TLC5949 Datasheet, PDF (18/51 Pages) Texas Instruments – 16-Channel, 12-Bit, ES-PWM, Full Self-Diagnosis LED Driver for 7-Bit Global BC LED Lamp

TLC5949
SBVS219A – DECEMBER 2012 – REVISED DECEMBER 2012
DETAILED DESCRIPTION
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MAXIMUM CONSTANT SINK CURRENT VALUE
The maximum output current value of each channel (I_OLCMax) is programmed by a single resistor (R_IREF) that is
placed between the IREF and GND pins. The current value can be calculated by Equation 1:
R=
IREF
V_IREF
I_OLCMax
´ 42.3
Where:
V_IREF = the internal reference voltage on IREF, typically 1.20 V when the global brightness control (BC) data
are at maximum
I_OLCMax = 2 mA to 45 mA with BC = 7Fh
(1)
I_OLCMax is the highest current for each output. Each output sinks I_OLCMax current when it is turned on, and the
global brightness control (BC) data are set to the maximum value of 7Fh (127). Each output sink current can be
reduced by lowering the BC value.
R_IREF must be between 1.13 kΩ and 25.4 kΩ in order to hold I_OLCMax between 45 mA (typ) and 2 mA (typ).
Otherwise, the output may be unstable. Output currents lower than 2 mA can be achieved by setting I_OLCMax to
2 mA or higher and then using global BC to lower the output current.
Table 1 shows the characteristics of the constant-current sink versus the external resistor, R_IREF.
Table 1. Maximum Constant-Current Output versus
External Resistor Value
I_OLCMax (mA)
45
40
35
30
25
20
15
10
5
2
R_IREF (kΩ, typ)
1.13
1.27
1.45
1.70
2.03
2.53
3.38
5.08
10.2
25.4
GLOBAL BRIGHTNESS CONTROL (BC) FUNCTION
The TLC5949 is capable of adjusting the output current of all constant-current outputs simultaneously. This
function is called global brightness control (BC). The global BC for all outputs (OUT0 to OUT15) is programmed
with a 7-bit word. The global BC adjusts all output currents in 128 steps from 25% to 100%, where 100%
corresponds to the maximum output current set by R_IREF. Equation 2 calculates the actual output current as a
function of R_IREF and global BC value. BC data can be set via the serial interface. When the device is powered
on, the BC data in the first and second control data latches contain random data. Therefore, BC data must be
written to the BC data latch before turning the constant-current output on.
The output current value controlled by BC can be calculated by Equation 2.
1 3/4 BC
I_OUTn =
+
4
127
´ I_OLCMax
Where:
I_OLCMax = the maximum constant-current value for each output determined by R_IREF
BC = the global brightness control value in the second control data latch (0h to 7Fh)
(2)
18
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