TLC5946_16 Datasheet, PDF (14/41 Pages) Texas Instruments – 16-Channel, 12-Bit PWM LED Driver with 6-Bit Dot Correction

TLC5946
SLVS824B – MARCH 2008 – REVISED JUNE 2008 .......................................................................................................................................................... www.ti.com
DETAILED DESCRIPTION
SETTING FOR THE MAXIMUM OUTPUT CURRENT VALUE
The maximum output current of each channel (I_OLCMax) is set by a single external resistor (R_IREF), placed between
the IREF pin and the GND pin. The voltage on IREF is made with an internal bandgap, V_IREF, which has a typical
value of 1.20 V. The R_IREF resistor value is calculated by Equation 1:
V_IREF (V)
R_IREF
()
W
=
42.5
´
I_OLCMax (mA)
(1)
Where:
• V_IREF = 1.20 V
• R_IREF = User-selected external resistor
I_OLCMax is the largest current for all outputs. Each output sinks the I_OLCMax current when it is turned on and its dot
correction is set to the maximum value of 3Fh (63d). The sink current for each output can be reduced by
lowering the respective output dot correction value.
R_IREF must be between 1.275 kΩ (typ) and 12.75 kΩ (typ) in order to keep I_OLCMax between 4 mA and 40 mA.
The output current may be unstable if I_OLCMax is less than 4 mA. Output currents lower than 4 mA can be
achieved by setting I_OLCMax to 4 mA or higher and then using dot correction.
Figure 14 illustrates the maximum output current versus R_IREF. R_IREF is the value of the resistor between the
IREF terminal to GND. A variable power supply may be connected to the IREF pin through a resistor to change
the maximum output current per output. The maximum output current is 42.5 times the current flowing out of the
IREF pin.
DOT CORRECTION (DC) FUNCTION
The TLC5946 is able to individually adjust the output current of each channel (OUT0 to OUT15). This function is
called dot correction (DC). The DC function allows the user to individually adjust the brightness and color
deviations of LEDs connected to the outputs OUT0 to OUT15. Each respective channel output current can be
adjusted in 64 steps from 0% to 100% of the maximum output current, I_OLCMax. The dot correction data are
entered into the TLC5946 via the serial interface.
The output current is calculated by Equation 2:
DCn
I_OUTn = I_OLCMax ´ ₆₃
(2)
Where:
• I_OLCMax = the maximum output current of each output
• DCn = the programmed dot correction value of output n (DCn = 0 to 63)
When MODE is high, the input shift register works as a DC shift register. The shift registers and data latches are
each 96 bits in length, and are used to individually adjust the constant current values for each constant current
driver. Each channel can be adjusted from 0% to 100% of the maximum LED current with 6-bit resolution.
Figure 23 illustrates the DC serial data configuration. Figure 12 illustrates the timing chart for writing data into the
shift registers and data latches. Each channel LED current is dot-corrected by the percentage corresponding to
the data in its DC data latch. DC data present on the SIN pin are clocked into the shift register with each rising
edge of the SCLK pin. Data are shifted in MSB first. The data are latched from the shift register into the DC data
latch with a rising edge on the XLAT pin.
The BLANK signal does not need to be high to latch in new data. When XLAT goes high, the new dot-correction
data immediately become valid and change the output currents if the output is on. When the IC is powered on,
the data in the shift register and DC data latch are not set to any default values. Therefore, DC data must be
written to the DC latch before turning on the constant current output.
14
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