Lech GRODZKI
Bialystok Technical University - Faculty of Electrical Engineering
Constant-current driving of the LEDs' group
Abstract. Some LED applications need parallel driving of the groups of diodes. The article presents few selected chips designed to constant-current
LED driving. The paper contains the description of main properties of those integrated circuits and some results of realised research works on their
application features. Because the presented devices have SPI interface, they can be controlled by supervising microcontroller. The paper also
contains the proposal of automotive application, as the component of modern vehicle electrical system.
Streszczenie. Artyku prezentuje kilka wybranych uk adów scalonych przeznaczonych do sta opr dowego sterowania diodami LED. Obok
prezentacji cech u ytkowych tych uk adów, zawarto tak e wybrane wyniki bada aplikacyjnych, zwi zanych z wykorzystaniem interfejsu SPI do ich
sterowania przez mikrokontroler. Artyku zawiera równie propozycj aplikacji uk adów tego typu w nowoczesnej instalacji elektrycznej samochodu.
Ci g o-pr dowe sterowanie grupowe diodami LED
Keywords: constant-current LED driving, LED group working, microcontrollers
S owa kluczowe: sta opr dowe sterowanie LED, praca grupowa diod LED, mikrokontrolery
Group driving of LED diodes 2. By sending via serial interface appropriate configuration
Some LED applications are based on simultaneous byte (Fig. 1). That byte determines the fraction of
control of few or more devices. For example, they are used maximum output current value IOUTMAX, defines by
for back-lighting other devices (like LCD displays, formula 1, which states the present efficiency of current
illuminating advertising panels, etc.) or they perform sources. That fraction, called current gain (CG), can take
information functions directly, like traffic lights, road signs, 256 discrete values among the range: 1/12-127/128,
information panels, etc. In simplest case, LED driving according to the following formulas.
circuits in those applications have to switch on and off the
whole groups of LED devices. More advanced applications
1 HC CC
1
(2) VG

need also the possibility of dimming the lighting sources.
4 64

That last feature allows to automatic control of emitting light
flux, according to external conditions, like external
(3) CG VG 3CM 1
illumination or the time of a day.
If group driving demands only simple switch on-off
where: CM, HC and 6-bit word CC are components of
features, without any brightness control, the driving circuits
the configuring byte.
can be reduced to bi-stable transistor valves. The
After power-up devices work with default values:
brightness control feature needs more complex circuits. To
CM = 1, HC = 1, CC = 111111b. It means that CG equals
achieve this feature, a PWM control of switching power
0,992 and causes maximum output current value
transistor has been applied, form many years [1, 2]. The
IOUTMAX, limited by external resistor REXT. Figure 2 shows
value of pulse width coefficient defines the switch-on and
the value of current gain CG depending on the value of
switch-off periods of LED group. Increasing of that
configuration byte with constant value of external
coefficient causes the higher brightness of emitting light.
resistor REXT.
This dimming control idea is based on human eyesight
sense features: integrating light pulses and limited fast
event notification. CM HC CC5 CC4 CC3 CC2 CC1 CC0
In last years some IC manufacturers started deliver the
integrated devices, which can control the constant current
Fig.1. Configuration byte of TLC5916/17
supplying the LED. Such devices allow to more precise
control of LED brightness. Especially, integrated circuits
CG [-]
controlling few or more LED simultaneously are very
interesting for group LED driving.
0,992
Group driving ICs - general characteristics
Texas Instruments elaborated the series of integrated 0,750
circuits dedicated to the control of work of groups of LED
diodes. Among others, company offers either chips with 8
0,500
output channels - TLC5916/17 or 16 output channels -
TLC5926/27 [3, 4]. The characteristic feature of those ICs is
0,250
application of N controlled current sources, with commonly
adjusted current efficiency. Output current adjustment can
0,083
be realised in two ways:
0
K [-]
1. By the change of the external resistor REXT we can
0 64 128 192 255
define the maximum value of current source efficiency,
Fig.2. Current gain (CG) versus value K of configuration byte
common for the all of outputs, according to formula:
The utilization of the second method is possible only in
1,25V 15
cooperation with supervising processor. Figure 3 shows the
(1) IOUTMAX
REXT
testing circuit, used in described research works. It's a
typical application circuit for that family of integrated
The internal construction and power dissipation
devices. Microcontroller can manage the devices via serial
possibilities limit the range of output current IOUTMAX to
synchronous interface looking like SPI. This interface is
the range: 5 - 120 mA.
PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 203
used to transfer to IC driver on-off control word, switching increase the luminous flux while the external luminance
outputs OUTX, and configuration byte. The same interface flux reaches high values.
allows to read out the device status word, containing the According to the application note [5], it s possible to
information about detected malfunctions of the output point one another way of the output current adjustment,
channels. Control circuits of current sources can detect especially useful for POWER LEDs. Even though the
following errors, separately for each channel: maximum output current of each channel is equal 120 mA,
for driving of POWER LED devices it can be insufficient. In
open circuit - too low value of output voltage for
such case, driver outputs can be connected in parallel, to
switched-on channel;
obtain controllable output current up to INOUTMAX =
over-temperature shutdown in working output channel -
N·120 mA. Using 8-channel TLC5916/17 ICs we can control
too high temperature of output circuit in given channel;
the 4, 2 or 1 POWER LED, up to 960 mA, according to their
LED short-circuit - too high value of voltage for
forward currents - figure 4.
switched-on output (only TLC5917 and TLC5927 ICs).
ULED a) b) c)
ULED ULED ULED
OUT0 OUT0 OUT0
OUT1 OUT1 OUT1
OUT2 OUT2 OUT2
OUT3 OUT3 OUT3
PB6/MISO
OUT4 OUT4 OUT4
PB5/MOSI SDI SDO
OUT5 OUT5 OUT5
PB7/SCK CLK VDD UCC OUT6 OUT6 OUT6
OUT7 OUT7 OUT7
100nF
TLC5916/17 TLC5916/17 TLC5916/17
PB4 LE
PB3 OE REXT
Fig.4. Possible POWER LED control configurations in order to their
forward current: a - up to 240 mA; b - up to 480 mA; c - up to
960 mA
TLC5916/17
ATmega8515
REXT
In this way, we achieve additional method of POWER
LED current adjustment: simultaneous change the current
Fig.3. Testing circuit applied during research works
gain and the number of working paralleled channels. For
fluent dimming in wide range of LED forward current it's
During simultaneous control of group of LED devices,
sufficient to select the number of switched on channels and
we try to achieve the same working points, as the condition
the value of configuration byte. That additional POWER
of steady brightness of several diodes. For presented IC
LED current adjustment method widens the application
drivers manufacturer guarantees quite good output current
fields of presented IC drivers.
accuracy: Ä…3% between channels in the same chip and
Ä…6% between different ICs. There are satisfied values,
Selected results of tests
looking at the typical divergence of LED working
Data sheets of presented IC drivers states, that
characteristics.
maximum frequency of serial interface clocking signal is
equal 30 MHz. In practice, according to defined transfer
Adjusting the LED forward current
frame, does not matching fully to SPI standard, data
In last capture two ways of output current adjustment
exchange between microcontroller and IC LED driver
are pointed:
should be realised under software control. The suitable
hardware method, by the changes of external resistor;
testing software, written in machine language, had been
software method, utilizing the configuration byte
prepared for testing circuit from figure 3. Programmes were
transferred to driver IC via serial interface.
debugged and tested in IDE AVRStudio. The realised tests
The both methods allow to precise adjustment of LED
with supervising microcontroller ATmega8515, clocked
forward current. But the second of them is more suitable,
frequency 10 MHz, allow to estimate software transfer
even though it needs the supervising microcontroller.
speed. Execution times of most important control operations
In practice, software method can realised in two ways:
for TLC5916/17 and TLC5926/27 are presented in table 1.
1. External resistor REXT is set to the value giving IOUTMAX
Table 1. Execution time of selected software-implemented
current equal the needed LED forward current IFLED. It
operations for TLC5916/17 i TLC5926/27
causes that any configuration byte with value differ to
Operation TLC5916/17 TLC5926/27
11111111b, written to IC driver will decrease the LED
control word transfer
current. We achieve the possibility of fluent control only
(8b for TLC5916/17, 16b for 11,1 s 23,1 s
during dimming of LED group connected to the chip
TLC5926/27)
output channels. configuring word (with current gain
coefficient) transfer
2. The value of the external resistor REXT is set less then it's
10,6 s 16,3 s
(8b for TLC5916/17, 16b for
described above. In such situation, the optimum LED
TLC5926/27)
forward current IFLED we can achieve at current gain CG
switching working mode from normal
4,1 s 4,1 s
less then 0,992 (IFLED < IOUTMAX). Then, before the
to special
switching on the outputs OUTX, we should send to the IC
switching working mode from
3,7 s 3,7 s
special to normal
driver appropriate configuration byte, decreasing the
setting special working mode and
value of output currents IOUT. Presented solution results
read out the error word
in the less number of dimming steps, but also allows for 16,6 s 26,2 s
(8b for TLC5916/17, 16b for
increasing the LED current over the chosen working
TLC5926/27)
value. The possibility of occasional increasing the LED
switching the LED group using /OE
0,3 s 0,3 s
luminance, can be very useful in some applications. For signal
minimum period of SPI clocking
example, if it is important to achieve the constant
0,4 s 0,4 s
signal SCLK
luminance contrast between the vehicle signalling lamp
and the external lighting, it would be possible to
204 PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010
:
:
:
OUT0
OUT1
OUT6
OUT7
GND
 The analysis of the table contents leads to a conclusion information should be transfer to the vehicle central
that: for 8-channel driver switching word and configuration computer. It can be also used locally to adjust the light
byte can be updated about 90000 times per second. The IC source, by increasing the currents in others active LED
with 16 outputs is served slower. supplying channels, to maintain the ordered luminance flux.
Three of the mentioned in table 1 operations can be Suggested automotive application of TLC59xx family chips
supported by hardware SPI interface. It give us a shorter can be applied also in classical electrical vehicle net. In that
execution times, as it is shown in table 2. case, local microcontroller of signal lamp should trace the
state of dedicated signal line instead the receiving
Table 2. Execution time of some operations using hardware SPI
messages via CAN bus.
interface for TLC5916/17 i TLC5926/27
Operation TLC5916/17 TLC5926/27
Conclusion
control word transfer
Integrated circuits providing constant current supplying
(8b for TLC5916/17, 16b for 5,7 s 9,4 s
LEDs are offered by others manufactures, too. For
TLC5926/27)
example, National Semiconductors offers several ICs, but
configuring word (with current
those chips have only single [8] or no more then few [9]
gain coefficient) transfer
6,6 s 10,3 s
controlled outputs. In addition their applications need some
(8b for TLC5916/17, 16b for
discrete external elements, like resistors, capacitors and
TLC5926/27)
setting special working mode and coils. Moreover, LED brightness adjustment is realised by
read out the error word
PWM signal provided to special terminal of ICs. Integrated
14,0 s 18,2 s
(8b for TLC5916/17, 16b for
LED drivers with 8 or 16 output channels are also offered by
TLC5926/27)
Allegro MicroSystems [10]. But embedded SPI interface
allows only for switching and checking LED diodes.
Application proposal
Dimming is possible only using additional PWM control
One of the possible applications of presented LED IC
signal.
drivers is their usage in vehicle signalling lamps. The aim is
Watching the broadening offer of LED constant current
designing, for instance, car STOP lights with ability of self-
IC drivers, it is possible to suppose, that those chips will find
adjusting their luminance, according to changing ambient
durable place for themselves in the wide range of LED
conditions, to guarantee the optimal luminance contrast
applications.
between activated signalling lamps and other vehicle lights
or ambient luminance. It should be noted, that adaptive
Presented results are the part of research work No S/WE/1/06.
signalling vehicle lamps are already allowed by international
standards [6]. The concept of such car lamp was described
REFERENCES
among other in [7]: proposed solution can be realised on
[1] Wojtkowski W.: Wielokana owa regulacja jasno ci wiecenia
the base of local analog-digital circuit, adjusting the light diod LED z wykorzystaniem uk adów programowalnych FPGA.
Przegl d Elektrotechniczny, 85 (2009), nr 11, 310-312.
source supply voltage. The usage of local microcontroller
[2] Wojtkowski W.: Wykorzystanie sprz towych bloków PWM
and IC LED driver really increases the construction
mikrokontrolerów AVR do regulacji jasno ci wiecenia diod
functionality.
LED du ej mocy. Przegl d Elektrotechniczny, 85 (2009), nr
11, 306-309.
12V
CAN
[3] Texas Instruments: TLC5916, TLC5917 8-bit constant-current
LED sink drivers. Datasheets on www.ti.com. 2008.
[4] Texas Instruments: TLC5926, TLC5927 16-bit constant-
SPI
current LED sink drivers. Datasheets on www.ti.com. 2008.
[5] Day M.: LED driver - Paralleled Outputs Provide High-Current
C TLC59xx
Outputs. Application report on www.ti.com. 2006.
[6] Mo ko W., Ka mierczak P., ukasik M.: Luminance contrast of
vehicle signalling lamps. Przegl d Elektrotechniczny, 85
LS1
LS2
(2009), nr 11, 272-275.
[7] ukasik M.: The concept of the wheeled vehicle's adaptive
stop lamps operation. Przegl d Elektrotechniczny, 84 (2008),
Fig.5. The proposal of intelligent vehicle signalling lamp
nr 8, 186-189.
[8] National Semiconductors: LM3402/LM3402HV 0.5A constant
In proposed solution (Fig. 5) local microcontroller is
Current Buck Regulator for Driving High Power LEDs.
connected with LED driver by SPI interface. Using that
Datasheets on www.national.com. 2008.
interface microcontroller sends to LED driver orders to [9] National Semiconductors: LM3432/LM3432B 6-Channel
Current Regulator for LED Backlight Application. Datasheets
control the light source (switch on, switch off, brightness
on www.national.com. 2008.
adjustment), according to orders, received via CAN bus
[10] Allegro MicroSystems: Constant-Current LED Drivers.
from supervising computer.
Application Information AN29503 on www.allegromicro.com.
Using the local light sensors LS1 and LS2 [7] and the
2007.
messages from CAN bus, microcontroller decides about
LED current adjustment. In addition, using the features of
TLC59xx chips, it can test the efficiency of group of LEDs.
Author: dr in . Lech Grodzki, Politechnika Bia ostocka, Wydzia
As it was described earlier, troubles, like short break,
Elektryczny, ul. Wiejska 45D, 15-351 Bia ystok, E-mail:
short-circuit, over-temperature shut down, in any output [e-mail]
channel are indicated in read out status word. This
PRZEGL D ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 86 NR 10/2010 205