JP2004022646A - LED drive circuit - Google Patents

Abstract

Provided is an LED drive circuit that requires a minimum number of external components, consumes less current, and enables a user to arbitrarily change the brightness of an LED with respect to the surrounding brightness.
An illuminance level detecting circuit for receiving an illuminance detection voltage corresponding to ambient brightness at predetermined time intervals to detect an illuminance level, and a plurality of PWM duty values stored therein and detected by the illuminance level detection circuit. A brightness setting register 18 for selecting one PWM duty value according to the illuminance level, and a brightness control for taking in the one PWM duty value selected by the brightness setting register and generating a PWM signal of the PWM duty value The circuit includes a circuit 19 and an LED drive output circuit 20 that PWM-drives an LED by the luminance control circuit.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an LED drive circuit such as a white LED used for a backlight of a liquid crystal panel of a mobile phone, and more particularly to an LED drive circuit capable of controlling dimming according to ambient brightness with low power consumption. is there.
[0002]
[Prior art]
The circuit shown in FIG. 5 is known as a white LED driving circuit. In FIG. 5, reference numeral 51 denotes a white LED, which is connected to a feedback terminal 52a of a one-chip switching regulator 52. 53 is a resistor connected between the white LED 51 and the ground. The switching regulator 52 operates so that the input voltage Vfb of the feedback terminal 52a becomes equal to the internal reference voltage Vref, and outputs a pulse to the switching output terminal 52b. This pulse is boosted and smoothed by the boosting / smoothing circuit 54 and applied to the white LED 51.
[0003]
Here, assuming that the current I _LED flowing through the white _LED 51 is R 53,
I _LED = Vref / R53
Is required. Therefore, the brightness of the white LED 51 can be set by appropriately setting the value R53 of the resistor 53, but dimming according to the surrounding brightness cannot be performed as it is.
[0004]
FIG. 6 is a diagram showing a white LED driving circuit in which the white LED 51 is dimmed according to the surrounding brightness. Reference numeral 55 denotes a phototransistor as an illuminance sensor. 57 is a voltage follower for detecting this voltage, and 58 and 59 are resistors. Others are the same as FIG.
[0005]
Here, assuming that the output voltage of the voltage follower 57 is Vsens and the values of the resistors 53, 58, 59 are R53, R58, R59, the current I _LED flowing through the white _LED 51 is:
I _LED ≒ (1 / R53) · [Vref− (Vsens−Vref) · R59 / R58] If R58 = R59,
I _LED ≒ (1 / R53) ・ (2Vref-Vsens)
Thus, the current I _{LED of the} white _LED 51 can be increased or decreased according to the brightness (Vsens) of the surroundings, and light can be adjusted.
[0006]
FIG. 7 is a diagram showing another example of a white LED driving circuit in which the white LED 51 is dimmed according to the surrounding brightness. Reference numeral 60 denotes an A / D converter that converts a voltage generated in the resistor 56 into a digital signal, and 61 denotes a PWM signal generation circuit that generates a pulse having a pulse width corresponding to the digital value of the converted voltage.
[0007]
Here, the switching regulator 52 is provided with an enable terminal 52c, and the output signal of the PWM signal generation circuit 61 is input to the enable terminal 52c to turn on / off the operation of the switching regulator 52. The current flowing through the white LED 51 is substantially proportional to the duty DUTY of the PWM signal generated by the PWM signal generation circuit 61, and the average current I _{LED (AVE)} of the white _LED 51 is the maximum value of the current flowing through the I _{LED (MAX)} through the white _LED 51. Then
I _{LED (AVE)} = I _{LED (MAX)}・ DUTY / 100
In the same manner as in FIG. 6, the current I _{LED (AVE)} of the white _LED 51 can be increased or decreased according to the surrounding brightness to perform light control.
[0008]
[Problems to be solved by the invention]
However, in the circuit shown in FIG. 6, since the voltage Vsens representing the brightness of the surroundings is a control factor of the current flowing through the white LED 51, it is necessary to always operate all the circuits, and it is difficult to reduce the current consumption. is there. If the circuit including the phototransistor 55, the resistor 56, and the voltage follower 57 is operated intermittently, current consumption can be reduced. However, a sample-hold circuit and a sample-and-hold circuit are provided between the circuit and the switching regulator 52. A new timer for controlling the hold circuit is required.
[0009]
In the circuit shown in FIG. 7, the current consumption can be reduced by controlling the A / D converter 60 with a timer at regular intervals, but the A / D converter and the PWM signal generation can be externally provided. Circuit is required.
[0010]
In each of the circuits in FIGS. 6 and 7, the relationship between the surrounding brightness and the white LED is fixed to a fixed relationship, and the relationship between the surrounding brightness and the brightness of the white LED is arbitrarily changed on the user side. I can't.
[0011]
SUMMARY OF THE INVENTION An object of the present invention is to provide an LED drive circuit that requires a minimum number of external components, requires less current consumption, and allows the user to arbitrarily change the brightness of the LED with respect to the surrounding brightness. To provide.
[0012]
[Means for Solving the Problems]
According to the first aspect of the present invention, an illuminance level detection circuit which takes in an illuminance detection voltage corresponding to ambient brightness at regular intervals and detects an illuminance level, and a plurality of dimming data stored therein, A luminance setting register for selecting one dimming data in accordance with the detected illuminance level, a luminance control circuit for taking in the one dimming data selected by the luminance setting register and generating a luminance control signal; And an LED drive output circuit for driving an LED by a brightness control signal from the brightness control circuit.
[0013]
According to a second aspect of the present invention, in the first aspect of the present invention, the illuminance level detection circuit sets a plurality of illuminance levels in which the illuminance detection voltage is divided into a plurality of ranges from “bright” to “dark”. The LED driving circuit is characterized by detecting which one of the above applies.
[0014]
According to a third aspect of the present invention, in the first aspect of the present invention, an external interface for externally rewriting a part or all of the plurality of light control data of the brightness setting register is provided. An LED drive circuit was used.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a block diagram showing the overall configuration of a white LED drive circuit according to an embodiment of the present invention. Reference numeral 10 denotes a one-chip white LED driving circuit. A phototransistor 11 and a resistor 12 as an illuminance sensor are connected to a sensor terminal 10a, and a white LED 13 is connected to an output terminal 10b. 10c and 10d are power terminals, 10e is an external control terminal, and 10f is a ground terminal. The power supply 14 receives the power supply voltage VDD input from the power supply terminal 10c and supplies power to the internal circuit, and also supplies power to the illuminance sensor 11 from the power supply terminal 10d. Reference numeral 15 denotes an oscillation circuit that generates the clock f1, and reference numeral 16 denotes a frequency division circuit that divides the frequency of the clock f1 generated by the oscillation circuit 15 to generate clocks f2 and f3. Reference numeral 17 denotes an illuminance level detection circuit which receives clocks f1 and f2 and operates intermittently only once every several seconds. The illuminance detection voltage generated in the resistor 12 is input, and the current illuminance is set to four predetermined levels. Is detected. A luminance setting register 18 selects one of four PWM duty values (light control data) stored in advance according to the illuminance level detected by the illuminance level detection circuit 17. Reference numeral 19 denotes a luminance control circuit which receives the PWM duty value selected by the luminance setting register 18 and generates a PWM signal (luminance control signal) having a cycle of 1 / f3, and 20 denotes a white LED 13 based on the PWM signal from the luminance control circuit 19. Is an LED drive output circuit for driving the LED. Reference numeral 21 denotes an external interface for externally rewriting part or all of the settings (PWM duty value: dimming data) of the luminance setting register 18.
[0016]
In this circuit, when a current corresponding to the surrounding brightness flows through the illuminance sensor 11, the current is converted into a voltage signal by the resistor 12 and taken into the illuminance level detection circuit 17, where the illuminance level is set to the first to fourth levels. Is detected at regular intervals. Then, a specific register is selected in the luminance setting register 18 according to the detected illuminance level, and the PWM duty value set in the register is set in the luminance control circuit 19, whereby the LED drive output circuit 20 switches the PWM drive output circuit 20 to the PWM. It operates, and the white LED 13 is driven.
[0017]
As described above, the white LED driving circuit 10 drives the white LED 13 according to the brightness of the surroundings, and thus is suitable as a backlight of a liquid crystal panel of a mobile phone. Further, the white LED driving circuit 10 can be constituted by one chip, and the number of external connection components is reduced to only the phototransistor 11, the resistor 12, and the white LED 13. Further, since the illuminance level detection circuit 17 operates intermittently once every several seconds, power consumption can be reduced. Further, since the content of the brightness setting register 18 can be changed from the outside by the external interface 21, PWM data can be set according to the use of the user, and the relationship between the surrounding brightness and the brightness of the white LED 13 can be arbitrarily adjusted. .
[0018]
FIG. 2 is a specific circuit diagram of the illuminance level detection circuit 17 described above. Reference numerals 31 to 34 denote resistors for dividing the power supply voltage VDD to generate reference voltages V1 to V3. The reference voltages V1 to V3 are input to the inverting input terminal (-) of the comparator 38 via the analog switches 35 to 37. I do. Reference numeral 39 denotes a shift register which operates only while the clock f2 of a predetermined duty value from the frequency dividing circuit 16 is "H", performs a shift operation with the clock f1 from the oscillation circuit 15, and sequentially converts the signals S1 to S4. The signal is set to “H”, and the signals S1 to S3 among them become control signals for the analog switches 35 to 37. Reference numeral 40 denotes a first latch for holding the data obtained by the comparator 38 in accordance with the signals S1 to S3, and reference numeral 41 denotes a second latch for holding the data held in the first latch 40 by a signal S4. Note that the register 39, the first latch 40, and the second latch 41 are reset by a reset signal RST. 42 is an OR gate to which the signals S1 to S3 are input, and 43 is a PMOS transistor controlled by the output of the OR gate 42. The PMOS transistor 42 is connected to the inverting input terminal (-) of the comparator 38. The non-inverting input terminal (+) of the comparator 38 is connected to a common connection point of the phototransistor 11 and the resistor 12 via the sensor terminal 10a.
[0019]
FIG. 3 is a time chart of the operation of the illuminance level detection circuit 17 of FIG. Here, VDD = 4V, V1 = 3V, V2 = 2V, V3 = 1V, and Vsens = 1.5V.
[0020]
In state 1 immediately after the input of the reset signal RST, since the signals S1 to S3 of the shift register 39 are all "L", the output of the OR gate 42 becomes "L", the PMOS transistor 43 is turned on, and the comparison is completed. The power supply voltage VDD (= 4 V) is applied to the inverting input terminal (−) of the detector 38. Since the voltage of the non-inverting input terminal (+) of the comparator 38 is Vsens = 1.5 V, the output Vcomp of the comparator 38 becomes “L”.
[0021]
In state 2, the signal S1 of the shift register 39 becomes “H”, the analog switch 35 is turned on, and the voltage V1 (= 3 V) is applied to the inverting input terminal (−) of the comparator 38. Therefore, also at this time, the output Vcomp of the comparator 38 becomes “L”.
[0022]
In state 3, the signal S2 of the shift register 39 becomes “H”, the analog switch 36 is turned on, and the voltage V2 (= 2V) is applied to the inverting input terminal (−) of the comparator 38. Therefore, also at this time, the output Vcomp of the comparator 38 becomes “L”.
[0023]
In state 4, the signal S3 of the shift register 39 becomes “H”, the analog switch 37 is turned on, and the voltage V3 (= 1V) is applied to the inverting input terminal (−) of the comparator 38. Therefore, at this time, the output Vcomp of the comparator 38 becomes “H”. When the output Vcomp of the comparator 38 becomes “H”, the first latch 40 holds the states of S1 = “L”, S2 = “L”, and S3 = “H”.
[0024]
In state 5, the signal S4 of the shift register 39 becomes “H”, and the register selection data corresponding to the combination of S1 to S3 held in the first latch 40 is held in the second latch 41. The above operation is intermittently repeated every few seconds when the clock f2 becomes "H", so that the current consumption can be greatly reduced.
[0025]
In the above, since the output Vcomp of the comparator 38 becomes "H" when the signal S3 is output, the comparator outputs corresponding to the signals S1 to S3 are fetched at this time. When the output Vcomp at 38 becomes "H", a signal of S1 = "H" is taken in at this time. When the output Vcomp of the comparator 38 becomes "H" when the signal S2 is output, the signals of S1 = "L" and S2 = "H" are taken in at this time. This is because when the signal S1 becomes “H”, the signals S2 and S3 must naturally become “H”, and when the signal S2 becomes “H”, the signal S3 also naturally becomes “H”. .
[0026]
FIG. 4 is a diagram showing the contents controlled according to the four states of the surrounding brightness “bright”, “dim 1”, “dim 2”, and “dark”. As described above, when the detection voltage Vsens = 1.5 V, the signals S1 to S3 held in the second latch 41 become S1 = “L”, S2 = “L”, S3 = “H”, and “ The register REG3 is selected in the luminance setting register 18 and the PWM control value is set to 60% by the luminance control circuit 19 in response to the control. An average current is applied to the white LED 13 so as to be “60%”.
[0027]
As described above, the contents of the REG1 to REG4 of the luminance setting register 18, that is, the duty value of the PWM can be changed from the outside, whereby the brightness of the white LED 13 to be controlled can be changed according to the surrounding brightness.
[0028]
The white LED driving circuit used as the backlight of the liquid crystal panel of the mobile phone has been described above. However, the driving target is not limited to the white LED, and it is needless to say that a colored LED can be similarly controlled. is there. In the above description, the PWM is used as the means for dimming the LED, and the PWM duty value is used as the dimming data. However, the present invention is not limited to this, and the digital dimming data selected by the luminance setting register 18 may be used. The brightness control circuit 19 generates an analog brightness control signal, and the LED drive output circuit 20 can drive the LED with the analog signal.
[0029]
【The invention's effect】
As described above, according to the present invention, the number of external components can be minimized only for illuminance detection or using only LEDs, and the illuminance level detection circuit operates at fixed time intervals, so that current consumption is reduced, and an external interface is provided. Accordingly, there is an advantage that the brightness of the LED with respect to the surrounding brightness can be arbitrarily changed on the user side.
[Brief description of the drawings]
FIG. 1 is a block diagram of a white LED driving circuit according to an embodiment of the present invention.
FIG. 2 is a circuit diagram of an illuminance level detection circuit in the white LED drive circuit of FIG. 1;
FIG. 3 is a time chart of the circuit of FIG. 2;
FIG. 4 is an operation explanatory diagram of the white LED driving circuit of FIG. 1;
FIG. 5 is a circuit diagram of a conventional white LED drive circuit without dimming.
FIG. 6 is a circuit diagram of a conventional white LED drive circuit that enables dimming.
FIG. 7 is a circuit diagram of another conventional white LED drive circuit that enables dimming.
[Explanation of symbols]
10: White LED drive circuit, 11: Phototransistor, 12: Resistance, 13: White LED, 14: Power supply circuit, 15: Oscillation circuit, 16: Frequency divider circuit, 17: Illuminance level detection circuit, 18: Brightness setting register, 19: brightness control circuit, 20: LED drive output circuit, 21: external interface 31-34: resistor, 35-37: analog switch, 38: comparator, 39: shift register, 40: first latch, 41: second Latch, 42: OR gate, 43: PMOS transistor

Claims (3)

An illuminance level detection circuit that takes in an illuminance detection voltage corresponding to the surrounding brightness at regular intervals and detects an illuminance level; A brightness setting register for selecting one dimming data, a brightness control circuit for taking in one dimming data selected by the brightness setting register and generating a brightness control signal; An LED drive output circuit that drives an LED according to a control signal. The LED drive circuit according to claim 1,
The LED drive circuit, wherein the illuminance level detection circuit detects which of the plurality of illuminance levels the illuminance detection voltage divides into a range from "bright" to "dark". . The LED drive circuit according to claim 1,
An LED drive circuit, comprising: an external interface for externally rewriting a part or all of the plurality of light control data of the luminance setting register.

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