JP5004700B2 - Light emitting element driving device - Google Patents

Description

  The present invention relates to a configuration of a light emitting element driving apparatus, particularly a light emitting element driving apparatus which is used for lighting and driving a light emitting element (LED) used in various portable devices and the like and boosts a voltage input from a power source.

  For portable devices such as cellular phones and PDAs (Personal Digital Assistants), for example, LEDs (light emitting diodes) are used as backlights for liquid crystal display units or for displaying incoming calls. Lithium ion batteries are often used. The output voltage of the battery varies depending on the state of charge, and is usually about 2.7 V to 4.2 V. However, a voltage higher than the output voltage of the battery may be required as the LED drive voltage. Therefore, when a voltage higher than the battery voltage is required, the battery voltage is boosted by using a boost type power supply circuit such as a charge pump circuit to obtain a voltage necessary for driving the LED as a load circuit. (Patent Document 1 below).

  Also, since the light emission luminance of the LED is determined according to the current flowing through the LED, when driving the LED with a charge pump circuit (boost circuit), a constant current circuit is connected on the LED drive path, and the LED is connected to the LED. In general, the flowing current is kept constant and the light emission luminance is stabilized.

FIG. 5 shows an example of a conventional light emitting element driving device. In this conventional example, the power source 1 is provided with a charge pump circuit 2 and a boosting magnification selection circuit 3 that boost the input voltage _VIN. A plurality of light emitting diodes LED1 to LEDn are connected to the charge pump circuit 2 in parallel. Each of the plurality of LEDs 1 to LEDn is provided with constant current circuits 4-1 to 4-n. The constant current circuits 4-1 to 4-n include an n-channel MOS field effect transistor M1, a resistor R1, an operational amplifier. (Comparator) 5 and a first reference voltage source 6 that outputs a reference voltage V _REF1 . Further, comparators 7-1 to 7-n for inputting cathode terminal voltages of the light emitting diodes LED1 to LEDn and outputting a signal for selecting a boosting factor and a second reference voltage source 8 for outputting a reference voltage _VREF2 are provided. .

According to such a light emitting element driving device, the input voltage _VIN is boosted by the charge pump circuit 2, and the output voltage _VOUT is supplied to each of the light emitting diodes LED1 to LEDn.
At this time, the forward current I _LED1 ~I _LEDn flowing through the LEDs LED1 ～LEDn is controlled by the constant current circuit 4-1 to 4-n. This I _LEDn is set by I _LEDn = V _REF1 / Rn.

Then, the comparator 7-1 to 7-n, a cathode terminal voltage of the light emitting diode LED1 ～LEDn and the reference voltage V _REF2 are compared, the comparison output are output to the boosting ratio selection circuit 3, thereby boosting magnification It can be switched between 1x, 1.5x and 2x. Further, the boosting ratio selection circuit 3, the input voltage V _IN is also monitored by the input, when the decrease of the input voltage V _IN is detected, the step-up ratio of the are switched. In such a conventional configuration, the burden voltage generated in the constant current circuits 4-1 to 4-n is reduced by setting the output voltage _VOUT boosted by the charge pump circuit 2 low, and the power supply power is reduced. Consumption efficiency is improved.

However, the in the charge pump circuit 2 of a conventional light emitting element driving apparatus, 1 times the input voltage V _IN, since only 1.5 times or twice the discrete output voltage, such as can not generate, change the input voltage V _IN In this case, an output voltage _{VOUT that} is higher than the minimum voltage required for driving the light emitting diodes LED1 to LEDn may be supplied, which causes a problem that power consumption efficiency is lowered. That is, the difference between the output voltage _VOUT and the forward voltage of LED1 to LEDn is a surplus voltage, and this surplus voltage is consumed as a burden voltage of the constant current circuits 4-1 to 4-n, thereby improving the consumption efficiency. descend.
JP 2006-254641 A Japanese Patent No. 3759134

On the other hand, although not from the viewpoint of the difference between the output voltage and the forward voltage of the light emitting diode, in the conventional light emitting element driving device, the second reference voltage V _REF2 of the comparators 7-1 to 7-n is constant. There is a problem that it is necessary to set the current circuit 4 higher by a margin in consideration of manufacturing variations, characteristic changes, and the like of components of the current circuit 4, thereby reducing the power consumption efficiency. That is, as shown in FIG. 4, assuming that the minimum drain-source voltage at which the field effect transistor M1 operates in the saturation region is V _DS-SAT , the cathode terminal voltages of the light emitting diodes LED1 to LEDn are at least V _REF1. If there is a voltage of + V _DS-SAT , normal operation is possible.

But in fact,
a) Influence of manufacturing variation of offset voltage of the first reference voltage source (V _REF1 ) 6, second reference voltage source (V _REF2 ) 8, resistor R 1, field effect transistor M 1, and operational amplifier 5, characteristic change due to temperature, etc.
b) Effects of changing the current setting value in the constant current circuit 4 by changing the value of the first reference voltage V _REF1 or R1.
Therefore, for the second reference voltage V _REF2 , the margin voltage must be added to the above V _REF1 + V _DS-SAT , and as a result, the constant current circuits 4-1 to 4- The burden voltage on n increases, and the consumption efficiency cannot be improved to the maximum.

  The present invention has been made in view of the above problems, and its purpose is to manufacture variations of the components of the constant current circuit, change in characteristics due to temperature, etc. with respect to the reference voltage when monitoring the cathode terminal voltage of the light emitting element. It is an object of the present invention to provide a light emitting element driving device that can improve the power consumption efficiency to the maximum without adding a margin voltage considering the above.

In order to achieve the above object, a light emitting element driving apparatus according to a first aspect of the present invention includes a booster circuit that boosts an input voltage from a power source by a selected boosting factor and outputs a driving voltage of the light emitting element . It has a first field effect transistor, a constant current circuit for controlling the current value of the light emitting element connected to the output terminal of the booster circuit, a second field effect aligned with the first field effect transistor in the constant current circuit A monitoring circuit for monitoring the gate-drain voltage (potential difference) of the first field effect transistor controlled by constant current in the constant current circuit using the transistor, and the boosting of the boosting circuit based on the determination result of the monitoring circuit And a boosting magnification selection circuit that supplies a signal for switching the magnification to the boosting circuit.
The invention according to claim 2 has a plurality of constant current circuits disposed against multiple light-emitting element, the monitoring circuit is characterized in that provided in each of the plurality of constant current circuits.
The invention according to claim 3 is characterized in that the boosting magnification selection circuit receives a power supply voltage and switches the boosting magnification even when the power supply voltage drops.

According to the configuration of the present invention, by using the second field effect transistor (having the same characteristics) that matches the first field effect transistor in the constant current circuit as the monitoring circuit , the current flowing to the light emitting element is kept constant. The voltage V _GD1 between the gate and the drain of the first field effect transistor in the constant current circuit is detected, and this V _GD1 is compared with the threshold voltage V _TH2 of the second field effect transistor as the monitoring circuit. The boost ratio is selected and set. According to the monitoring of the cathode terminal voltage of the light emitting element by comparing V _GD1 and V _TH2 , the first reference voltage source, resistor, field effect transistor, and operational amplifier in the constant voltage circuit are manufactured as in the past. Variations (individual differences) or characteristic changes due to the temperature of these elements do not appear, and even if the first reference voltage source and resistance are made variable, there is no effect, and the second reference voltage source is not used. There is no need to add a margin voltage to the reference voltage.

According to the third aspect of the present invention, the boosting magnification is selected by both monitoring the cathode terminal voltage of the light emitting element and monitoring the input voltage from the power source, and the boosted voltage is efficiently supplied. .

According to the light emitting element driving device of the present invention, each of the first reference voltage source, the resistor, the field effect transistor, the operational amplifier, and the like constituting the constant current circuit with respect to the reference voltage when the cathode terminal voltage of the light emitting element is monitored. There is no need to add a margin voltage in consideration of manufacturing variations of elements, temperature characteristic changes, etc., and no second reference voltage source is used, so that the power consumption efficiency can be maximized. There is.
In addition , since the field effect transistor of the monitoring circuit matches the field effect transistor of the constant current circuit (having the same characteristics), the consumption efficiency is improved without being affected by individual differences of each transistor or the temperature characteristic change. be able to. Further, as compared with the configuration of a conventional comparator or the like, since it can be configured with only a field effect transistor and a constant current source, it is possible to manufacture a device with a smaller number of elements than in the past.

According to the second aspect of the present invention, fine power can be supplied to all the light emitting elements by individual monitoring, and when the forward voltages of the light emitting elements vary or light emitting elements having different forward voltages are arranged in parallel. Even when driving, the required minimum voltage can be supplied, and the overall consumption efficiency can be improved satisfactorily.
According to the invention described in claim 3, by both monitoring of the monitoring and the power supply voltage of the cathode terminal voltage of the light emitting element, the supply of the boost voltage is performed efficiently, the advantage that it is possible to obtain a practical device is there.

FIG. 1 shows a configuration of a light emitting element driving apparatus according to a reference example of the present invention. As shown in FIG. 1, in the reference example, a power source 1 such as a lithium ion battery is connected to a power source 1 via a capacitor C1. A charge pump circuit (boost circuit) 2 and a boost magnification selection circuit 3 are provided. This boost magnification selection circuit 3 selects 1 ×, 1.5 ×, and 2 × magnification, and the charge pump circuit 2 is selected and set. The input voltage _VIN is boosted by the magnification. In the charge pump circuit 2, a plurality of light emitting diodes LED1 to LEDn are connected in parallel via a capacitor C2, and constant current circuits 4-1 to 4-n are provided for the plurality of LEDs 1 to LEDn, respectively. The constant current circuits 4-1 to 4-n are provided with an n-channel MOS field effect transistor M1, a resistor R1, an operational amplifier (comparator) 5, and a first reference voltage source 6 for outputting a reference voltage _VREF1 .

The light emitting diodes LED1 ～LEDn the cathode terminal is connected to a gate terminal connected to the drain terminal of the field effect transistor M1, V _GD monitoring circuit 10-1 for monitoring the gate-drain voltage (potential difference) V _GD -10-n. The V _GD monitoring circuits 10-1 to 10-n use, for example, a comparator as shown in FIG. 5, and compare V _GD1 of the field effect transistor M1 with a reference voltage equivalent to the threshold voltage V _{TH1 of the} transistor M1. As a result, it is determined whether the field effect transistor M1 operates in the saturation region and maintains constant current, and a boosting magnification switching signal is output to the boosting magnification selection circuit 3 according to the determination result.

That is, in order for the field effect transistor M1 to operate in the saturation region and maintain constant current characteristics,
V _DS1 > V _DS-SAT (1)
Should be satisfied. In the n-channel MOS field effect transistor M1, the following relational expression holds.
V _DS-SAT = V _GS1 -V _TH1 (2)
V _DS1 = V _GS1 −V _GD1 (3)
Here, V _DS1 is the drain-source voltage of the transistor M1, V _DS-SAT is the minimum drain-source voltage at which the field effect transistor M1 operates in the saturation region _, V _GS1 is the gate-source voltage of the transistor M1, V _GD1 is a gate-drain voltage of the transistor M1, and V _TH1 is a threshold voltage of the transistor M1.

And when the above formulas (1) to (3) are arranged,
V _GD1 <V _TH1 (4)
Is obtained.
From this equation (4), when the gate-drain voltage V _GD1 of the transistor M1 is higher than the threshold voltage V _TH1 of the transistor M1 (V _GD1 > V _TH1 ), it can be determined that the transistor M1 is not operating in the saturation region. I understand.

Therefore, in the reference example, the operating state of the n-channel MOS field effect transistor M1 is determined by the V _GD monitoring circuits 10-1 to 10-n, and the boosting factor of the charge pump circuit 2 is switched based on the determination result. That is, when V _GD1 is higher than V _TH1 , a switching signal for increasing the boosting ratio is output to the boosting ratio selecting circuit 3, and the boosting ratio selecting circuit 3 switches the boosting ratio.

Further, in the reference example, the input voltage _VIN is supplied to the boosting magnification selection circuit 3, and the decrease of the input voltage _VIN is also monitored at the same time to determine the switching of the boosting magnification in the charge pump circuit 2. . That is, when the input voltage _VIN is higher than the voltage necessary for driving the light emitting diodes LED1 to LEDn, the input voltage _VIN is not boosted by the charge pump circuit 2 and is supplied to the light emitting diodes LED1 to LEDn as they are. . On the other hand, when the input voltage _VIN is lower than the required drive voltage, the input voltage _VIN is boosted by the charge pump circuit 2 and supplied to the light emitting diodes LED1 to LEDn.

By the operations of the V _GD monitoring circuits 10-1 to 10-n, the second reference voltage source (V _REF2 ) 8 that has been conventionally used and the first reference voltage sources (constant current circuits 4-1 to 4-n) ( V _REF1 ) 5, resistance R1, characteristics such as offset voltage of the field effect transistor M1 and the operational amplifier 5, etc., even if there are manufacturing variations, or even if these characteristics fluctuate due to temperature changes, etc., the constant current circuit 4 Even when the current setting value is changed by changing the value of the first reference voltage V _REF1 or R1, the constant current circuits 4-1 to 4-1 are always operated. Since it works to detect the minimum burden voltage of 4-n, the consumption efficiency can be improved.

That is, in the conventional configuration, the influence of the manufacturing variation of the circuit element and the temperature characteristic change appears in the input voltage of the comparator (the cathode terminal voltage of the LED). The temperature characteristic fluctuation does not affect the monitored signal. Therefore, unlike the prior art, it is not necessary to add the margin voltage considering the circuit elements to the reference voltage (V _REF2 ) for comparing the monitoring signals, and the consumption efficiency can be improved satisfactorily.

  Further, even when the forward voltages of the light emitting diodes LED1 to LEDn are varied or when the light emitting diodes LED1 to LEDn having different forward voltages are driven in parallel, the light emitting diodes LED1 to LEDn having the maximum forward voltage are detected. Since the voltage with the minimum necessary boosting magnification is supplied, the consumption efficiency can be improved satisfactorily.

2 shows a configuration is shown of the light emitting element driving apparatus according to the actual施例, real施例this is the V _GD monitoring circuit, consistent with field effect transistor (first field-effect transistor) M1 n A channel MOS field effect transistor (second field effect transistor) M2 is used. In FIG. 2, V _GD monitoring circuits 12-1 to 12-n have their source terminals connected to the drain terminals of the field effect transistors M1 in the constant current circuits 4-1 to 4-n, and the gate terminals of the transistors M1. And a constant current source I1 connected to the source terminal of the field effect transistor M2 and a field effect transistor M2 matched with the transistor M1. The matching of the transistors M2 and M1 is manufactured under the same manufacturing conditions and has the same characteristics such as variations in manufacturing of the threshold voltage and temperature change (assuming that the threshold voltage of M2 is V _TH2 , V _TH2 = V _TH1 ).

According to such V _GD monitoring circuits 12-1 to 12-n, by detecting the gate-drain voltage V _GD1 of the transistor M1, and comparing this V _GD1 and the threshold voltage V _{TH2 of the} transistor M2, The determination of boosting magnification switching is executed. As described above, this is equivalent to comparing the gate-drain voltage V _GD1 of the transistor M1 with the threshold voltage V _TH1 of the transistor M1 because V _THl = V _TH2 .

That is, when V _GD1 <V _TH2 , the transistor M2 is off and the transistor M1 operates in the saturation region and maintains constant current, but as the cathode terminal voltages of the light emitting diodes LED1 to LEDn decrease, When the monitored gate-drain voltage V _GD1 becomes high and V _GD1 > V _TH2 is satisfied, the transistor M2 is turned on, and a determination signal for switching the boost magnification is output to the boost magnification selecting circuit 3. As a result, the boosting magnification selection circuit 3 selects the boosting magnification switched from 1.5 times to 2 times, for example, so that the voltage boosted up to 2 times is supplied from the charge pump circuit 2 to the light emitting diodes LED1 to LEDn. The

In FIG. 3, there is shown the characteristics of the output voltage _{V OUT} to the input voltage _{V IN} in the Reference Examples and Examples, for example, when the input voltage _{V IN} drops to 3.75V, boosting for monitoring the input voltage _{V IN} When the voltage is switched from 1 × to 1.5 × by the magnification selection circuit 3 and further decreased to 2.5V, the step-up magnification is increased from the V _GD monitoring circuits 10-1 to 10-n and 12-1 to 12-n. A switching signal is output to the boosting magnification selection circuit 3, and the boosting magnification selection circuit 3 switches from 1.5 times to 2 times. This switching from 1.5 times to 2 times is conventionally performed at a voltage higher than 2.5 V, as indicated by arrow 100, and the embodiment emits light at a lower input voltage _VIN . The diodes LED1 to LEDn can be driven.

In the above real施例_{at V GD} monitoring circuit 12-1 to 12-n, the transistor M2 which is aligned with the transistor M1 a reference voltage which is necessary for comparison with the gate-drain voltage _{V GD1} of the transistor M1 By realizing the threshold voltage V _TH2 , the constant current circuits 4-1 to 4-n are always used even when the reference voltage V _TH2 of the V _GD monitoring circuits 12-1 to 12-n varies due to manufacturing variations, temperature changes, and the like. It works to detect the minimum burden voltage, and it is possible to improve the consumption efficiency. Furthermore, since the V _GD monitoring circuits 12-1 to 12-n are composed of only the transistor M2 and the constant current source I1, it can be configured with a smaller number of elements and a chip area compared to the conventional device or the reference example. Since it can reduce, there exists an advantage that manufacturing cost becomes cheap.

It is a circuit diagram which shows the structure of the light emitting element drive device which concerns on the reference example of this invention. Is a circuit diagram showing a configuration of a light emitting element driving apparatus according to the actual施例. It is a figure which shows the characteristic of the output voltage with respect to the input voltage in the light emitting element drive device of a reference example and an Example. It is a figure which shows the operating characteristic of the MOS field effect transistor by which constant current control was carried out. It is a circuit diagram which shows the structure of the conventional light emitting element drive device.

Explanation of symbols

1 ... Power supply, 2 ... Charge pump circuit,
3 ... Boost ratio selection circuit, 4-1 to 4-n ... Constant current circuit,
5 ... operational amplifier, 6 ... first reference voltage source,
10-1 to 10-n, 12-1 to 12-n... V _GD monitoring circuit,
M1, M2 ... n-channel MOS field effect transistor,
I1 is a constant current source.

Claims (3)

A booster circuit that boosts an input voltage from a power source at a selected boosting factor and outputs a drive voltage of the light emitting element;
A constant current circuit having a first field effect transistor and controlling a current value of the light emitting element connected to the output terminal of the booster circuit;
A monitoring circuit for monitoring a gate-drain voltage of the first field effect transistor controlled in constant current in the constant current circuit using a second field effect transistor matched with the first field effect transistor in the constant current circuit; ,
A light emitting element driving device comprising: a boosting magnification selection circuit that supplies a signal for switching the boosting magnification of the boosting circuit to the boosting circuit based on a determination result of the monitoring circuit. 2. The light emitting element driving device according to claim 1 , further comprising a plurality of constant current circuits arranged for the plurality of light emitting elements, wherein the monitoring circuit is provided in each of the plurality of constant current circuits . 3. The light emitting element driving device according to claim 1 , wherein the step-up magnification selection circuit inputs a power supply voltage and switches the step-up magnification even when the input voltage decreases .

Images