LED Driver

The present invention relates to an LED driver and to an associated method of driving an LED.

In conventional circuitry, LEDs are typically driven from a power source, such as a battery or a mains supply. The current input to the LED is limited by the use of a series resistor to prevent overloading. The resistor however dissipates power. At higher driving voltages (when a higher voltage supply is used), the LED will draw more current and accordingly, the power dissipated in the resistor is also increased. This dissipation reduces the energy efficiency of the LED driver.

If reduced power consumption is required, a known alternative is to drive the

LED with a pulsed voltage. The dissipation can then be maintained within a target range by adjusting the on off ratio of the pulsing. Whilst this solves the problem, this solution is not applicable in all situations, in particular where the use of a pulsed power supply will induce unwanted noise in related circuitry.

One example of a system where a pulsed supply is inappropriate is an automotive system. In such systems there are tight legislative requirements for the light intensity output of certain lamps on the vehicle. To ensure that an acceptable light intensity output is maintained over the entire range of potential automotive battery voltages can result in excessive dissipation within the driver when used at higher driving voltages. Additionally in the present day there is a constant pressure to

increase energy efficiency of vehicle and hence all methods of power saving are important.

Accordingly it is an object of the present invention to provide an LED driver and method of operating the same which at least partially overcomes or alleviates the above problems.

According to a first aspect of the present invention there is provided a method of driving an LED comprising the following steps: providing a constant current power source; monitoring the voltage level of the power source; and varying the current supplied by the power source dependent upon the voltage level of the power source.

By use of the above method, the output light intensity of an LED can be maintained within a desired range even when the supply voltage varies over a wide range without undue power being lost due to dissipation in a series resistor.

In one embodiment, the voltage of the power source is monitored to determine whether it is above or below a threshold value. In such cases, when the voltage of the power source is above the threshold value the current supplied by the power source may be reduced. When the voltage of the power source is below the threshold value the current supplied by the power source may be unvaried. In an alternative arrangement, the voltage level of the power source is monitored to determine whether it is within or outside a range having upper and lower threshold values. In such embodiments, when the voltage level is within the upper and lower threshold values,

the current supplied by the power source is unvaried but when the voltage level is above the upper value or below the lower value the current supplied by the power source is varied. In particular, when the voltage level is above the upper value or the current supplied by the power source may be reduced and when the voltage level is below the lower value the current supplied by the power source may be increased.

The variation in the current may be a stepwise increase or decrease.

Alternatively the variation in the current may be in accordance with a function of voltage. A suitable function of voltage may be a linear function of voltage. A different step or a different function of voltage may be utilised when the voltage level is above the upper value to when the voltage level is below the lower value.

The method may include the additional step of relaying status information relating to the voltage level to related circuitry for monitoring and/or diagnostic purposes. The output light level may also be monitored and relayed to related circuitry for monitoring and/or diagnostic purposes.

The method may be utilised to drive an LED within any suitable circuitry. In a particular embodiment the LED may be provided within an automotive system. In such an automotive system, the upper and lower threshold values of the range may be around say 16V and around say 9 V respectively.

According to a second aspect of the present invention there is provided an LED driver operable according to the method of the first aspect of the present invention comprising: a constant current power source, for outputting a constant

current power supply to the LED; a series resistor, provided between the power source and the LED; monitoring means for monitoring the voltage level of the power source to determine whether it is above or below a threshold value; and control means operable in response to the monitoring means to vary the current supplied by the power source if the voltage level of the power source is above the threshold value.

The LED driver of the second aspect of the present invention may incorporate any or all features of the method of the first aspect of the present invention as are desired or required.

In order that the invention may be more clearly understood one embodiment is described below, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a schematic block diagram of a known LED driver; and

Figure 2 is a schematic block diagram of an LED driver according to the present invention.

Referring now to figure 1, a conventional driver 100 for an LED 110 comprises a power source 101 and a series resistor 102, the series resistor 102 connected to the LED 110. The series resistor 102 limits the current input to the LED 110 and thus helps to prevent overload. Power is dissipated in the series resistor 102 and this reduces the efficiency of the driver 100. When the supply voltage increases, the dissipated power also increases, reducing the efficiency of the driver 100. This

lack of efficiency is a particular problem when it is necessary to maintain a desired light output level from LED 110, for instance when the LED 110 is utilised in an automotive application wherein there are tight legislative requirements on light output level.

Turning now to figure 2, an LED driver 200 according to the present invention is shown. In this example, the LED driver 200 comprises a power source 201 and a series resistor 202, the series resistor 202 connected to the LED 210. The series resistor 202 limits the current input to the LED 210 and thus helps to prevent overload. To help reduce the dissipated power in the driver 200, the power source 201 is a constant current power source 201 and the driver additionally comprises a voltage monitoring means 203 and a control means 204. The voltage monitoring means 203 is operable to monitor the voltage level of the constant current power source 201. The control means 204 is connected to the voltage monitoring means 203 and is operable to control the current output by the constant current power source 201 in response to variation in the voltage level of the constant current power source 201 as determined by the voltage monitoring means 203.

In use, the voltage monitoring means 203 are operable to determine when the voltage level of the constant current power source 201 falls outside of a preset normal range. If the voltage is in the normal range, then the control means is not operable to vary the current output by the constant current power source 201. If the voltage exceeds the upper limit of the normal range, the control means 204 is operable to reduce the current output by the constant current power source 201. If the voltage

drops below the lower limit of the normal range, the control means 204 may be operable to increase the current output by the constant current power source 201.

In a typical example, the LED 210 is used in an automotive application. The normal range of voltage is preset at 9V- 16V. By maintaining a steady current within the normal operating range an output light level can be maintained within around 1% of a desired level. By reducing the current when the voltage exceeds 16V and decreasing the current when the voltage drops below 9V an output light level can be maintained within around 10% of a desired output level when the supply voltage is in the wider range 8V- 18 V.

Both above and below the 8 - 18V range it is difficult to maintain such an even output intensity however monitoring of the voltage still takes place. In the 18V and over ranges this is also necessary to prevent overheating. Such a danger can be avoided by decreasing the current. In the case of a supply voltage below 8V, a decrease in the current is used to maintain as long as possible a case where the forward voltage of the LED 210 is not higher than the supply voltage minus the voltage drop across the series resistor 202. This enables controlled performance to be maintained down to as low a supply voltage as is possible. By undertaking such monitoring, different LEDs 210 may be driven by a common driver so as to output as even a light intensity as is possible.

The driver 200 can provide a diagnostic tool should the LED 210 fail. In such circumstances the variation in current caused by a failed LED 210 would differ from

that caused by a short circuit. This is similar in operation to the common automotive arrangement wherein an ordinary incandescent lamp is connected parallel with a resistor such that a determination may be made between a broken bulb or a wiring fault.

The control means 204 may have connections to external components or circuits. The control means 204 may thus be operable to relay status information relating to the driver 200, including voltage level to such external components or circuitry for monitoring and/or diagnostic purposes.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiment which is described by way of example only.