1. Field of Invention
The present invention relates to a light emitting device array driver circuit, and a current splitter circuit and a method of splitting current for the light emitting device array driver circuit. In particular, the present invention relates to such driver circuit, current splitter circuit and method of splitting current which simplify the wiring and control of a power regulator circuit by providing a total supply current to the light emitting device array.
2. Description of Related Art
Light emitting devices, of which one typical example is LED (light emitting diode), are used in many applications. One application of light emitting devices is to arrange them in a array to provide backlight to a LCD (liquid crystal display). Referring to FIG. 1, to driver the light emitting device array 20, a power regulator 10 is required to provide a suitable supply voltage and supply currents to multiple strings of light emitting devices in the light emitting device array 20.
More specifically, as shown in FIG. 1, the power regulator 10 provides a supply voltage VLED to the light emitting device array 20. The light emitting device array 20 includes N light emitting device strings and each light emitting device string includes M light emitting devices, wherein M and N are positive integers. Each light emitting device string has a first end electrically connected to the power regulator 10 in common, and a second end electrically connected to a corresponding current source circuit 301 in a current control circuit 30. The current source circuit 301 controls the current of the light emitting device string connected thereto, so that the light emitting devices generate uniform light.
However, due to variations in manufacture, the voltages across light emitting devices are often different from one another and the difference may be as high as 10%, which is about 0.3V. Since the difference between the total voltage drop in one light emitting device string and the total voltage drop in another light emitting device string may be as high as 10%, if there are 20 light emitting devices in one light emitting device string, there may be a total difference of as high as 6V. For the current source circuits 301 to operate normally, the supply voltage VLED must be able to sustain the operation of the light emitting device string having the highest voltage drop. To this end, one method is to obtain a feedback signal from every light emitting device string and send the feedback signals to the power regulator 10, as shown in FIG. 1. However, this requires N signal wires between the light emitting device strings and the power regulator 10, and the power regulator 10 needs N additional pins, which are disadvantageous. In certain applications, there can only be two wires between the light emitting unit (including the light emitting device strings 20 and the current control circuit 30) and the power regulator 10, and in this case the prior art of FIG. 1 can not function.
FIGS. 2A-2C show several prior art circuit arrangements under the limitation that there can only be two wires between the light emitting device strings and the power regulator 10. In the prior art of FIG. 2A, the power regulator 11 provides the supply voltage VLED to the light emitting device array 20, and the current control circuit 31 only includes resistors R. As compared with the prior art of FIG. 1, the prior art of FIG. 2A reduces the number of wires connected to the power regulator 11, but it can not precisely control the uniformity of currents through all the light emitting device strings, that is, the currents through different light emitting device strings are very possibly different.
In the prior art of FIG. 2B, the current control circuit 32 includes a master current source circuit 322 which is connected to one of the light emitting device strings, and the other light emitting device strings are connected to circuits 321. The current of the master current source circuit 322 is mirrored to the circuits 321 so that currents through all the light emitting device strings are kept the same and uniform if the supply voltage VLED is sufficient to keep the current control circuit 32 normally working. However, this prior art does not properly feedback control the supply voltage VLED; if supply voltage VLED is not above a proper level, one or more of the circuits 321 may not operate normally, and the corresponding light emitting device string or strings can not operate normally to emit light with designed brightness. On the other hand, if the supply voltage VLED is set too high, it will result in undesired power loss and heat in the current control circuit 32. Furthermore, if the power regulator 11 supplies a regulated total current, the voltage VLED will be automatically established by the master current source circuit 322 and its corresponding LED string (referred to as the “master LED string”). In such a scenario, if the voltage drop of the master LED string is smaller than the voltage drop of any other LED string, the voltage VLED may not be sufficient for all LED strings and all current mirror circuits to work normally.
The prior art of FIG. 2C is similar to the prior art of FIG. 2B except that its current control circuit 33 further includes a circuit 332 for better controlling the circuits 321. In this circuitry, the master LED string is the LED string with connection to circuit 322, and its corresponding current source circuit is the master current source circuit. However, no matter whether the power regulator 11 supplies a regulated voltage or a regulated total current, the same problems happening in the prior art of FIG. 2B will happen in this prior art of FIG. 2C in the same manner.
In view of the above, the present invention provides a light emitting device array driver circuit, and a current splitter circuit and a method of splitting current for the light emitting device array driver circuit, wherein the supply voltage VLED is properly established by local feedback control, and the wiring of the power regulator is simplified.