plik

200 PRZEGL!D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010

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

The main aims of LED RGB driving circuit are the

controlling the value of luminance flux and its colour. They
are two basic features of LED RGB diode, as a light source
illuminating architectural elements, art objects, etc. To
achieve

the

changes

these

parameters,

the

monochromatic  components  of  RGB  diode  should  be
controlled.  The  main  electrical  factors  deciding  about  the
work  of  RGB  diode  are  forward  currents  flowing  by
component diodes. By controlling of these currents we can:

change the ratios between monochromatic luminance
fluxes !

, !

to change the colour of resultant flux;

change total luminous flux by simultaneous changes
forward currents I

, I

and I

in component diodes;

correct the influences on brightness and luminance
colour such environment factors as ambient temperature
or ageing the LED diode as a semiconductor devices.

Very popular method of LED luminance control is

applying the pulse width modulation. It is based on human
eyesight sense features: integrating light pulses and limited
fast event notification. By appropriate selecting the duty
cycle coefficients it is possible to control total luminance flux
as the sum of monochrome

component fluxes:

(1)

The same principle is used to control the LED RGB colour -
the changes of ratios between component luminance fluxes
!

, !

result in the colour changes.

PWM controlled LED RGB drivers can be built using

analog  circuits  [1],  microcontrollers  [2]  and  even  FPGA
devices  [3].  Unquestionable  disadvantage  of  described
above  method  is  the  necessity  of  simultaneous,  multi-
channel  generating  rectangle  waveforms  with  suitable  high
frequency and adjustable duty cycle coefficients. Therefore,
the  appearance  of  new  solutions  in  driving  LED  devices  is
worthy of notice.

Constant-current IC drivers - general characteristic

Texas Instruments offers a several ICs, dedicated to

constant-current driving of 16 LED diodes, with separate
forward current correction. There are the chips: TLC5922,
TLC5923 and TLC5924. They have some common features
[4, 5, 6], like:

16 output channels, each with controlled current source
with the current range 0-80 mA and accuracy ±1%;

7-bit current correction coefficient, separately for each
channel;

individual switching of output channels;

IC supply voltage range: 3,0-5,5 V;

LED device supply voltage up to 17 V, what allows to
control LED connected in series;

serial synchronous SPI interface, with speed up to
30 MHz, allowing supervising controller to control the
work of IC driver;

input of common blanking all the output channels;

the possibility of cascading IC devices on SPI bus.

Except the common features, presented ICs have some
differences each other, as it is shown in table 1.

Table 1. Selected feature differences of TLC5922/23/24

Feature

TLC5922 TLC5923 TLC5924

operation temperature range -20÷85°C -40÷85°C -40÷85°C
LED open detection and
indication

over-temperature indication

maximum LED supply
voltage U

LEDMAX

17 V

16 V

pre-charge FET circuit

Autonomous work of IC drivers

Presented IC drivers are able to work autonomous,

without  supervising  microcontroller,  but  in  that  case  they
would  drive  the  LEDs  with  a  stable  forward  current,  the
same  for  all  output  channels.  Then,  the  only  method  of
changing the forward currents is to change external resistor
R

IREF

. That discrete element is placed between the power

ground and built-in band gap with a typical value of 1,24 V.
The maximum output current per channel can be calculated
by formula 2.

(2)

IREF

OUTMAX

The value of R

IREF

should not be smaller than 600 #. The

above relation is illustrated by figure 1.

The second way of controlling the LED devices,

connected to autonomous working IC driver, is the usage of
input terminal BLANK, which high logic level switches off
simultaneously all output channels.

In addition, TLC5923/24 devices have output signal

XERR, informing about detection of their malfunction, like
open LED circuit or over-temperature condition of IC.

TLC5924 chip has also built-in pre-charge FET circuit,

that enables an improvement in image quality of the
dynamic-drive LED display. That circuit uses LED supply

PRZEGL!D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 201

voltage U

LED

provided to selected terminal of driver

package. If the current-carrying capacity of single output
channel, equal 80 mA, is insufficient, there is a possibility of
parallel connecting several

outputs. That solution is

recommended by manufacturer in one of the application
notes [7]. In that case the resultant maximum current of
paralleled outputs is calculated by formula:

(3)

OUTMAX

OUTN

500

1000

1500

2000

2500

3000

IREF

[ ]

OUTMAX

[mA]

Fig.1. The relation between the value of external resistor R

IREF

and

maximum output current I

OUTMAX

Because presented IC drivers have 16 current outputs

each, then to drive POWER LED RGB, it is possible to
group outputs into 3 groups, with 5 paralleled outputs per
each group. In such circuit, the maximum forward current of
monochromatic components would be equal 400 mA.

Co-operation with microprocessor system

The proper application circuit of TLLC5922/23/24

devices

uses

the

connection

with

supervising

microcontroller. Allowable supply  voltage of ICs (3,3V - 5V)
lets  for  collaboration  with  either  older  or  up-to-date
microcontrollers.  The  example  circuit,  with  AVR  controller,
is shown in figure 2.

SIN
SCLK
MODE
XLAT
BLANK
XERR

SOUT

VCC

GND

IREF

LED

IREF

100nF

ATmega8515

TLC5923

Fig.2.

Typical

application

connection

with

supervising

microcontroller

For co-operation between LED IC drivers and controller

the built-in SPI-slave interface is used. Using this interface
the supervising microcontroller can transfer to IC driver:

16-bit switching word, which bits switch on or off
independently all output channels (Fig.3a);

112-bit word, called dot-correction word, containing

sixteen 7-bit length coefficients DC

, programming each of

the 16 output current I

OUTX

(Fig.3b).

For distinguishing those both transmissions the

additional  control  input  terminal,  called  MODE,  is  used.
Each  transfer  ends  with  the  pulse  on  latching  input  XLAT.
The  rising  edge  on  XLAT  input  causes  storing  the  serial
transferred  data  in  appropriate  internal  register.  Moreover,
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
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.

Fig.3. Control words for TLC5922/23/24 devices:
a - 16-bit switching word; b - 112-bit dot correction word with
coefficients DC

The open-drain (OD) output XERR is used to report

detected  malfunctions  in  TLC5923/24  chips.  The  circuit-
break  at  least  in  one  off  enabled  output  channels  or  over-
temperature condition of IC structure pulls down that output.
Open-drain  configuration  allows  to  connect  many  such
outputs  to  a  single  line,  to  inform  the  microcontroller  about
working errors of many LED drivers. This line can be either
periodically  checked  by  microcontroller  or  used  as  its
external interrupt.

Especially useful, using supervising microcontroller, is

the possibility of individual output current adjustment. Each
of the 16 channel has its own 7-bit coefficient word DC

which value determines the actual output current of it. The
current adjustment range is from 0 mA up to I

OUTMAX

defined by external resistor R

IREF

(formula 2). That

adjustment range is divided into 128 equal steps. It means,
that if I

OUTMAX

is equal 80 mA the value of adjustment step is

equal 625 $A. This is a satisfied value for precise control of
RGB LED component currents. By appropriate changes of
coefficients DC

, 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
scaling of DC

coefficients for monochrome components. In

that

case

additional

calculations,

using

working

characteristics ! = f(I

) for each monochrome component,

are necessary.

During driving POWER RGB LED with connected in

parallel  N  output  channels,  due  to  higher  working  currents,
we  achieve  very  wide  adjustment  range  of  component
currents  I

, I

. Because each of paralleled output

currents I

OUTX

, according to value of DC

coefficient, can

have independently any of 128 values from the range
<0 mA ; I

OUTMAX

>, the resultant currents can be adjusted in

the range <0 mA ; N · I

OUTMAX

>, in N · 128 steps each -

formula 4.

(4)

OUTX

)

GCB

(

DC15.6 . . . DC15.0 DC14.6

DC1.0 DC0.7

DC0.0

. . .

OUT15 OUT14 OUT13

OUT2 OUT1 OUT0

. . .

202 PRZEGL!D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010

Selected results of tests

Data sheets of presented IC drivers states, that

maximum  frequency  of  serial  interface  clocking  signal  is
equal  30 MHz.  In  practice,  data  transfer  can  have  either
software  or  hardware  implementation.  The  second  one  is
possible,  if  used  microcontroller  have  built-in  SPI-master
interface.  The  suitable  testing  software,  written  in  machine
language,  had  been  prepared  for  test  circuit  from  figure  2.
Programmes were debugged and tested in IDE AVRStudio.
The  realised  tests  with  supervising  microcontroller
ATmega8515, clocked frequency 10 MHz, allow to estimate
transfer  execution  times  of  most  important  control
operations for TLC5922/23/24. The results are presented in
table 2.  The  utilization  of  high  level  programming  language
(like C) can cause little longer service times. It depends on
either applied programming techniques or selected compiler
optimization options.

Table

Execution

times

software

and

hardware

implementations the main control operations for TLC5922/23/24
devices

Operation

Implementation

software

hardware

16b control word transfer

22,3 $s

9,1 $s

112b dot-correction word transfer

144,1 $s

82,1 $s

16b control word transfer with
simultaneous read out the error word
(doesn't match TLC5922)

25,7 $s

9,7 $s

switching the LED group using BLANK
signal

0,3 $s

minimum SPI clocking period

0,4 $s

0,2 $s

The analysis of the table 2 contents leads to a

conclusion  that:  the  usage  of  hardware  SPI  interface
increases the LED driver speed twice. Using hardware SPI
interface  it's  possible  to  update  switching  word  to  100000
times  per  second,  and  current  adjustment  word  can  be
transferred  about 10000 times per second. There are quite
satisfied  values,  according  to  human  eyesight  sense
perception  ability.  So  if  we  don't  operate  with  maximum
possible speed, we have free processor time, which can be
used  for  additional  calculations,  needed  for  example  to
appropriate  correction  of  current  coefficients  DC

during

brightness control.

The possible high speed of LED driver service via

hardware SPI is an important observation due to sometimes
a big number of controlled drivers. Microcontroller
supervising of few or more drivers like TLC5922/23/24
becomes easier, because these chips can be cascaded on
SPI bus, as it's shown in figure 4.

SIN
SCLK
MODE
XLAT
BLANK
XERR

SOUT

VCC

GND

IREF

MISO
MOSI

SCK

PB2
PB4
PB3
PB1

LED

UD1

SIN
SCLK
MODE
XLAT
BLANK
XERR

SOUT

VCC

GND

IREF

LED

UDn

Fig.4. Cascading TLC5922/23/24 LED drivers on SPI bus

Conclusion

Constant current driving of LED RGB diodes becomes

the  alternative  solution  to  the  usually  used  driving  circuits,
like  switching  supplying  or  PWM  control.  It  is  possible  due
to  new  specialized  integrated  circuits  -  LED  drivers,
provided  controllable  constant  current.  Presented  in  the
paper idea of adjusting monochrome component currents of
RGB  diode,  can  be  also  widened  to  quad-colour  LEDs,
which  become  to  be  more  often  applied  in  illuminating  by
the  reason  of  better  colour  rendering.  Presented  in  the
paper ICs from Texas Instruments are not a unique devices,
also other firms offer constant current drivers.

Presented results are the part of research work No S/WE/1/06.

REFERENCES

1. Gilewski M., Karpiuk A.: Elektroniczna stabilizacja parametrów

%wietlnych LED RGB. Przegl"d Elektrotechniczny, 84 (2008),
nr 8, 194-198.

2. Wojtkowski W.: Wykorzystanie sprz&towych bloków PWM

mikrokontrolerów AVR do regulacji jasno%ci %wiecenia diod
LED du'ej mocy. Przegl"d Elektrotechniczny, 85 (2009), nr 11,
306-309.

3. Wojtkowski W.: Wielokana!owa regulacja jasno%ci %wiecenia

diod LED z wykorzystaniem uk!adów programowalnych FPGA.
Przegl"d Elektrotechniczny, 85 (2009), nr 11, 310-312.

4. Texas Instruments: TLC5922 LED driver. Datasheets on

www.ti.com. 2009.

5. Texas Instruments: TLC5923 16-channel LED driver with dot

correction. Datasheets on www.ti.com. 2005.

6. Texas Instruments: TLC5924 16-channel LED driver with dot

correction and pre-charge FET. Datasheets on www.ti.com.
2006.

7. Day M.: LED driver - Paralleled Outputs Provide High-Current

Outputs. Application report on www.ti.com. 2006.

Author: dr in#. Lech Grodzki, Politechnika Bia!ostocka, Wydzia!
Elektryczny, ul. Wiejska 45D, 15-351 Bia!ystok, E-mail:
lgrodzki@we.pb.edu.pl