Abstract:
The present invention discloses a backlight control circuit, comprising: a voltage supply circuit, which is a boost converter circuit for receiving an input voltage from an input terminal and generating an output voltage to an output terminal, the output voltage being provided as an operating voltage for a plurality of light emitting devices; at least one input capacitor electrically connected between the input terminal and ground; and at least one output capacitor electrically connected between the output terminal and the input terminal.

Description:
FIELD OF INVENTION 
   The present invention relates to a backlight control circuit. More particularly, the present invention relates to a backlight control circuit which uses a low voltage rating capacitor to provide a high output voltage. 
   BACKGROUND OF THE INVENTION 
   In a liquid crystal display, a backlight control circuit is used which controls light emitting diodes (LEDs) to illuminate from the back side of a liquid crystal screen, so that a user can observe an image from the front side of the liquid crystal screen. 
   In early days, LED backlight is used only in a small size screen, which does not require high backlight brightness. Therefore, the LEDs can be connected all in series or all in parallel.  FIG. 1  shows a prior art circuit wherein all LEDs are connected in series. As shown in the figure, a backlight control circuit  1  comprises a backlight control integrated circuit  10  which includes an input terminal and an output terminal, wherein the input terminal is connected with an input capacitor Cin to receive an input voltage Vin, and the output terminal is connected with an output capacitor Cout to provide an output voltage Vout. (Besides the backlight control integrated circuit  10  and the two above-mentioned capacitors, other devices irrelevant to the spirit of the present invention, such as magnetic devices, are omitted for simplicity.) 
   The backlight control integrated circuit  10  provides output voltage Vout to a plurality of LEDs L 1 -LN connected in series, and the output voltage Vout is provided via a voltage supply circuit  11  according to a signal  15  which is outputted from an error amplifier circuit  13 . A resistor R is provided on a path of the LEDs connected in series, and a voltage at a node Vsense 1  is compared with a reference voltage Vref to check whether a current through the path satisfies a predetermined condition. If the current is lower than a predetermined value and the voltage at the node Vsense 1  decreases, the error amplifier circuit  13  sends the signal  15  to the voltage supply circuit  11  to pull up the output voltage Vout, so that the current flowing through the LEDs increases. Additionally, to avoid the voltage supply circuit  11  from unlimitedly increasing the output voltage Vout (for example, when the error amplifier circuit  13  malfunctions, or when the path of the LEDs is open), an over voltage protection circuit  12  is provided in the backlight control integrated circuit  10 , which detects the output voltage Vout and sends a signal to stop the voltage supply circuit  11  from increasing Vout if the output voltage Vout is excessively high. (Depending on circuit design, the voltage supply can be totally stopped, or kept at an upper limit value. The latter is more popular in a backlight control circuit.) 
     FIG. 2  shows a typical structure of an over voltage protection circuit  12 , wherein the output voltage Vout is monitored by comparing the voltage at the node Vsense 2  with a reference voltage Vovp. The result of comparison determines a signal for controlling the voltage supply circuit  11 . 
   Referring to  FIG. 3 , it shows a conventional backlight control circuit with LEDs all connected in parallel. As shown in the figure, a backlight control circuit  2  comprises a backlight control integrated circuit  20 , wherein the currents passing through LEDs L 1 -LN are respectively controlled by the current sources CS 1 -CSN. The backlight control integrated circuit  20  comprises a minimum voltage selection circuit  21  which chooses a lowest voltage value among all voltages at cathode ends of the LEDs L 1 -LN, and the error amplifier circuit  13  compares the lowest voltage value with a reference voltage to generate a signal controlling the voltage supply circuit  11 . Thus, the output voltage Vout is under control so that all current source circuits are provided with sufficient operating voltage for normal operation, and all LEDs can illuminate normally thereby. 
   Similarly, the backlight control integrated circuit  20  can further comprise an over voltage protection circuit  12  as the one described above. 
   The number of LEDs that are allowed to be connected all in series or all in parallel in the above conventional arrangements is limited, and naturally this leads to connecting the LEDs partially in series and partially in parallel (series-parallel connection).  FIG. 4  shows a prior art arrangement of such series-parallel connection in which the backlight control integrated circuit  10  shown in  FIG. 1  is employed to provide voltage to a series-parallel connection circuit of LEDs. However, it only checks the current on the path of LEDs L 1 -LN but does not check those on the other paths. 
   Another prior art arrangement is shown in  FIG. 5  which employs the backlight control integrated circuit  20  shown in  FIG. 3  to compose a series-parallel connection circuit for LEDs. 
   In the above circuits shown in  FIGS. 1 ,  4 , and  5 , the larger the number of the series-connected LEDs is, the higher the required output voltage Vout is. Correspondingly, a higher voltage rating capacitor is required for the output capacitor, which will increase the total cost of the backlight control circuit. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is therefore an objective of the present invention to provide a backlight control circuit capable of supplying a relatively high output voltage by means of a relatively low voltage rating capacitor, to solve the above-mentioned cost and other issues. 
   In accordance with the foregoing and other objectives of the present invention, and as disclosed by an embodiment of the present invention, a backlight control circuit is provided, which comprises a voltage supply circuit, which receives an input voltage from an input terminal and generates an output voltage to an output terminal, wherein the output voltage being provided as an operating voltage for a plurality of light emitting devices; at least one input capacitor electrically connected between the input terminal and ground; and at least one output capacitor electrically connected between the output terminal and the input terminal. 
   Preferably, the voltage supply circuit further comprises a noise filtering circuit to avoid a noise problem from the electrical connection between the output capacitor and the input terminal. 
   Moreover, a power supply with a low internal impedance is preferred for providing the input voltage; in other words, a power supply having a low impedance for both current sourcing and current sinking is preferred. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1  is a schematic circuit diagram showing a prior art circuit including LEDs which are all connected in series and a backlight control circuit thereof; 
       FIG. 2  is a schematic circuit diagram showing a conventional over voltage protection circuit; 
       FIG. 3  is a schematic circuit diagram showing a prior art circuit including LEDs which are all connected in parallel and a backlight control circuit thereof; 
       FIG. 4  is a schematic circuit diagram showing a prior art circuit including LEDs which are connected partially in series and partially in parallel, and a backlight control circuit thereof; 
       FIG. 5  is a schematic circuit diagram showing another prior art circuit including LEDs which are connected partially in series and partially in parallel, and a backlight control circuit thereof; 
       FIG. 6  is a schematic circuit diagram showing a backlight control circuit according to an embodiment of the present invention; 
       FIG. 7  is a diagram for explaining the internal working model of a power supply; 
       FIGS. 8 and 9  are schematic circuit diagrams showing the arrangement of a noise filtering circuit in the voltage supply circuit  11 ; 
       FIGS. 10A-10D  are diagrams showing four embodiments of regulator circuits; 
       FIGS. 11A and 11B  are diagrams showing two embodiments of low-pass filter circuits; and 
       FIGS. 12A and 12B  are diagrams showing two embodiments of spike voltage clamper circuits. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The voltage of a white or blue LED may vary in a range from 3.3V to 4V due to manufacture deviation. To cope with it, in circuit design, the necessary output voltage Vout is calculated by 4V multiplied by the number of LEDs connected in series in a path. That is, if the number of LEDs in a path is more than or equal to 13, the Vout is higher than 50V. (4*13=52&gt;50) 
   Considering the demand for thin thickness, small size, low parasitic resistance, environmental protection, and cost effectiveness, ceramic capacitor is currently the best choice for an LED backlight circuit. The nominal voltage ratings of ceramic capacitors are classified as: 6.3V/10V/16V/25V/50V/100V/200V/ . . . , and the corresponding cost greatly increases as the rating goes higher (i.e., using a higher voltage rating capacitor). For example, the cost of a 100V rating capacitor is twice more than that of a 50V rating capacitor. In the prior art circuits shown in  FIGS. 1 ,  4 , and  5 , if the number of LEDs in the series-connection path is more than or equal to 13, a 100V rating capacitor must be used as the output capacitor Cout. 
   The present invention is more cost-saving because it can use a relatively low voltage rating capacitor as the output capacitor Cout.  FIG. 6  shows a circuit diagram according to an embodiment of the present invention, wherein a backlight control circuit  3  comprises a backlight control integrated circuit  30  and two external capacitors Cin and Cout electrically connected therewith. The input voltage Vin is provided by a power supply  5 . One feature of the present invention is that the output capacitor Cout is electrically connected to the input terminal instead of ground. Therefore, the span voltage of the output capacitor Cout becomes Vout-Vin, and a capacitor with voltage rating lower than Vout can be used. 
   The input voltage Vin to a white LED backlight control circuit in currently popular applications, such as notebook computers or other products, is probably provided by 3 or 4 Li-ion batteries or Li-polymer batteries connected in series, which is under about 24V (charger voltage included) and typically between about 10V to about 24V; however, when the battery energy is close to running out, it can be under 10V. The maximum output voltage Vout is about 40V to about 60V, for 10-15 white LEDs connected in series. In some other applications, the input voltage Vin is provided by two Li-ion batteries or Li-polymer batteries, which is under about 15V (charger voltage included) and typically between about 6.6V to about 15V; however, when the battery energy is close to running out, it can be under 6.6V. The maximum output voltage Vout is about 24V to about 32V for 6-8 white LEDs connected in series. (In other words, the voltage supply circuit  11  is usually a boost converter circuit.) Referring to the prior art circuits shown in  FIGS. 1 ,  4 , and  5 , these circuits must use a 100V rating capacitor as its output capacitor when the output voltage Vout is higher than 50V. However in contrast, according to the embodiment of the present invention under the same condition, the input capacitor Cin can be a 25V rating capacitor and the output capacitor Cout can be a 50V rating capacitor. (Or, the output capacitor Cout can even be a 25V rating capacitor or a capacitor of other lower ratings, depending on the difference between the output voltage Vout and the input voltage Vin.) Thus, it is not required to use a capacitor having a rating equal to or higher than the output voltage Vout. 
   Because the output terminal is connected to the input terminal via the output capacitor Cout, a noise in the output terminal (for example, a ripple noise) may be transmitted into the backlight control circuit  3  through the input terminal. The present invention discloses a solution thereto, as described below. 
   Preferably, the power supply providing the input voltage Vin is a power supply having a low internal impedance.  FIG. 7  shows a working model of the power supply for providing the input voltage Vin, wherein the power supply  5  comprises an ideal voltage supply source Vs and two paths: a current sourcing path  51  composed of an ideal diode  52  (having a conductive span voltage of zero) and a resistor Rs 1 , and a current sinking path  53  composed of an ideal diode  54  and a resistor Rs 2 . (Rs 1 , Rs 2  are referred to as “internal impedances”.) 
   According to the inventor&#39;s analysis, when a noise at the output terminal is coupled to the input terminal via the output capacitor Cout, the noise coupling effect correlates to the Cout/Cin ratio, and the resistances of Rs 1  and Rs 2 . The larger the Cout/Cin ratio, or the resistances of Rs 1  and Rs 2  are, the more obvious the noise coupling effect is. 
   Consequently, according to the present invention, the power supply  5  which provides input voltage Vin is preferably a power supply with low internal impedance, i.e., low Rs 1  and Rs 2  resistances. Preferred power supplies include: Li-ion batteries, Li-polymer batteries, NiCd batteries, NiMH batteries, fuel cells, and a power supply connected in parallel with a super capacitor (having a capacitance higher than 0.1 F), etc. 
   Further, to avoid the noise influence on the voltage supply circuit  11 , the backlight control circuit  30  preferably comprises a circuit with noise filtering function, such as a regulator circuit, a filter circuit such as a low-pass filter circuit, or a spike voltage damper circuit. The input voltage Vin is transmitted into the voltage supply circuit  11  only after it has been subject to noise filtering. Such noise filtering circuit can be disposed inside or outside the integrated circuit  30 . 
     FIG. 8  better illustrates the noise filtering concept described above, wherein the voltage supply circuit  11  comprises a group of devices which are sensitive to noises (noise sensitive device group  70 ) and a group of devices which are insensitive to noises (noise insensitive device group  80 ). The noise sensitive device group  70  includes, e.g., a reference voltage supplier circuit, a current bias circuit, an error amplifier circuit, a comparator circuit, an oscillator circuit, a voltage sensor circuit, a current sensor circuit, and a temperature sensor circuit, etc. The noise insensitive device group  80  includes, e.g., a level shifter circuit, a power stage circuit, etc. (The details of a voltage supply circuit is well known to the people skilled in the art, so the detailed circuit structure is omitted for simplicity.) The input voltage Vin at the input terminal passes through a noise filtering circuit  60  to be subject to noise filtering, and afterwards supplied to the noise sensitive devices of the group  70 , while the noise insensitive devices of the group  80  directly receive the unfiltered input voltage Vin. As an alternative, referring to  FIG. 9 , the noise insensitive devices of the group  80  can also receive the filtered input voltage Vin. The noise filtering circuit  60  is disposed inside the voltage supply circuit  11  in  FIGS. 8 and 9 , yet the noise filtering circuit  60  certainly can be disposed outside the voltage supply circuit  11  or even outside the backlight control integrated circuit  30 . 
   As described in the above, the noise filtering circuit  60  can be a regulator circuit, a filter circuit such as a low-pass filter circuit, or a spike voltage clamper circuit.  FIGS. 10-12  illustrate several possible embodiments of such circuits. 
     FIGS. 10A-10D  show four embodiments of the regulator circuits according to the present invention, each of which can regulate the input voltage Vin into a noiseless internal voltage Vinternal for operation of internal devices inside the voltage supply circuit  11 . 
     FIGS. 11A and 11B  show two embodiments of low-pass filter circuits according to the present invention, each of which can filter high frequency noises in the input voltage Vin and transform it into an internal voltage Vinternal for operation of internal devices inside the voltage supply circuit  11 . 
     FIGS. 12A and 12B  show two embodiments of spike voltage damper circuits according to the present invention, each of which can filter voltage spikes in the input voltage Vin and transform it into an internal voltage Vinternal for operation of internal devices inside the voltage supply circuit  11 . 
   Other embodiments of regulator circuits, low-pass filter circuits, and spike voltage clamper circuits are achievable by the persons skilled in the art under the spirit and within the scope of the present invention, based on respective circuit design requirements. 
   The present invention has been described in considerable detail with reference to certain preferred embodiments thereof, but they are only for illustration of the spirit, rather than for limiting the claim scope of the present invention. For those who are skilled in the art, modifications and variations are readily achievable. For example, although the present invention is more advantageous in the situation where high output voltage is required because of series connection of LEDs, it can similarly apply to the situation where LEDs are all connected in parallel, as shown in  FIG. 2 . Further, in all of the embodiments, one can insert a circuit which does not affect the primary function, such as a switch circuit, a diode circuit, a resistor circuit and so on, between any two devices which are shown to be directly connected. Furthermore, the embodiments described above show only one capacitor at each of the input terminal and the output terminal, but of course one can provide more than one capacitor at either the input terminal or the output terminal. Moreover, the input capacitor Cin and the output capacitor Cout are shown to be discrete devices in the above, yet Cin and Cout can be integrated in the backlight control integrated circuit  30 . In addition, the backlight control integrated circuit  30  of the above embodiments comprises current source circuits, a minimum voltage selection circuit, and an error amplifier circuit to provide a signal  15  to control the voltage supply circuit  11 , which is only one example of the possible arrangements of the backlight control integrated circuit  30 ; there can be other arrangements to control the voltage supply circuit  11  for the backlight control integrated circuit  30 . Still further, the light emitting device, although shown as LED in the above, are not limited thereto but can be other light emitting devices such as an organic light emitting diode. And the word “backlight” in the term “backlight control circuit” is not to be taken in a narrow sense that the circuit has to control the backlight of a screen; the present invention can be applied to “active light emission display”, or “LED illuminator”, or other apparatuses that employ light emitting devices. Therefore, all modifications and variations based on the spirit of the present invention should be interpreted to fall within the scope of the following claims and their equivalents.