TLC5949PWPR datasheet(18/51 Pages) TI

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127

3/4 BC

OUTn

´ OLCMax

IREF

OLCMax

IREF

´ 42.3

TLC5949

SBVS219A – DECEMBER 2012 – REVISED DECEMBER 2012

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DETAILED DESCRIPTION

MAXIMUM CONSTANT SINK CURRENT VALUE

The maximum output current value of each channel (IOLCMax) is programmed by a single resistor (RIREF) that is

placed between the IREF and GND pins. The current value can be calculated by Equation 1:

Where:

VIREF = the internal reference voltage on IREF, typically 1.20 V when the global brightness control (BC) data

are at maximum

IOLCMax = 2 mA to 45 mA with BC = 7Fh

(1)

IOLCMax is the highest current for each output. Each output sinks IOLCMax 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.

RIREF must be between 1.13 kΩ and 25.4 kΩ in order to hold IOLCMax 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 IOLCMax 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, RIREF.

Table 1. Maximum Constant-Current Output versus

External Resistor Value

IOLCMax (mA)

RIREF (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 RIREF. Equation 2 calculates the actual output current as a

function of RIREF 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.

Where:

IOLCMax = the maximum constant-current value for each output determined by RIREF

BC = the global brightness control value in the second control data latch (0h to 7Fh)

(2)

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