Lech GRODZKI
Bialystok Technical University - Faculty of Electrical Engineering
Constant-current driving of the RGB LED
Abstract. LED RGB driving circuit should allow to stabilise the chosen colour of light according to the changing working conditions. Usually it is done
by the PWM control of the monochrome diode components. Some new integrated circuits widen the methods of LED control. Those ICs, working
under processor control, realise the constant-current supplying of LED diodes. The paper contains the description of main properties of such
integrated circuits and some results of realised research works on their application features.
Streszczenie. Uk ady steruj ce diodami LED RGB powinny umo liwia stabilizowanie barwy emitowanego wiat a przy zmieniaj cych si
warunkach zewn trznych. Zwykle wykorzystuje si do tego sygna y PWM. Nowoczesne uk ady scalone umo liwiaj zastosowanie tak e sterowania
sta opr dowego. Artyku zawiera prezentacj wybranych uk adów tego typu, ich cech u ytkowych i w a ciwo ci aplikacyjnych.
Ci g o-pr dowe sterowanie diod LED RGB
Keywords: constant-current LED RGB driving, LED group working, microcontrollers
S owa kluczowe: sta opr dowe sterowanie diod LED RGB, praca grupowa diod LED, mikrokontrolery
LED RGB driving
individual switching of output channels;
The main aims of LED RGB driving circuit are the
IC supply voltage range: 3,0-5,5 V;
controlling the value of luminance flux and its colour. They
LED device supply voltage up to 17 V, what allows to
are two basic features of LED RGB diode, as a light source
control LED connected in series;
illuminating architectural elements, art objects, etc. To
serial synchronous SPI interface, with speed up to
achieve the changes of these parameters, the
30 MHz, allowing supervising controller to control the
monochromatic components of RGB diode should be
work of IC driver;
controlled. The main electrical factors deciding about the
input of common blanking all the output channels;
work of RGB diode are forward currents flowing by
the possibility of cascading IC devices on SPI bus.
component diodes. By controlling of these currents we can:
Except the common features, presented ICs have some
change the ratios between monochromatic luminance
differences each other, as it is shown in table 1.
fluxes R, G, B to change the colour of resultant flux;
change total luminous flux by simultaneous changes
Table 1. Selected feature differences of TLC5922/23/24
forward currents IFR, IFG and IFB in component diodes;
Feature TLC5922 TLC5923 TLC5924
correct the influences on brightness and luminance
operation temperature range -20÷85°C -40÷85°C -40÷85°C
colour such environment factors as ambient temperature
LED open detection and
- + +
or ageing the LED diode as a semiconductor devices. indication
over-temperature indication - + +
Very popular method of LED luminance control is
maximum LED supply
applying the pulse width modulation. It is based on human
17 V 17 V 16 V
voltage ULEDMAX
eyesight sense features: integrating light pulses and limited
pre-charge FET circuit - - +
fast event notification. By appropriate selecting the duty
cycle coefficients it is possible to control total luminance flux
Autonomous work of IC drivers
as the sum of monochrome component fluxes:
Presented IC drivers are able to work autonomous,
without supervising microcontroller, but in that case they
(1) R G B
would drive the LEDs with a stable forward current, the
same for all output channels. Then, the only method of
The same principle is used to control the LED RGB colour -
changing the forward currents is to change external resistor
the changes of ratios between component luminance fluxes
RIREF. That discrete element is placed between the power
R, G, B result in the colour changes.
ground and built-in band gap with a typical value of 1,24 V.
PWM controlled LED RGB drivers can be built using
The maximum output current per channel can be calculated
analog circuits [1], microcontrollers [2] and even FPGA
by formula 2.
devices [3]. Unquestionable disadvantage of described
above method is the necessity of simultaneous, multi-
1,24V 40
channel generating rectangle waveforms with suitable high
(2) IOUTMAX
RIREF
frequency and adjustable duty cycle coefficients. Therefore,
the appearance of new solutions in driving LED devices is
worthy of notice. The value of RIREF should not be smaller than 600 . The
above relation is illustrated by figure 1.
Constant-current IC drivers - general characteristic The second way of controlling the LED devices,
Texas Instruments offers a several ICs, dedicated to connected to autonomous working IC driver, is the usage of
constant-current driving of 16 LED diodes, with separate input terminal BLANK, which high logic level switches off
forward current correction. There are the chips: TLC5922, simultaneously all output channels.
TLC5923 and TLC5924. They have some common features In addition, TLC5923/24 devices have output signal
[4, 5, 6], like: XERR, informing about detection of their malfunction, like
open LED circuit or over-temperature condition of IC.
16 output channels, each with controlled current source
TLC5924 chip has also built-in pre-charge FET circuit,
with the current range 0-80 mA and accuracy Ä…1%;
that enables an improvement in image quality of the
7-bit current correction coefficient, separately for each
dynamic-drive LED display. That circuit uses LED supply
channel;
200 PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010
voltage ULED provided to selected terminal of driver 112-bit word, called dot-correction word, containing
package. If the current-carrying capacity of single output sixteen 7-bit length coefficients DCX, programming each of
channel, equal 80 mA, is insufficient, there is a possibility of the 16 output current IOUTX (Fig.3b).
parallel connecting several outputs. That solution is For distinguishing those both transmissions the
recommended by manufacturer in one of the application additional control input terminal, called MODE, is used.
notes [7]. In that case the resultant maximum current of Each transfer ends with the pulse on latching input XLAT.
paralleled outputs is calculated by formula: The rising edge on XLAT input causes storing the serial
transferred data in appropriate internal register. Moreover,
(3) IOUTN N IOUTMAX TLC5923 and TLC5924 devices allow read out 16-bit error
word. It depends on reading the states of LED open
detection circuits in output channels, during falling edge of
IOUTMAX [mA]
XLAT signal into internal shifting register. That shifting
register can be read via serial data output SOUT using
sixteen clocking pulses on SCK input. According to SPI
standard, the simultaneous writing of switching word and
reading of error word is possible, too.
a)
OUT15 OUT14 OUT13 . . . OUT2 OUT1 OUT0
b)
. . . . . . . . .
DC15.6 DC15.0 DC14.6 DC1.0 DC0.7 DC0.0
Fig.3. Control words for TLC5922/23/24 devices:
a - 16-bit switching word; b - 112-bit dot correction word with
coefficients DCX
RIREF [ ]
500 1000 1500 2000 2500 3000
The open-drain (OD) output XERR is used to report
Fig.1. The relation between the value of external resistor RIREF and
detected malfunctions in TLC5923/24 chips. The circuit-
maximum output current IOUTMAX
break at least in one off enabled output channels or over-
temperature condition of IC structure pulls down that output.
Because presented IC drivers have 16 current outputs
Open-drain configuration allows to connect many such
each, then to drive POWER LED RGB, it is possible to
outputs to a single line, to inform the microcontroller about
group outputs into 3 groups, with 5 paralleled outputs per
working errors of many LED drivers. This line can be either
each group. In such circuit, the maximum forward current of
periodically checked by microcontroller or used as its
monochromatic components would be equal 400 mA.
external interrupt.
Especially useful, using supervising microcontroller, is
Co-operation with microprocessor system
the possibility of individual output current adjustment. Each
The proper application circuit of TLLC5922/23/24
of the 16 channel has its own 7-bit coefficient word DCX,
devices uses the connection with supervising
which value determines the actual output current of it. The
microcontroller. Allowable supply voltage of ICs (3,3V - 5V)
current adjustment range is from 0 mA up to IOUTMAX,
lets for collaboration with either older or up-to-date
defined by external resistor RIREF (formula 2). That
microcontrollers. The example circuit, with AVR controller,
adjustment range is divided into 128 equal steps. It means,
is shown in figure 2.
that if IOUTMAX is equal 80 mA the value of adjustment step is

equal 625 A. This is a satisfied value for precise control of
ULED
RGB LED component currents. By appropriate changes of
coefficients DCX, we can control the LED brightness with
simultaneous stabilising the ordered colour of luminance.
Because LED devices are non-linear semiconductor
elements, brightness control can't be achieved by simple
SIN SOUT
scaling of DCX coefficients for monochrome components. In
SCLK VCC UCC
that case additional calculations, using working
MODE
100nF
characteristics = f(IF) for each monochrome component,
XLAT GND
are necessary.
BLANK IREF
During driving POWER RGB LED with connected in
XERR
parallel N output channels, due to higher working currents,
TLC5923
we achieve very wide adjustment range of component
ATmega8515
RIREF
currents IFR, IFG, IFB. Because each of paralleled output
currents IOUTX, according to value of DCX coefficient, can
have independently any of 128 values from the range
UCC
<0 mA ; IOUTMAX>, the resultant currents can be adjusted in
Fig.2. Typical application - connection with supervising
the range <0 mA ; N · IOUTMAX>, in N · 128 steps each -
microcontroller
formula 4.
For co-operation between LED IC drivers and controller
N
the built-in SPI-slave interface is used. Using this interface
(4) IFR( GCB ) DCX IOUTX

the supervising microcontroller can transfer to IC driver:
X 1
16-bit switching word, which bits switch on or off
independently all output channels (Fig.3a);
PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 201
:
:
:
OUT0
OUT1
OUT14
OUT15
PGND
ULED ULED
Selected results of tests
Data sheets of presented IC drivers states, that
C
maximum frequency of serial interface clocking signal is
MISO
equal 30 MHz. In practice, data transfer can have either
MOSI SIN SOUT SIN SOUT
software or hardware implementation. The second one is
SCK SCLK VCC UCC VCC UCC
SCLK
possible, if used microcontroller have built-in SPI-master
PB2 MODE C1 MODE Cn
PB4 XLAT GND XLAT GND
interface. The suitable testing software, written in machine
PB3 BLANK IREF BLANK IREF
language, had been prepared for test circuit from figure 2.
PB1 XERR XERR
Programmes were debugged and tested in IDE AVRStudio.
UD1 UDn
U1
The realised tests with supervising microcontroller
R1 Rn
ATmega8515, clocked frequency 10 MHz, allow to estimate
UCC
transfer execution times of most important control
operations for TLC5922/23/24. The results are presented in
Fig.4. Cascading TLC5922/23/24 LED drivers on SPI bus
table 2. The utilization of high level programming language
(like C) can cause little longer service times. It depends on Conclusion
either applied programming techniques or selected compiler Constant current driving of LED RGB diodes becomes
optimization options. the alternative solution to the usually used driving circuits,
like switching supplying or PWM control. It is possible due
Table 2. Execution times of software and hardware
to new specialized integrated circuits - LED drivers,
implementations the main control operations for TLC5922/23/24
provided controllable constant current. Presented in the
devices
paper idea of adjusting monochrome component currents of
Implementation
Operation
RGB diode, can be also widened to quad-colour LEDs,
software hardware
which become to be more often applied in illuminating by
16b control word transfer 22,3 s 9,1 s
the reason of better colour rendering. Presented in the
112b dot-correction word transfer 144,1 s 82,1 s
paper ICs from Texas Instruments are not a unique devices,
16b control word transfer with
also other firms offer constant current drivers.
simultaneous read out the error word 25,7 s 9,7 s
(doesn't match TLC5922)
switching the LED group using BLANK Presented results are the part of research work No S/WE/1/06.
0,3 s 0,3 s
signal
minimum SPI clocking period 0,4 s 0,2 s
REFERENCES
1. Gilewski M., Karpiuk A.: Elektroniczna stabilizacja parametrów
The analysis of the table 2 contents leads to a wietlnych LED RGB. Przegl d Elektrotechniczny, 84 (2008),
nr 8, 194-198.
conclusion that: the usage of hardware SPI interface
2. Wojtkowski W.: Wykorzystanie sprz towych bloków PWM
increases the LED driver speed twice. Using hardware SPI
mikrokontrolerów AVR do regulacji jasno ci wiecenia diod
interface it's possible to update switching word to 100000
LED du ej mocy. Przegl d Elektrotechniczny, 85 (2009), nr 11,
times per second, and current adjustment word can be
306-309.
transferred about 10000 times per second. There are quite
3. Wojtkowski W.: Wielokana owa regulacja jasno ci wiecenia
satisfied values, according to human eyesight sense
diod LED z wykorzystaniem uk adów programowalnych FPGA.
perception ability. So if we don't operate with maximum
Przegl d Elektrotechniczny, 85 (2009), nr 11, 310-312.
possible speed, we have free processor time, which can be 4. Texas Instruments: TLC5922 LED driver. Datasheets on
www.ti.com. 2009.
used for additional calculations, needed for example to
5. Texas Instruments: TLC5923 16-channel LED driver with dot
appropriate correction of current coefficients DCX during
correction. Datasheets on www.ti.com. 2005.
brightness control.
6. Texas Instruments: TLC5924 16-channel LED driver with dot
The possible high speed of LED driver service via
correction and pre-charge FET. Datasheets on www.ti.com.
hardware SPI is an important observation due to sometimes
2006.
a big number of controlled drivers. Microcontroller
7. Day M.: LED driver - Paralleled Outputs Provide High-Current
supervising of few or more drivers like TLC5922/23/24
Outputs. Application report on www.ti.com. 2006.
becomes easier, because these chips can be cascaded on
SPI bus, as it's shown in figure 4.
Author: dr in . Lech Grodzki, Politechnika Bia ostocka, Wydzia
Elektryczny, ul. Wiejska 45D, 15-351 Bia ystok, E-mail:
[e-mail]
202 PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010
:
:
OUT0
OUT1
OUT0
OUT1
OUT14
OUT15
OUT14
OUT15
PGND
PGND