1. Field of the Invention
The present invention relates to an electronic driving apparatus, and particularly to a driving circuit for light emitting diodes (LEDs).
2. Description of Prior Art
Referring to
VF=Von+RSIF+(ΔVF/ΔT)(T−25° C.)
where Von is a threshold voltage of the LED 121
When a temperature of the environment is constant, the above equation can be simplified as follows:
VF=Von+RSIF
Thus the electrical characteristic of the LED driving circuit can be expressed as:
U−mVon=I(R+RS(m/n))
Simplifying the above equation yields:
U−VX=I(R+RX)
where
VX=m*Von, RX=RS(m/n)
Because of the existence of Von, the current I does not change proportionally with the voltage U. For example, when the voltage U changes to 2U, the current I does not double but instead changes to less than 2I. The LED driving circuit cannot regulate the current I proportionally by linearly changing the voltage U provided to the LED array 12. Therefore it is difficult to precisely control the current I.
Similarly, when a quantity of the LEDs 121 or when a form of the LED array 12 is changed (i.e., VX and/or RX is varied), alterations of the voltage U and the resistor R are required in order to control the current I. However, for the reasons described above, such alterations to precisely control the current I are difficult.
Therefore, it is desirable to provide an improved driving circuit which overcomes the above-described disadvantages of the conventional driving circuit.
An object of the present invention is to provide a driving circuit that allows precise control of current.
In order to achieve the above-described object, a light emitting diode driving circuit in accordance with the present invention includes a current limiting resistor, an FET (Field Effect Transistor), and a feedback circuit. The FET has a drain electrode connected to the current limiting resistor to provide current to a load such as an LED, and the feedback circuit has a variable reference voltage source for controlling a current of the drain electrode of the FET.
Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
Reference now will be made to the drawings to describe the present invention in detail.
Referring to
The feedback circuit 23 includes a first differential amplifier 231, a second differential amplifier 232, an integrating circuit 233, and a variable reference voltage source 234. The first differential amplifier 231 has two input terminals, which are connected to two terminals of the current limiting resistor 24, respectively. An output terminal of the amplifier 231 and the variable reference voltage source 234 are connected to two input terminals of the second differential amplifier 232. An output terminal of the second differential amplifier 232 is connected to the integrating circuit 233, and an output terminal of the integrating circuit 233 is connected to the gate electrode G of the FET 22.
A voltage provided by the power supply 21 is VDD, a voltage drop of the current limiting resistor 24 is VR1, an output voltage of the first differential amplifier 231 is V0, an output voltage of the second differential amplifier 232 is V1, a voltage provided by the variable reference voltage source 234 is VREF, and an output voltage of the integrating circuit 233 is VG. Thus VG is a voltage of the gate electrode G of the FET 22. An operating current of the drain electrode D of the FET 22 is iD. That is, a current in the current limiting resistor 24 and the LED 25. A resistance of the current limiting resistor 24 is R1, a resistance of a resistor (not labeled) between the power supply 21 and the source electrode S is R4, and resistances of resistors (not labeled) of the feedback circuit 23 are R2, R3 and R5˜R12 respectively. A capacitance of a capacitor (not labeled) of the integrating circuit 233 is C1.
It would be desirable to use the feedback circuit 23 for stabilizing the operating current iD in the LED 25. In order to achieve this object, keeping VR1 constant is all that is required.
The two input terminals of the first differential amplifier 231 are connected to the two terminals of the current limiting resistor 24 respectively; therefore VR1 is an input signal of the first differential amplifier 231. When R12/R3=R5/R2, V0 is expressed as:
V0=−R5VR1/R2 (1)
The output terminal of the amplifier 231 and the variable reference voltage source 234 are connected to the two input terminals of the second differential amplifier 232. When R11/R10=R8/R9, V1 can be expressed by the following equation:
The output voltage V1 is input to the integrating circuit 233, and the output voltage of the integrating circuit 233 is VG. That is, the voltage of the gate electrode G of the FET 22 becomes:
When the resistances of the resistors R2, R3 and R5˜R12 are equal, Eq. (3) can be simplified as follows:
VG=−(1/RC1)∫(VR1+VREF)dt (4)
Because VG and iD have a linear relationship, this can be expressed as: VG=KiD, where K is a constant. Also, VR1=iDR1. Substituting these two equations into Eq. (4) yields:
KiD=−(1/RC1)∫(iDR1+VREF)dt (5)
Differentiating Eq. (5) gives:
Ki′D=−(1/RC1)(iDR1+VREF) (6)
Solving for i′D yields:
i′D=−AiD+BVREF (7)
where
A=(1/RC1)R1/K
and
B=(1/RC1)/K
Then the solution of the Eq. (7) is expressed as:
iD=(BVREF/A)(1−Exp(−At)) (8)
When time t increases, Exp(−At) approaches zero, and finally the following equation is obtained:
iD=BVREF/A=VREF/R1 or
iDR1=VREF (9)
Equation (9) expresses the linear relationship between the operation current in the LED 25 and the variable reference voltage source 234. Thus the operation current in the LED 25 can be precisely regulated by adjusting the variable reference voltage source 234.
In this embodiment, the resistances of the resistors R2, R3 and R5˜R12 in the feedback circuit are equal. If the resistances of said resistors are not equal, then equation (9) is modified to:
iDR1KR=VREF
Where KR is a constant, which is determined by the resistances of said resistors. That is, KR does not affect the linear relationship between the variable reference voltage source 234 and the operation current of the LED 25.
An LED or an LED array driven by the driving circuit 2 of the present invention can be used as a light source in a field of display or a like apparatus.
The main advantage of the described embodiment over the prior art is that the driving circuit 2 includes a feedback circuit which stabilizes the operating current under different loads, and which provides precise current control.
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
Number | Date | Country | Kind |
---|---|---|---|
92104987 A | Mar 2003 | TW | national |
Number | Name | Date | Kind |
---|---|---|---|
4160934 | Kirsch | Jul 1979 | A |
5025204 | Su | Jun 1991 | A |
5175748 | Takahashi | Dec 1992 | A |
6097360 | Holloman | Aug 2000 | A |
6690146 | Burgyan et al. | Feb 2004 | B2 |
6734639 | Chang et al. | May 2004 | B2 |
20020047642 | Miyagawa | Apr 2002 | A1 |
20030086457 | Theodoras | May 2003 | A1 |
Number | Date | Country | |
---|---|---|---|
20040183478 A1 | Sep 2004 | US |