBVS114A – JULY 2008 – REVISED SEPTEMBER 2008
DETAILED DESCRIPTION
SETTING FOR THE CONSTANT SINK CURRENT VALUE
The constant current value for all channels is set by an external resistor (R_IREF) placed between IREF and GND.
The resistor (R_IREF) value is calculated by Equation 1.
R_IREF (W) = 41 ´
V_IREF (V)
I_OLC (mA)
(1)
Where:
V_IREF = the internal reference voltage on the IREF pin (typically 1.20 V).
I_OLC must be set in the range of 2 mA to 30 mA. The constant sink current characteristic for the external resistor
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
I_OLC (mA, Typical)
30
25
20
15
10
5
2
R_IREF (Ω)
1640
1968
2460
3280
4920
9840
24600
GRAYSCALE (GS) CONTROL FUNCTION
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
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