Lamp Control Circuit Having a Brightness Condition Controller Having 2.sup. N .sup.rd and 4.sup.th Current Paths

1. A back light circuit for a liquid crystal display, said circuit comprising: an alternating current adapter for receiving alternating current and converting said alternating current to generate a direct current; a battery for providing said direct current when said alternating current adapter is not being used to generate said direct current; an adapter detecting unit for determining whether said alternating current adapter is being used to generate said direct current; a direct current/direct current converter for receiving said direct current and providing output of a switched signal via a switching regulator; a direct current/alternating current converter for driving a cold cathode fluorescent lamp by generating an increased electric potential after converting said switched signal output from said direct current/direct current converter; said cold cathode fluorescent lamp operated by said direct current/alternating current converter for providing back light for said liquid crystal display; an on/off controller for controlling driving of said direct current/direct current converter to interrupt operation of said cold cathode fluorescent lamp; a brightness controller having a first current channel for controlling a degree of brightness of said back light provided by said cold cathode fluorescent lamp; a feedback unit for receiving an output signal from said brightness controller and providing said output signal from said brightness controller to said direct current/direct current converter; a brightness condition detector for determining said degree of brightness of said back light based on said output signal from said brightness controller; a battery signal transmitter for generating signals indicating a residual amount of battery power when said battery is being used to provide said direct current; and a brightness condition controller having second, third and fourth current channels for determining and controlling said degree of brightness of said back light in response to signals output from said adapter detector, said battery signal transmitter and said brightness condition detector.

2. The back light circuit as defined in claim 1, wherein said adapter detecting unit comprises a Zener diode for enabling generation of a constant reference voltage and a comparator for comparing said constant reference voltage with an input voltage.

3. The back light circuit as defined in claim 1, wherein said brightness condition detector is operated based on a voltage level of a signal input to said brightness controller.

4. The back light circuit as defined in claim 1, wherein said first current channel of brightness controller comprises a variable resistor and controls said degree of brightness of said back light based on a resistance value exhibited by said variable resistor.

5. The back light circuit as defined in claim 1, wherein each of said first to fourth current channels are operated when said back light is in a brightest mode, and two of said first to fourth current channels are operated when said back light is in a darkest mode.

6. The back light circuit as defined in claim 1, wherein each of said first to fourth current channels are operated when said alternating current adapter is being used to generate said direct current, and less than each of said first to fourth current channels are operated when said battery is being used to generate said direct current.

7. A circuit for controlling back lighting of a liquid crystal display, said circuit comprising: an alternating current adapter for receiving alternating current and converting said alternating current to generate a direct current used to produce said back lighting of said liquid crystal display; battery means for providing said direct current used to produce said back lighting of said liquid crystal display when said alternating current adapter is not being used to generate said direct current; means for determining whether said alternating current adapter is being used to generate said direct current; and control means for automatically controlling said back lighting of said liquid crystal display based on whether said alternating current adapter is being used to generate said direct current, and further based on a detected level of residual power of said battery means and a voltage indicative of a present degree of brightness of said back lighting, said control means enabling said back lighting of said liquid crystal display to be exhibited at a brightest level when said alternating current adapter is being used to generate said direct current, said control means enabling said back lighting of said liquid crystal display to be exhibited at one of three different brightness levels having a lower intensity than said brightest level when said battery means is being used to generate said direct current, wherein none of said three different brightness levels represent an off state of said liquid crystal display.

8. The circuit as defined in claim 7, wherein said control means enables said back lighting of said liquid crystal display to be exhibited at a darkest level when said residual power of said battery means is at a lowest level.

9. The circuit as defined in claim 7, further comprising a cold cathode fluorescent lamp for generating said back lighting of said liquid crystal display.

10. A circuit for controlling back lighting of a liquid crystal display, said circuit comprising: an alternating current adapter for receiving alternating current and converting said alternating current to generate a direct current used to produce said back lighting of said liquid crystal display; battery means for providing said direct current used to produce said back lighting of said liquid crystal display when said alternating current adapter is not being used to generate said direct current; means for generating said back lighting of said liquid crystal display in response to said direct current; and control means for automatically controlling said back lighting of said liquid crystal display based on whether said alternating current adapter is being used to generate said direct current, and further based on a detected level of residual power of said battery means and a voltage indicative of a present degree of brightness of said back lighting, said control means enabling said back lighting of said liquid crystal display to be exhibited at a brightest level when said alternating current adapter is being used to generate said direct current, said control means enabling said back lighting of said liquid crystal display to be exhibited at one of three different brightness levels having a lower intensity than said brightest level when said battery means is being used to generate said direct current, wherein none of said three different brightness levels represent an off state of said liquid crystal display.

11. The circuit as defined in claim 10, wherein said plurality of brightness levels comprises three different brightness levels.

12. The circuit as defined in claim 10, wherein said control means enables said back lighting of said liquid crystal display to be exhibited at a darkest level when said residual power of said battery means is at a lowest level, wherein said darkest level does not represent an off state of said liquid crystal display.

13. The circuit as defined in claim 12, wherein said means for generating said back lighting of said liquid crystal display comprises a cold cathode fluorescent lamp.

14. A back light circuit for a liquid crystal display, said circuit comprising: an alternating current adapter for receiving alternating current and converting said alternating current to generate a direct current; a battery for providing said direct current when said alternating current adapter is not being used to generate said direct current; an adapter detecting unit for determining whether said alternating current adapter is being used to generate said direct current; a direct current/direct current converter for receiving said direct current and providing output of a switched signal via a switching regulator; a direct current/alternating current converter for driving a cold cathode fluorescent lamp by generating an increased electric potential after converting said switched signal output from said direct current/direct current converter; said cold cathode fluorescent lamp operated by said direct current/alternating current converter for providing back light for said liquid crystal display; an on/off controller for controlling driving of said direct current/direct current converter to interrupt operation of said cold cathode fluorescent lamp; a brightness controller having a first current channel comprising a variable resistor for controlling a degree of brightness of said back light provided by said cold cathode fluorescent lamp based on a resistance value exhibited by said variable resistor; a feedback unit for receiving an output signal from said brightness controller and providing said output signal from said brightness controller to said direct current/direct current converter; a brightness condition detector for determining said degree of brightness of said back light based on said output signal from said brightness controller; a battery signal transmitter for generating signals indicating a residual amount of battery power when said battery is being used to provide said direct current; and a brightness condition controller having second, third and fourth current channels for determining and controlling said degree of brightness of said back light from in response to signals output from said adapter detector, said battery signal transmitter and said brightness condition detector; said second current channel comprising a first field effect transistor having a gate terminal connected to an output terminal of said adapter detector and a first resistor connected between said first field effect transistor and said variable resistor; said third current channel comprising a second field effect transistor having a gate terminal connected to a first output terminal of said battery signal transmitter and a second resistor connected between said second field effect transistor and said variable resistor; and said fourth current channel comprising a third field effect transistor having a gate terminal connected to a second output terminal of said battery signal transmitter and a third resistor connected between said third field effect transistor and said variable resistor.

15. The circuit as set forth in claim 14, said brightness condition controller further comprising: a fourth resistor connected between an output terminal of said brightness condition detector and said gate terminal of said second field effect transistor; and a fifth resistor connected between said output terminal of said brightness condition detector and said gate terminal of said third field effect transistor.