TLC5970 Datasheet, PDF (20/48 Pages) Texas Instruments – 3-Channel, 12-Bit, PWM LED Driver with Buck DC/DC Converter and Differential Signal Interface

TLC5970
SBVS140 – MARCH 2010
www.ti.com
APPLICATION INFORMATION
MAXIMUM CONSTANT SINK CURRENT VALUE
The TLC5970 maximum constant sink current value for each channel, I_OLCMax, is determined by an external
resistor, R_IREF, placed between IREFn and GND. IREFn determines the maximum current of OUTn, where n
represents outputs 0, 1, or 2. The R_IREF resistor value is calculated with Equation 1:
R_IREF (kW) =
V_IREF (V) _{´ 125}
I_OLCMax (mA)
Where:
V_IREF = the internal reference voltage on the IREF pin (1.20 V, typically).
(1)
I_OLCMax is the largest current for each output. Each output sinks the I_OLCMax current when it is turned on, the dot
correction is set to the maximum value of 7Fh (127d), and global brightness control data are 7Fh (127d). Each
output sink current can be reduced by lowering the output dot correction and brightness control values.
R_IREF must be between 1 kΩ (typical) and 15 kΩ (typical) to keep I_OLCMax between 10 mA and 150 mA. The
output may be unstable when I_OLCMax is set lower than 10 mA. Output currents lower than 10 mA can be
achieved by setting I_OLCMax to 10 mA or higher and then using dot correction and global brightness control. The
constant sink current versus external resistor, R_IREF, characteristics are shown in Figure 13 and Table 1.
Table 1. Maximum Constant Current versus External Resistor Value
I_OLCMax (mA)
150
140
120
100
80
60
40
20
10
R_IREF (kΩ, Typical)
1.00
1.07
1.25
1.50
1.88
2.50
3.75
7.50
15.0
DOT CORRECTION (DC) AND GLOBAL BRIGHTNESS CONTROL (BC) FUNCTION (CURRENT
CONTROL)
The TLC5970 has the capability to adjust the output current of each channel (OUT0 to OUT2) individually. This
function is called dot correction (DC). The DC data are seven bits long, which allows each channel output current
to be adjusted in 128 steps from 0% to 100% of the maximum output current, I_OLCMax. The DC data are entered
into the TLC5970 via the serial interface and can be stored into the internal EEPROM. When the IC is powered
on, DC data are automatically loaded into the DC data latch from the EEPROM.
The TLC5970 also has the capability to adjust all output currents at the same time. This function is called global
brightness control (BC). The BC data are seven bits long, which allows all three output channel currents to be
adjusted in 128 steps from 0% to 100% of the maximum output current, I_OLCMax. The BC data are entered into the
TLC5970 via the serial interface. The brightness control data cannot be stored into EEPROM. When IC is
powered on, BC data are automatically set to 7Fh (127d).
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