Driver circuit and method of operating the same

Details Driver circuit and method of operating the same Driver circuit and method of operating the same

1998-01-02

2001-03-06

Crosland, Donnie L. (Department: 2736)

Communications: electrical

Visual indication

Using light emitting diodes

C340S331000, C340S691100, C340S384720, C340S384710

Reexamination Certificate

active

06198405

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention refers to a driver circuit having an inductor, first and second connection points for connection of a voltage source, switching means which when in a first state allows an electrical current to flow from the first connection point and through the inductor to thereby charge the inductor with energy and when in a second state substantially prevents an electrical current from flowing from the first connection point to the inductor. It also refers to a method of operating the same.
DESCRIPTION OF THE PRIOR ART
Drivers for Light Emitting Diodes, LED's, are well known in the prior art.
A first type of LED driver comprises a resistor, an LED and a switch connected to a voltage source. A first electrode of the resistor is connected to the anode of the LED. The cathode of the LED is connected to a first electrode of the switch. The electrode of the voltage source having the most positive potential, “plus-pole”, is connected to the second electrode of the resistor and the electrode of the voltage source having the most negative potential, “minus-pole”, is connected to the second electrode of the switch. The switch may be an n-type bipolar transistor where its first electrode is the collector and its second electrode is the emitter.
In operation, when the switch is closed, i.e. conducting, a current flows from the “plus-pole” of the voltage source through the resistor, the LED and the switch to the “minus-pole” of the voltage source. If the value of the resistor and the voltage of the voltage source is chosen properly, the LED will emit light. This occurs when the voltage over the LED is greater than the threshold voltage of the diode when forward biased. This voltage, referred to as V
F
, is about 1 to 2 V. The resistor is employed to limit the current in the circuit. The switch may be realized by, for example, a bipolar transistor or a Field Effect Transistor, FET.
A drawback with the first type of LED driver is that the LED requires a minimum voltage in the forward direction to emit light. Furthermore, the current limiting resistor will consume power which will be wasted. These drawbacks become more pronounced when the voltage source is a battery where the maximum voltage supplied is limited and the energy stored in the battery is a scarce resource. If V
F
is 1.4 V and a bipolar transistor, for which the potential between the collector and the emitter is 0.2 V when the transistor is conducting, is used as a switch, the voltage of the voltage source needs to be more than 1.6 V (1.4+0.2). In this case it would not be possible to use a battery providing a voltage of 1.5 V. The situation becomes even worse if two or more LED's are connected in series. Even if the voltage of the voltage source is sufficiently high to allow the LED to emit light, energy is wasted in the resistor. This is undesired since the available amount of energy which is stored in the battery is limited.
A first solution to the above mentioned problems is presented in DE-A-22 55 822. Disclosed herein is a driver which comprises an LED, a bipolar transistor acting as a switch and an inductor connected to a voltage source. The LED and the inductor are connected in parallel. The anode of the LED is connected to the collector of an n-type bipolar transistor. The electrode of the voltage source having the most positive potential, “plus-pole”, is connected to the cathode of the LED and the electrode of the voltage source having the most negative potential, “minus-pole”, is connected to the emitter of the bipolar transistor.
In operation, the transistor is used as a switch which is alternately closed and opened. This is achieved by the application of an appropriate signal on the base of the transistor. During the period when the switch is closed energy is stored in the inductor. Thereafter, when the switch is opened, the stored energy is released through the LED. If the parameters of the inductor are appropriately chosen, the voltage over the LED in the forward direction will reach the threshold voltage V
F
and the LED will emit light. The switch is then closed again to repeat the sequence described above. It should be noted that the maximum voltage over the LED in the forward direction may have a greater nominal value than the nominal value of the voltage supplied by the voltage source. It is thereby possible to drive an LED using a voltage source supplying a voltage which has a smaller nominal value than the threshold voltage V
F
of the LED. Furthermore, this solution does not include any current limiting resistor in which power is wasted.
A second solution to the above mentioned problems is disclosed in U.S. Pat. No. 3,944,854. Disclosed herein is a driver which comprises an LED, a bipolar transistor acting as a switch and an inductor connected to a voltage source. In this case the LED is connected in parallel with the switch. The operation of the driver is thus similar to the operation of the driver disclosed in DE-A-22 55 822 above.
A driver of an electroluminescent lamp, EL-lamp, comprising a switching circuit and an inductor is disclosed in U.S. Pat. No. 5,313,141.
Drivers for buzzers are well known in the prior art.
A buzzer comprises an inductor and a membrane. In operation an electrical potential, which alternates periodically, is applied over the inductor and a magnetic field having a periodically changing strength is thereby created in the vicinity of the inductor. The membrane, which is physically placed adjacent to the inductor, is made to vibrate due to these changes in the strength of the magnetic field. These vibrations of the membrane generates an acoustic signal. The operation of a buzzer is thus similar to the operation of a loudspeaker.
A prior art buzzer driver comprises a buzzer, a transistor, a resistor, a diode and an n-type bipolar transistor connected to a voltage source. A first electrode of the buzzer is connected to a first electrode of the resistor and to the anode of the diode. The second electrode of the resistor is connected to the collector of the transistor. The electrode of the voltage source having the most positive potential, “plus-pole”, is connected to a second electrode of the buzzer and to the cathode of the diode. The electrode of the voltage source having the most negative potential, “minus-pole”, is connected to the emitter of the transistor.
In operation, the transistor may be used as a switch which is alternately closed and opened. This is achieved by the application of an appropriate signal on the base of the transistor. A current will flow through the inductor of the buzzer when the transistor is conducting and energy will be stored in the inductor. The stored energy will be released as a current through the diode when the transistor is not conducting. The current through the inductor of the buzzer will generate a magnetic field around the inductor. The physical position of the membrane within the buzzer will depend on the strength of the magnetic field. Since the strength of the magnetic field will periodically vary as a function of time dependent on the switching of the transistor, the membrane will vibrate and thereby generate an acoustic wave. The frequency of the acoustic wave will depend on the frequency of the switching of the transistor. Other kinds of periodical signals such as a sine curve may, of course, also be used when driving the transistor.
To fully understand the background of the invention a number of prior art circuits will now be discussed.
An LED driver may be employed to drive a number of LED's. This is frequently used in the prior art when the LED's are intended to generate background light for example to a Liquid Crystal Display (LCD) or to the pads of a keyboard. One kind of a prior art LED driver for a plurality of LED's comprises a constant current generator and a plurality of LED's connected to a voltage source. A group of LED's may be connected in series or in parallel. A number of groups of LED's may then be connected in series or in parallel.
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