The present invention relates to a circuit arrangement for operating at least one LED, having an operational amplifier with a non-inverting input and an inverting input as well as an output, a desired value predefining apparatus which is coupled to the non-inverting input of the operational amplifier, a first connection and a second connection for the at least one LED, wherein the first connection is coupled to a connection for a DC supply voltage, a transistor which is coupled in series with the first and second connections for the at least one LED and can be operated in an analog manner, wherein the transistor has a control electrode, a reference electrode and a working electrode, wherein the control electrode of the transistor is coupled to the output of the operational amplifier, wherein the working electrode of the transistor is coupled to the connection for a DC supply voltage, and a current measuring resistor which is coupled in series between the reference electrode of the transistor and a reference potential, wherein the voltage dropped across the current measuring resistor is coupled to the inverting input of the operational amplifier. The invention also relates to a corresponding method for operating at least one LED.
A generic circuit arrangement, known from the prior art, is shown in
Generic circuit arrangements are used, for example, in LED projection applications, in particular in so-called back projection. Signals are applied from the desired value predefining apparatus to the operational amplifier, which may have very short turn-on pulses, up to 4 μs, and very short dark periods, likewise up to 4 μs. As corresponding analyses have shown, operation of the generic circuit arrangements was unsatisfactory in particular in the case of very short turn-on pulses or dark periods. This leads to projection results of lower quality.
The object of the present invention is therefore to develop a circuit arrangement described in the introduction or a method described in the introduction such that higher quality projection applications are made possible.
This object is achieved by a circuit arrangement with the features of claim 1 and a method with the features of claim 6.
The present invention is based on the understanding that within the entire current range which can flow through the LED and which ranges from 0 A to ILEDmax, at a specified current in the range of 0 A, the linear regulator has undesirable, strongly deteriorating properties. The reason for this is that in practice it is never possible to control a current of 0 A with complete precision, i.e. positive or negative current, even if very low, always flows. As the circuit arrangement used permits no negative currents at all, in this case the operational amplifier would saturate and abandon linear regulator operation. As a result, the control characteristics would deteriorate inadmissibly greatly. The regulator therefore displays various dynamic behavior within the entire current range. A detailed analysis of the processes with currents close to 0 A can be found below, reference being made to
As a result of at least one other load being connected in parallel to the LED, at appropriate dimensioning the voltage over the LED remains so low that the LED still does not emit light, although the analog transistor is already in linear operation, as a positive control voltage is applied to it. The transistor can be switched on quickly so that a time lag is avoided by the slew rate of the analog-operable transistor. As a result extremely short turn-on pulses and dark periods can be achieved, resulting in very high-quality projection applications.
In the circuit arrangement according to the invention an LED can therefore be operated regardless of manufacturer or color or manufacturing lot such that it does not yet light up at a specified current of 0 A, but the current regulator is already operative, i.e. in linear operation. At now predefined current steps the regulator can react with extremely small time constants.
A further advantage of having at least one load connected to an LED in parallel is that this results in the discharge of the capacity of the LED and its cables after the current through the LED has been switched off. This avoids an after-glow of the LED, which in the prior art may be up to 1 μs. Furthermore, negative current spikes on account of line inductivities, which may be up to 1 V and can therefore result in the failure of the LED, are reliably eliminated.
Preferably the load connected to at least one LED in parallel represents at least one or more elements of the following selection: ohmic resistor, current sink, constant-current diode.
It is particularly preferable that the load includes at least one first and one second partial load, an electronic switch being assigned in series to at least the second partial load. This opens up the possibility of changing the load as a function of the color emitted by the LED or to take manufacturing tolerances into consideration, in order to take account of different cut-off voltages. By this means the current as of which the LED emits light can be selected. Ageing of the LED or a change in the temperature of the LED, for example, can also be taken into account in this way.
Furthermore, it is particularly preferable that the circuit arrangement includes a microcontroller which is designed to determine the forward voltage of at least one LED coupled between the first and the second connection for the at least one LED and to control the electronic switch(es) accordingly. This opens up the possibility of always automatically connecting the most appropriate load or the most appropriate loads of the at least one LED in parallel, i.e. in particular, also dynamically during operation of the at least one LED.
Furthermore, it is preferable if a feedback network is connected between the output and the inverting input of the operational amplifier. By this means the control parameters of the linear regulator and consequently the circuit arrangement can be selected.
Further advantageous embodiments emerge from the subclaims.
The preferred embodiments and their advantages presented in connection with a circuit arrangement according to the invention apply accordingly, insofar as applicable, to the method according to the invention.
In a preferred embodiment of the method according to the invention the step of coupling takes place such that as a result when operating the circuit arrangement a positive current constantly flows through the at least one LED.
We will now describe in more detail an exemplary embodiment of a circuit arrangement according to the invention with reference to the attached drawings. These show:
The reference characters inserted with reference to
In
However, it is even more critical if the offset voltage UOF is negative. For clarification, reference is made to the chronological sequences of the voltage Usoll, UGS and Ushunt in
If the operational amplifier 12 is now to be moved out of this position again into an area with positive current ILED, the operational amplifier 12 initially finds itself “so affected”, i.e. in such a saturated condition, that dynamically it is very slow. This is shown by the curved line at the bottom of
Increasing the voltage Usoll such that it is always greater than UOF, regardless of whether UOF is positive or negative, would result in UGS always being greater than zero and thus a current flow ILED takes place through the LED, even if this is not desired. In order to prevent this, see
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP08/58753 | 7/7/2008 | WO | 00 | 2/23/2011 |