CN100538474C - Backlight Control Circuit - Google Patents

Abstract

The present invention provides a backlight control circuit, comprising: a voltage supply circuit, which is a boost circuit, receiving an input voltage from an input terminal and generating an output voltage to an output terminal, the output voltage being used as a working voltage for a light-emitting element; at least one input terminal capacitor electrically connected between the input terminal and ground; and at least one output terminal capacitor electrically connected between the output terminal and the input terminal, wherein the withstand voltage specification of the output terminal capacitor is lower than the output voltage. The present invention uses capacitors with lower withstand voltage specifications to achieve a high output voltage backlight control circuit, which can reduce the overall cost of the backlight control circuit.

Description

Backlight Control Circuit

technical field

The present invention relates to a backlight control circuit (Backlight Control Circuit), in particular to a backlight control circuit that achieves high output voltage with capacitors of relatively low withstand voltage specifications.

Background technique

In the liquid crystal display device, the backlight control circuit is used to control the light-emitting diodes to emit light from the back of the liquid crystal screen, so that users can watch the images on the screen.

In the early days, since LED backlights were only used in small-sized screens, the required backlight brightness did not need to be too strong, so all LEDs could be connected in series or in parallel. Taking full series as an example, as shown in Figure 1, the backlight control circuit 1 in the prior art includes a backlight control integrated circuit 10, and the backlight control integrated circuit 10 has an input terminal and an output terminal, and the input terminal is connected to a backlight control integrated circuit 10. The input terminal capacitor Cin is connected to receive the input voltage Vin; the output terminal is connected to an output terminal capacitor Cout to provide the output voltage Vout. (In addition to the integrated circuit 10 and the two capacitors, according to the current integrated circuit integration technology, it is still necessary to cooperate with some external components, such as magnetic components, etc., which are not related to this case, so they are omitted.) The backlight control integrated circuit 10 passes through its internal The voltage supply circuit 11 provides an output voltage Vout to the LEDs L1 - LN connected in series according to the signal 15 of the error amplifier circuit 13 . At the same time, a resistor R is provided on the path of the LEDs connected in series. By extracting the voltage at the node Vsense1 and comparing it with the reference voltage Vref, it is checked whether the current passing through the LEDs connected in series meets the requirements. When the current is low At the default value, the voltage at the node Vsense1 drops, and the signal 15 sent by the error amplifier circuit 13 at this time will control the voltage supply circuit 11 to pull up the output voltage Vout, that is, pull up the current on the LED series path. In addition, in order to prevent the voltage supply circuit 11 from pulling up the voltage without limit (for example, the error amplifier circuit 13 fails or the LED series path is disconnected), an overvoltage protection circuit 12 is usually added in the backlight control circuit 10, which detects the output voltage Vout, and when the output voltage Vout is too high, send a signal to control the voltage supply circuit 11 to stop pulling up the voltage (depending on the circuit design, the supply voltage can be completely stopped, or the voltage can be kept at a certain upper limit; in In the backlight control circuit, the second approach is generally adopted.)

The general practice of the overvoltage protection circuit 12 is shown in Figure 2. It can extract the divided voltage from the output voltage Vout, compare the voltage at the node Vsense2 with the preset reference voltage Vovp, and send a signal to control the voltage supply circuit according to the comparison result. 11.

Please refer to FIG. 3 again, which is an example of the prior art backlight control circuit when the light emitting diodes are fully connected in parallel. As shown in the figure, the backlight control circuit 2 includes a backlight control integrated circuit 20. The currents of the LEDs L1-LN in the backlight control integrated circuit 20 are respectively controlled by current sources CS1-CSN. The backlight control integrated circuit 20 includes a lowest voltage selection circuit 21 for selecting the lowest voltage among the cathode terminals of all light emitting diodes L1-LN, and comparing the selected voltage with the reference voltage Vref, thereby controlling the voltage supply circuit 11 . In this way, the output voltage Vout will be controlled, so that all the current source circuits have enough working voltage to work normally, and also make all the light emitting diodes normally illuminate.

The backlight control integrated circuit 20 may also include an over-voltage protection circuit 12, and its method is the same as that described above, so it is omitted.

In the above-mentioned all-series or all-parallel arrangements, the number of light-emitting diodes is limited, so it is natural to think that both series and parallel can be used. In this regard, one of the prior art is shown in FIG. 4, in which the known backlight control integrated circuit 10 shown in FIG. The current on the other LED series paths will not be detected.

Another approach in the prior art is to use the known backlight control integrated circuit 20 shown in FIG. 3 to form a series-parallel circuit of light emitting diodes as shown in FIG. 5 .

In the circuits shown in FIGS. 1, 4, and 5 above, when the number of light-emitting diodes connected in series is higher, it means that the output voltage Vout must increase correspondingly. At this time, the capacitor Cout at the output terminal must also use a capacitor with a relatively high withstand voltage specification; thus, the overall cost of the backlight control circuit will inevitably be increased.

Contents of the invention

In view of this, the present invention aims at the shortcomings of the above-mentioned prior art, and proposes a backlight control circuit capable of achieving a high output voltage with a capacitor with a lower withstand voltage specification, so as to solve the above-mentioned problems.

To achieve the above purpose, in one embodiment of the present invention, a backlight control circuit is provided, including: a voltage supply circuit, which receives an input voltage from an input terminal and generates an output voltage to an output terminal, This output voltage provides an operating voltage as a light-emitting element; 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, wherein the output capacitor The withstand voltage specification is lower than the output voltage.

In the above-mentioned embodiments, in order to prevent possible noise problems caused by the connection of the capacitor at the output terminal to the input terminal, a noise filter circuit may be further included in the voltage supply circuit.

In addition, the power supply for supplying the input voltage preferably has a low internal impedance, that is, the resistance value of both current sourcing and current sinking is preferably low.

The specific embodiments will be described in detail below, so that it is easier to understand the purpose, technical content, characteristics and effects of the present invention.

Description of drawings

Graphic description:

FIG. 1 is a schematic circuit diagram of a full series LED circuit and a backlight control circuit in the prior art.

FIG. 2 is a schematic circuit diagram of an overvoltage protection circuit in the prior art.

FIG. 3 is a schematic circuit diagram of a fully parallel LED circuit and a backlight control circuit in the prior art.

FIG. 4 is a schematic circuit diagram showing an example of a series-parallel LED circuit and a backlight control circuit in the prior art.

FIG. 5 is a schematic circuit diagram showing another example of a series-parallel LED circuit and a backlight control circuit in the prior art.

FIG. 6 is a schematic circuit diagram of a backlight control circuit according to an embodiment of the present invention.

Fig. 7 is a schematic circuit diagram illustrating the internal model of the power supply.

8 and 9 illustrate how to set up the noise filter circuit 60 in the voltage supply circuit 11 .

10A-10D illustrate four embodiments of voltage regulator circuits.

11A and 11B illustrate two embodiments of low-pass filter circuits.

12A and 12B illustrate two embodiments of spike voltage clamping circuits.

Explanation of symbols in the figure

1, 2, 3 Backlight control circuit

5 power supply

10 Backlight control integrated circuit

11 Voltage supply circuit

12 Overvoltage protection circuit

13 Error amplifier circuit

15 signal

20 backlight control integrated circuit

21 Minimum voltage selection circuit

30 backlight control integrated circuit

51, 52 path

53, 54 ideal diode

60 Noise filter circuit

70 Group of components sensitive to noise

80 component groups that are not sensitive to noise

Cin input capacitor

Cout output capacitor

CS1-CSN Current Source

L1-LN LED

R, Rs1, Rs2 resistance

Vs ideal voltage source

Detailed ways

In general, white or blue LEDs may have a voltage across the range of 3.3V to 4V due to variations in LED manufacturing. In circuit design, the variation of each LED must be considered conservatively. Therefore, the required output voltage Vout is usually calculated by multiplying 4V by the number of LEDs connected in series on the LED path. In other words, assuming that the number of LEDs connected in series on each LED path exceeds (including) 13, Vout should be greater than 50V. (4*13=52>50)

Considering the overall efficiency and cost ratio of low thickness, small volume, low parasitic series resistance, and environmental protection requirements, ceramic capacitors are the first choice for LED backlight circuit applications. However, the current commonly used withstand voltage specifications of ceramic capacitors are: 6.3V/10V/16V/25V/50V/100V/200V/.... Capacitors), the cost is relatively increased several times. For example, the cost of a capacitor with a withstand voltage specification of 100V is more than twice that of a capacitor with a withstand voltage specification of 50V. Therefore, assuming that the number of LEDs connected in series on each LED path exceeds (including) 13, according to the prior art circuits shown in Figures 1, 4, and 5, a capacitor with a withstand voltage specification of 100V must be used as the output Capacitor Cout.

However, in the present invention, a more economical method can be adopted, using a capacitor with a lower withstand voltage specification as the output capacitor Cout. Please refer to FIG. 6 , which shows an embodiment of the present invention in a schematic circuit diagram. In the present embodiment, the backlight control circuit 3 includes a backlight control integrated circuit 30 and externally connected capacitor elements Cin and Cout. The input voltage Vin is supplied by the power supply 5 . As shown, one of the features of the present invention is to connect the output capacitor Cout to the input terminal instead of ground. Therefore, the voltage across the output capacitor Cout is only Vout-Vin, and a capacitor with a withstand voltage specification lower than Vout can be used.

Generally speaking, in the current common application occasions of white light emitting diode backlight control circuits on notebook computers or similar products, the input voltage Vin may be supplied by 3 to 4 lithium batteries or lithium polymer batteries connected in series, and the voltage is about Below 24V (including the charger voltage), usually about 10V-24V, but when the battery energy is almost exhausted, it may drop below 10V; the maximum output voltage Vout is about 40V-60V, which may be 10-15 white light-emitting diodes in series. In addition, in some applications, the input voltage Vin may be supplied by two lithium batteries or lithium polymer batteries connected in series, which is about 15V or less (including the charger voltage), usually 6.6V-15V, but when the battery energy is exhausted , it may drop below 6.6V; the maximum output voltage Vout is about 24V-32V, which may be 6 to 8 white light-emitting diodes connected in series. (That is, in most cases, the voltage supply circuit 11 is a step-up circuit.) If the prior art circuits shown in Figures 1, 4, and 5 are used, when the output voltage Vout is greater than 50V, the output terminal must use a A capacitor with a voltage specification of 100V. However, if the embodiment of the present invention is applied to the above-mentioned occasions, the input terminal can use a capacitor with a withstand voltage specification of 25V, and the output terminal can use a capacitor with a withstand voltage specification of 50V; Capacitors with a voltage specification of 25V or other specifications, etc., depend on the difference between the output voltage Vout and the input voltage Vin, and it is not necessary to use a capacitor whose withstand voltage specification is the same as the output voltage Vout or higher than Vout.

If the embodiment shown in FIG. 6 is adopted, since the output terminal is connected to the input terminal through the output capacitor Cout, the noise (such as ripple noise, ripple noise) on the output terminal will also be transmitted from the input terminal to the backlight control circuit. within 3. To this end, the present invention also proposes a solution.

First, a power supply that provides the input voltage Vin is preferably a power supply with low internal impedance. See Figure 7, which represents a model of a power supply supplying an input voltage Vin. The power supply 5 in the figure comprises an ideal voltage source Vs and two paths: a path 51 providing current (current sourcing) has an ideal diode 52 (defined here as its conduction voltage is zero) and resistance Rs1, and another path 53 for current sinking, has an ideal diode 54 and resistor Rs2 on it (Rs1, Rs2 are generally referred to as internal resistance or internal impedance).

According to the derivation of the inventor of this case, when the noise on the output terminal is coupled to the input terminal through the output capacitor Cout, the coupling effect is related to the ratio of Cout/Cin, the resistance values of Rs1 and Rs2; when the ratio of Cout/Cin, Rs1 When the resistance value of and Rs2 is larger, the coupling effect is larger.

Therefore, according to the present invention, the power supply for providing the input voltage Vin is preferably one with low internal impedance, that is, one with low resistance values of Rs1 and Rs2 . According to the present invention, the power sources that meet this condition include lithium ion (Li-ion) batteries, lithium polymer (Li-polymer) batteries, nickel-cadmium batteries (NiCd), nickel-metal hydride batteries (NiMH), properly equipped fuel cells ( FuelCell), and a power supply with a super capacitor (Super Capacitor, generally with a capacitance value of 0.1F or more) connected in parallel.

Secondly, in order to prevent the voltage supply circuit 11 from being affected by noise, the backlight control circuit 3 should include a circuit with a function of filtering noise, such as a regulator circuit (regulator), a filter circuit such as a low-pass filter circuit (low-pass filter) , or a spike voltage clamper, etc., after the noise is filtered, it is then input into the voltage supply circuit 11. These circuits with the function of filtering noise can be installed inside or outside the IC.

The concept of the above approach can be referred to FIG. 8 . Inside the voltage supply circuit 11 , there are a noise-sensitive component group 70 and a noise-insensitive component group 80 . Components sensitive to noise are, for example, a reference voltage supplier, a current bias circuit, an error amplifier, a comparator, an oscillator, and a voltage sensor. sensor), current sensor (current sensor), temperature sensor (temperature sensor) and so on. Components that are not sensitive to noise are, for example, a level shifter, a power stage, and the like. (Because the voltage supply circuit 11 is well known to those skilled in the art, only a few examples are given here for illustration, and not shown in detail one by one.) According to the present invention, the voltage input from the input terminal can first pass through the noise filter circuit 60 , After the noise is filtered, it is supplied to the noise-sensitive components; as for the noise-insensitive components, it can directly receive the input voltage. But of course, as shown in FIG. 9, the components that are not sensitive to noise can also receive the voltage after filtering the noise; and although the noise filter circuit 60 is shown inside the voltage supply circuit 11 in the figure, it can also be The noise filter circuit 60 is provided outside the voltage supply circuit 11 and even outside the backlight control integrated circuit 30 .

As mentioned above, the circuit 60 with the function of filtering noise can be, for example, a regulator circuit (regulator), a filter circuit such as a low-pass filter circuit (low-pass filter), or a spike voltage clamper circuit (spike voltage clamper). For the specific method of each circuit mentioned above, please refer to Fig. 10-12.

10A-10D identify four embodiments of voltage regulator circuits. The circuit shown in the figure can convert the input voltage Vin into a noise-free internal voltage Vinternal after regulation, which is used by the internal components of the voltage supply circuit 11 .

Figures 11A and 11B show two embodiments of low-pass filter circuits. The circuit shown in the figure can filter the high-frequency noise in the input voltage Vin and convert it into an internal voltage Vinternal for use by the internal components of the voltage supply circuit 11 .

Figures 12A, 12B illustrate two embodiments of spike voltage clamping circuits. The circuit shown in the figure can filter the high-voltage surge suddenly appearing in the input voltage Vin, and convert the filtered voltage into an internal voltage Vinternal, which is used by the internal components of the voltage supply circuit 11 .

In addition to the above, there are other various implementations of the voltage regulator circuit, low-pass filter circuit, and peak voltage clamping circuit. Those familiar with the art can make corresponding circuit changes according to the needs of circuit design. included within the concept of the present invention.

The present invention has been described above with reference to preferred embodiments, and the above description is only for making the content of the present invention easy for those skilled in the art, and is not intended to limit the scope of rights of the present invention. As mentioned above, those skilled in the art should immediately conceive of various equivalent changes within the spirit of the invention. For example, although the present invention is designed for the situation where high output voltage must be provided due to series connection of light emitting diodes, the arrangement of the present invention can also be used in the full parallel connection structure of light emitting diodes as shown in FIG. 2 . As another example, between the two elements shown in all embodiments that are directly connected, a circuit that does not affect the main function can be inserted therebetween, such as a switch circuit, a diode circuit, a resistor circuit, and the like. For another example, in the illustrated embodiment, only one capacitor is provided at the input terminal and the output terminal, but it is of course possible to provide more than one capacitor at the input terminal or the output terminal. For another example, although the capacitors Cin and Cout are used as independent components in the embodiment and other parts are integrated into the integrated circuit 30 , other integration methods are also possible. For another example, in the illustrated embodiment, the backlight control integrated circuit 30 includes a current source, a minimum voltage selection circuit, an error amplifier, etc. to generate a signal 15 to control the voltage supply circuit 11, but this is only an example of the backlight control integrated circuit One of the implementation manners of 30; in the backlight control integrated circuit 30, other ways can be used to control the voltage supply circuit 11. In addition, although the light-emitting element shown is a light-emitting diode, it may also be other light-emitting elements, such as an organic light-emitting diode; the "backlight" control circuit may not necessarily control the "backlight", and may also be used in an active light-emitting panel, or LED lighting and more. Therefore, all equivalent changes or modifications made according to the concept and spirit of the present invention shall be included in the scope of the claims of the present invention.

Claims (17)

1. backlight control circuit comprises:

Voltage supply circuit, it is a booster circuit, accepts an input voltage from an input end, and an output terminal is produced an output voltage, this output voltage provides the operating voltage as light-emitting component;

At least one is connected electrically in the input end capacitor device between input end and the ground connection; And

At least one is connected electrically in the output terminal capacitor between output terminal and the input end, the withstand voltage specification of this output terminal capacitor wherein, and specific output voltage is low.

2. backlight control circuit as claimed in claim 1, wherein this input voltage is below 24V, and output voltage is in the scope of 40V-60V.

3. backlight control circuit as claimed in claim 1, wherein this input voltage is supplied by the lithium ion battery or the lithium polymer battery of 3 or 4 series connection, and output voltage is in the scope of 40V-60V.

4. backlight control circuit as claimed in claim 1, wherein this input voltage is below 15V, and output voltage is in the scope of 24V-32V.

5. backlight control circuit as claimed in claim 1, wherein this input voltage is supplied by the lithium ion battery or the lithium polymer battery of 2 series connection, and output voltage is in the scope of 24V-32V.

6. backlight control circuit as claimed in claim 1, wherein this output voltage is greater than 50V, and the withstand voltage specification of this output terminal capacitor is for being less than or equal to 50V.

7. backlight control circuit as claimed in claim 1, wherein this output voltage is greater than 25V, and the withstand voltage specification of this output terminal capacitor is for being less than or equal to 25V.

8. backlight control circuit as claimed in claim 1, wherein this output voltage provides the operating voltage as at least one group of series connection light-emitting component, and the number of this group series connection light-emitting component is more than or equal to 13.

9. backlight control circuit as claimed in claim 1, wherein this input end is electrically connected with a power supply, and this power supply has low internal driving.

10. backlight control circuit as claimed in claim 9, wherein this power supply is selected from one of following power supply: lithium ion battery, lithium polymer battery, nickel-cadmium battery, Ni-MH battery, fuel cell and the power supply that is parallel with super capacitor.

11. backlight control circuit as claimed in claim 1 wherein has the noise filtering circuit in this voltage supply circuit, after this noise filtering circuit received input voltage, the inner member that produces builtin voltage voltage supplied supply circuit used.

12. backlight control circuit as claimed in claim 11 wherein has the element to noise-sensitive in this voltage supply circuit, this element to noise-sensitive receives the builtin voltage that the noise filtering circuit is produced.

13. backlight control circuit as claimed in claim 11, wherein this noise filtering circuit is that voltage regulator circuit, filtering circuit or peak voltage pincers end circuit.

14. backlight control circuit as claimed in claim 12, wherein this element to noise-sensitive comprises one of following element or many persons: reference voltage supplies circuit, current setting circuit, error amplifier, comparer, oscillator, voltage sensor, current sensor, temperature sensor.

15. backlight control circuit as claimed in claim 1, wherein said light-emitting component are light emitting diode.

16. backlight control circuit as claimed in claim 1, wherein said light-emitting component are white light emitting diode.

17. backlight control circuit as claimed in claim 1, wherein said light-emitting component are Organic Light Emitting Diode.

Images