TECHNICAL FIELD
[0001] The invention relates to the field of driver circuits for light emitting diodes (LEDs)
, especially multi-colour light emitting diodes.
BACKGROUND
[0002] The brightness of light emitting diodes (LEDs) is directly dependent on the load
current flowing through the diode. To vary the brightness of a LED it is known to
use a controllable current source that is set to a current representing a desired
brightness. In digitally controlled applications a digital-to-analog converter (DAC)
may be used to set the current of the controllable current source.
[0003] Since the human eye cannot resolve high frequency brightness fluctuations of approximately
100 hertz or higher, it is known to supply the LED with a pulse width modulated current.
In this case the human eye low-pass filters the resulting pulse width modulated brightness
of the LED, i.e. the eye can only sense a mean brightness that depends on the mean
LED current which is proportional to the duty cycle of the pulse width modulation.
[0004] It is known to combine light of different colours (e.g. red, green, and blue) and
different brightness to generate nearly any colour sensation in the visible spectrum
of light. In modern illumination systems or displays a combination of at least three
LEDs of different colours are used to provide a multi-colour illumination. The LED-triples
may be arranged in a matrix like structure thus forming a display where each "pixel"
of the display is represented by a LED-triple typically comprising a red, a green,
and a blue LED. To vary the colour of a pixel the brightness of the different LEDs
has to be individually adjustable. Each of the three LEDs may therefore be driven
by a pulse-width modulated current signal of a sufficiently high frequency, for example
400 hertz.
[0005] However, the resolution requirements are quite high for modern illumination systems
or displays. That is, the brightness of a single LED should be adjustable to at least
4096 different brightness values which corresponds to a brightness resolution of 12
Bit. When using pulse width modulation for controlling the brightness, a time resolution
of approximately 600 nanoseconds has to be provided in order to be able to resolve
a PWM period of, for example, 2.5 milliseconds (corresponds to 400 hertz) with 12
bits. This entails the need for very fast switching currents with all the known problems
coming therewith. Particularly, the electromagnetic compatibility (EMC) is low when
switching currents with rise and fall times in the sub-microsecond range.
[0006] Driving the LEDs with a continuous current whose value is controlled by a DAC is
also not satisfying since the wavelength of the colour of a single LED may vary over
the LED current. This entails a very complex brightness control in multi-colour LED
systems since the colour has to be corrected when changing the brightness of a three
LED pixel.
[0007] There is a need for an alternative concept for driving LED, particularly improving
the electromagnetic compatibility compared to PWM driven LED systems.
SUMMARY
[0008] One example of the invention relates to a driver circuit for driving a light emitting
diode. The circuit comprises: a controllable current source connected in series to
the light emitting diode, the current source being configured to provide a load current
to the light emitting diode dependent on a control signal; and a modulator unit configured
for generating the control signal, where the control signal is dependent on a pulse-density
modulated signal.
[0009] A further example of the invention relates to a method for driving a light emitting
diode. The method comprises: providing a signal representing a desired brightness
or colour; modulating the desired signal thus generating a pulse-density modulated
signal; providing a current to the light emitting diode dependent on the pulse-density
modulated signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The invention can be better understood with reference to the following drawings and
description. The components in the figures are not necessarily to scale, instead emphasis
being placed upon illustrating the principles of the invention. Moreover, in the figures,
like reference numerals designate corresponding parts. In the drawings:
- FIG. 1
- is a block diagram of a LED driver circuit for driving multi-colour LEDs;
- FIG. 2
- is a block diagram of a digital sigma-delta modulator providing a pulse-density modulated
output signal;
- FIG. 3
- is a block diagram of a LED driver circuit comprising the sigma-delta modulator of
FIG. 2;
- FIG. 4
- is a block diagram of a LED driver circuit comprising a sigma-delta modulator with
a multi-bit output;
- FIG. 5
- is a block diagram of a LED driver circuit corresponding to FIG. 3 but with a dither
noise added to the input for preventing limit cycles;
- FIG. 6
- is a block diagram of a LED driver circuit for driving multi-colour LEDs with a sigma-delta
modulator comprising three times the driver circuit of FIG. 3;
- FIG. 7
- is a block diagram of a further LED driver circuit, where the load current passing
through a LED is controlled by means of a bypass current source;
- FIG. 8
- is a block diagram of the LED driver of FIG. 7 where MOS transistors operate as switchable
bypass current sources.
DETAILED DESCRIPTION
[0011] FIG. 1 illustrates a known LED driver circuit for driving a LED triple, where each
LED has a different colour. Such LED triples can be - if adequately controlled - used
for generating any colour of the visible spectrum by means of additive mixture of
colours. For this purpose usually a red LED LD
R, a green LED LD
G, and a blue LED LD
B are used. For controlling the brightness of each LED LD
R, LD
G, LD
B each LED is connected in series to a respective controllable (in the present example
switchable) current source Q
R, Q
G, and Q
B. If, for example, yellow light is to be generated, then the load current through
the red LED LD
R has to be zero and the load currents through the green LED LD
G and the red LED LD
R have to be approximately the same, where the absolute current value depends on the
desired brightness of the yellow light.
[0012] However, the wavelength of the light emitted by the LEDs will vary dependent on the
load current passing through the LEDs. This dependency entails a change in hue when
changing the load current for adjusting the brightness value. To avoid this effect
it is known to use switchable current sources Q
R, Q
G, Q
B each controlled by a pulse width modulated (PWM) control signal. The hue of the LEDs
is does not change since the brightness value is not adjusted by continuously adjusting
the load currents but by adjusting the duty cycle of the PWM control signal. The "averaging"
of the PWM signal is performed by the human eye.
[0013] In driver circuit of FIG. 1 the hue is selected by a pointer CS that identifies an
entry of a calibration table 10 where the corresponding load current values S
R, S
G, S
B for the three LEDs are stored. The stored values S
R, S
G, S
B are calibrated for maximum brightness and are multiplied (multiplier 11) with a brightness
value S
B for a reduced brightness. The resulting desired average current values I
R=S
RS
BR, I
G=S
GS
BR, I
B=S
BS
BR are fed to the pulse width modulators PWM
R, PWM
G, PWM
B that generate a respective PWM control signal having the desired mean value for driving
the LEDs.
[0014] In digitally controlled systems the desired average current values I
R, I
G, I
B are typically provided as 12 bit words. The repetition frequency of the PWM pulses
is typically 400Hz which is high enough that the human eye does not sense any flickering.
However, PWM frequencies ranging from 100 Hz to 600 Hz are commonly used for this
purpose. As already discussed above a very fast switching of the load currents is
necessary for providing the desired 12 bit resolution which entails, for example,
EMC problems.
[0015] FIG. 2 illustrates a sigma-delta modulator 1 (Σ-Δ modulator, often also denoted as
delta-sigma modulator) for providing a pulse density modulated signal PDM for driving
a LED LD, respectively the corresponding current source Q. A pulse density modulated
signal is a generally non periodic bit-stream with an average value corresponding
to the input signal, i.e. the desired average load current I in the present example.
In the present example the input signal I is a sequence of 12 bit words. The bit-stream
is a sequence of equally spaced bits, i.e. a high level represents a binary "1" and
a low level a binary "0". The density of "1" in the pulse density modulated signal
is high if the level of the input signal of the sigma-delta modulator is high. However,
the length of one bit symbol ("1" or "0") is always the same and is equal to the period
of the bit-rate frequency. For example at a bit-rate of 40 kHz, the length of a bit
symbol is 25 µs.
[0016] The sigma-delta modulator 1 comprises a forward path comprising an integrator 30
and a quantiser 20. It further comprises a feedback path comprising a delay element
21. The delay element receives the 1-bit output signal PDM [k] of the quantiser 20
and provides the signal at its output delayed by a sample and as a 12 bit word, i.e.
the bit value of the 1-bit input signal of the delay element 21 is copied to the most
significant bit of the respective output signal. "k" thereby is a time index. The
delayed output signal PDM[k-1] is subtracted (subtractor 22) from the input signal
I[k] and the resulting difference I[k]-PDM[k-1] is supplied to the integrator 30 that
has its output connected to the quantiser 20.
[0017] In the present example the integrator 30 is a standard first-order digital integrator
with a delay element 32 in a feedback path and an adder 31. The transfer function
of the integrator in the z-domain is 1/(1-z
-1). However higher order integrators may also be applied. The quantiser 20 may be a
simple comparator element. In the present example the quantiser provides the most
significant bit of its 12-bit input signal value at its output. However, also multi-bit
quantiser 20 are applicable for providing an N-bit output PDM signal which is a stream
of N-bit words or a set of N "parallel" bit-streams.
[0018] For proper operation of the sigma-delta modulator 1 the input signal has to be strongly
over-sampled. Then the quantisation noise is "shifted" towards higher frequencies
an can therefore be removed by a simple low-pass filtering which is - in the present
case - advantageously performed by the human eye. The noise shaping properties of
sigma delta modulators a well known and not further discussed here. For a bandwidth
of the input signal I
R of 400 Hz an over-sampling frequency of 40 kHz is sufficient to provide an signal-to-noise
ratio (SNR
dB) of at least 74dB which corresponds to an effective resolution of 12 bits. The effective
number of bits (ENOB) may be calculated as
whereby the signal-to-noise ratio SNR
dB may be calculated as
for a sigma-delta modulator 1 with a first order integrator 30, a over-sampling rate
OSR (ratio of sampling rate and bandwidth) and a N-bit quantiser 20 (N=1 in the present
example). For a sigma-delta modulator 1 with a second order integrator 30 the signal-to-noise
ratio SNR
dB is given by
[0019] From the discussion above it can be seen, that at a given resolution, for example
12 bit, and moderate frequencies of about 40 kHz a sigma-delta modulator provides
a pulse-density modulated output signal which may be used for controlling the current
sources Q
R, Q
G, Q
B connected to the LEDs LD
R, LD
G, LD
B in a LED driver circuit such as the circuit of FIG. 1.
[0020] For a stable operation within the desired resolution the sigma-delta modulator may
comprise an anti-aliasing filter for limiting the bandwidth of its input signal to
a predefined bandwidth of, for example, 400 Hz.
[0021] Compared to the circuit of FIG. 1 that uses PWM modulators for driving the LEDs the
rise and fall times of the switching can be much longer when using a sigma-delta modulator
instead, since the bit-stream comes at relatively low frequencies of about 40 kHz.
Longer rise and fall times entail less electromagnetic interferences (EMI) and a better
electromagnetic compatibility (EMC).
[0022] FIG. 3 shows the application of the sigma-delta modulator 1 of FIG. 2 in a LED driver
circuit. Only one LED LD connected in series to one current source Q is depicted in
FIG. 3. However, the circuit of FIG. 3 may be tripled to form a driver circuit for
three LEDs LD
R, LD
G, LD
B of different colours analogous to the circuit of FIG. 1. The sigma-delta modulator
1 receives a desired average current value I and provides a corresponding pulse bit-stream
which is a pulse-density modulated control signal supplied to the switchable current
source Q. The input I of the sigma-delta modulator 1 may be derived from a calibration
table analogous to the circuit of FIG. 1.
[0023] FIG. 4 illustrates another example of how to apply a sigma delta modulator in a LED
driver circuit. This example is especially useful when using a sigma-delta modulator
1 with a multi-bit quantiser 20, e.g. a 3-bit quantiser. In this case the sigma-delta
modulator 1 does not provide a single bit output signal PDM but a stream of 3-bit
words, i.e. three parallel bit-streams. For transforming this three bit-streams into
one control signal for driving the current source Q a second modulator 2 may be employed,
for example, a pulse-width modulator (PWM) or a pulse frequency modulator (PFM). In
the present example a PWM is used as second modulator. In contrast to the example
of FIG. 1 the PWM needs only to resolve 8 different positions (3 Bits) in time during
the PWM period of, for example, 25 µs. As a consequence the steepness of the switching
edges may be lower by a factor of five due to the sigma-delta modulator 1 arranged
upstream to the sigma delta modulator while maintaining or even increasing the resolution.
Alternatively a 3-bit digital-to-analog converter may be used as second modulator
2. In this case the sigma-delta modulator 1 arranged upstream to the digital-to-analog
converter (DAC) has the advantage that a low resolution DAC is sufficient. Compared
to the circuit of FIG. 3 the present example allows for even slower switching frequencies
which may be advantageous in case the connection between the LED and the driver circuit
comprises long cables. Furthermore switching losses are lower.
[0024] When modulating a constant input signal I then the pulse density modulated output
signal of the sigma-delta modulator 1 (bit-stream) may exhibit some periodicity. This
undesired effect is due to limit cycles and the spectrum of the bit-stream has so-called
idle-tones, i.e. peaks at certain discrete frequencies. To avoid the idle tones a
low power noise signal n[k] having zero mean and, for example, a triangular or a rectangular
probability density function may be added to the input signal I as depicted in FIG.
5 by means of an adder 12. This technique is also referred to as "dithering". Due
to the noise-shaping properties of sigma-delta modulators 1 the power is of the dither
noise n[k] is "shifted" towards higher frequencies that cannot be resolved by the
human eye. That is, the human eye performs a low-pass filtering of the bit-stream.
The dithering technique results in a lower signal-to-noise ratio but, however, the
desired resolution of the sigma-delta modulator can be achieved regardless of the
lower signal-to-noise ratio. Furthermore, the idle tones are suppressed and the undesired
periodicity of the bit-stream is destroyed.
[0025] FIG. 6 illustrates, by means of a block diagram, a LED driver circuit for driving
multi-colour LEDs with a sigma-delta modulator 1, the LED driver circuit comprising
three times the driver circuit of FIG. 3. Of course driver circuits with a sigma-delta
modulator 1 having a second modulator connected downstream thereof as depicted in
FIG. 4 are also applicable for building up a multi-colour LED driver. In the present
example one driver circuit according to FIG. 3 is employed for each colour channel
(red, green, and blue). Furthermore a dither noise may be added to the input signals
I
R, I
G, I
B of each colour channel as discussed with reference to FIG. 5. Apart from the sigma-delta
modulator 1 the further components of the multi-colour LED driver circuit correspond
to the components of the circuit discussed with reference to FIG 1.
[0026] FIG. 7 illustrates another driver circuit for driving a plurality of light emitting
diodes LD
1, LD
2, ..., LD
N. However, the driver circuit of FIG. 7 may be usefully employed for driving at least
two light emitting diodes LD
1, LD
2. The driver circuit comprises a main current source Q
M providing a main current I
QM. A plurality of bypass current sources Q
1, Q
2 ..., Q
N are connected in series to the main current source Q
M and have terminals for connecting one light emitting diode LD
1, LD
2, ... LD
N in parallel to each bypass current source Q
1, Q
2 ..., Q
N. Each bypass current source Q
1, Q
2 ..., Q
N drives a bypass current I
Q1, I
Q2 ..., I
QN.
[0027] Each bypass current source Q
1, Q
2 ..., Q
N and the respective light emitting diode LD
1, LD
2, ... LD
N form a parallel circuit, wherein all these parallel circuits are connected in series.
[0028] A sigma-delta modulator 1 is connected to each bypass current source Q
1, Q
2 ..., Q
N and configured to control the respective bypass current I
Q1, I
Q2 ..., I
QN passing through the respective bypass current source Q
1, Q
2 .... Q
N. As a result, the effective load current I
LD1, that passes through a certain light emitting diode LD
1 of the plurality of light emitting diodes, equals to the difference between the main
current I
QM and the respective bypass current I
Q1, that is:
whereby i is an index ranging from 1 to N denoting the number of the bypass current
source Q
i driving the bypass current I
Qi and the light emitting diode LD
i with the load current I
LDi.
[0029] Similar to the examples of FIGs. 3, 4, and 5 the brightness of each single LED LD
i may be adjusted to a desired value by appropriately controlling the bypass currents
I
Qi and thus the load currents I
LDi by means of the sigma-delta modulators 1.
[0030] Each sigma delta-modulator 1 may comprise an digitally addressable bus interface,
for example a serial bus interface for connecting a serial bus 30. The desired current
or brightness value may be received from the bus 30 as a binary word. For multi-colour
illumination the brightness values may be taken from a calibration table as illustrated
in the example of FIG. 1. Of course the sigma-delta modulators 1 of the present example
may be followed by a second modulator 2, e.g. a pulse-width modulator, as discussed
with reference to FIG. 4.
[0031] FIG. 8 illustrates an example similar to the example of FIG. 7, where semiconductor
switches, i.e. transistors, e.g. MOS-FETs, are employed as bypass current sources
Q
i. Except of the bypass current sources the example of FIG. 8 is identical to the example
of FIG. 7.
[0032] In multi-colour applications, for example an illumination device comprising a red
LED LD
1, a green LED LD
2, and a blue LED LD
3, and a driver circuit as shown in FIG. 8, the colour generated by mixing the light
of the different LEDs may be adjusted by appropriately adjusting the brightness of
each single LED LD
1, LD
2, LD
3 by means of the sigma-delta modulators 1. Additionally, the overall brightness may
be adjusted by varying the main current I
QM.
1. A driver circuit for driving a light emitting diode comprising:
a controllable current source for connecting to the light emitting diode, the current
source being configured to regulate a load current passing through the light emitting
diode dependent on a control signal;
a modulator unit configured for generating the control signal, where the control signal
is dependent on a pulse-density modulated signal.
2. The driver circuit of claim 1, where the modulator unit comprises a sigma-delta modulator
for providing the pulse-density modulated signal.
3. The driver circuit of claim 1 or 2, where the pulse-density modulated signal is directly
supplied to the current source as the control signal.
4. The driver circuit of claim 2, where the sigma-delta modulator comprises a multi-bit
comparator for providing a multi-bit pulse-density modulated signal.
5. The driver circuit of claim 4, where the modulator unit comprises a second modulator
receiving the multi-bit pulse-density modulated signal and providing the control signal
to the current source.
6. The driver circuit of claim 5, where the second modulator is a pulse-width modulator
or a pulse-frequency modulator.
7. The driver circuit of claim 6, where the second modulator is a digital-to-analog converter.
8. The driver circuit of one of the claims 1 to 7, where the current source is a transistor.
9. The driver circuit of one of the claims 1 to 8 further comprising an adder being connected
upstream to the sigma-delta modulator, where the adder is configured to receive a
first signal representing a desired brightness or colour and to add a dither noise
signal thereto, the sum of the signals being supplied to the sigma-delta modulator.
10. A method for driving a light emitting diode comprising:
providing a first signal representing a desired brightness or colour;
generating a pulse-density modulated signal representing the first signal; and
providing a current to the light emitting diode dependent on the pulse-density modulated
signal.
11. The method of claim 10, where the modulating is performed by a sigma-delta modulator.
12. The method of claim 10 or 11, where the current provided to the light emitting diode
is proportional to the pulse-density modulated signal.
13. The method of claim 11, where the pulse-density modulated signal is a multi-bit pulse-density
modulated signal.
14. The method of claim 11 further comprising:
providing the multi-bit pulse-density modulated signal to a second modulator that
generates a control signal representing the multi-bit pulse-density modulated signal,
where the current provided to the light emitting diode is proportional to the control
signal.
15. The method of claim 14, where the second modulator is a pulse-width modulator or a
pulse-frequency modulator.
16. The method of claim 14, where the second modulator is a digital-to-analog converter.
17. The method of claim 10, where the pulse-density modulated signal is a stream of bit-symbols
of equal length, each bit-symbol representing either the binary symbol "1" or "0".
18. The method of claim 10, where the pulse-density modulated signal is a set of streams
of bit-symbols of equal length, each bit-symbol representing either the binary symbol
"1" or "0".
19. The method of one of the claims 10 to 18, where a dither noise signal is added to
the first signal before being supplied to the sigma-delta modulator for generating
a pulse-density modulated signal therefrom.