JP2006261160A - Inductive led driver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inductive LED driver capable of independently controlling each of light-emitting diodes. <P>SOLUTION: In the inductive LED driver (10A), an input voltage (V<SB>IN</SB>) is supplied to one end of an inductor (L), the other end of the inductor (L) is repeatedly grounded and opened by a switching circuit (11) turned on/off at a switching frequency, thereby accumulating a voltage combining the input voltage (V<SB>IN</SB>) and voltages of both ends of the inductor (L) in an output capacitor (C<SB>OUT</SB>) as an output voltage (Vo), and applying the output voltage to first-fourth light-emitting diodes (LED1-LED4) connected in series. In this LED driver, first-fourth switches (SW1-SW4) each connected in parallel to the first-fourth light-emitting diodes each control currents flowing through the first-fourth light-emitting diodes. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an LED driver for driving an LED such as a white LED (light emitting diode), and more particularly to an inductive type LED driver.

  This type of LED driver is used, for example, to drive a white LED used as a backlight of a liquid crystal display device of a mobile phone. Some liquid crystal display devices for mobile phones include a main display and a sub display.

  As this type of LED driver, an inductive type LED driver and a charge pump type LED driver are known. An inductive type LED driver is disclosed in Patent Document 1, for example.

  Hereinafter, a conventional inductive type LED driver 10 will be described with reference to FIG. Hereinafter, the inductive type LED driver 10 is also simply referred to as “LED driver”.

A battery voltage is applied to the LED driver 10 from a battery 20 such as a lithium ion battery as an input voltage _VIN . An input capacitor _CIN is connected to the battery 20 in parallel, and the input voltage _VIN is held across the input capacitor _CIN .

  The LED driver 10 has an input voltage pin VIN, a switching pin SW, an LED drive pin DRV, a shutdown pin SHDN, and a ground pin GND.

Next, the external configuration of the LED driver 10 will be described. The input voltage pin VIN Input voltage _{V IN} is applied. An inductor L is connected between the input voltage pin VIN and the switching pin SW. An anode of a Schottky barrier diode SBD is connected to the switching pin SW. The first to fourth white light emitting diodes LED1 to LED4 are connected in series between the cathode of the Schottky barrier diode SBD and the LED drive pin DRV. Between the LED drive pin DRV and ground pin GND resistor _{R 1} is connected. The ground pin GND is grounded. An output capacitor _COUT is connected between the cathode of the Schottky barrier diode SBD and the ground pin GND. A PWM signal as will be described later is supplied to the shutdown pin SHDN from a control circuit (not shown).

Next, the internal configuration of the LED driver 10 will be described. The LED driver 10 includes a switching circuit 11 in order to generate a necessary voltage in the first to fourth white light emitting diodes LED1 to LED4. This switching circuit 11 is for periodically shorting the inductor L to the ground via the switching pin SW. The switching frequency of the switching circuit 11 is, for example, 1 MHz, but may be in the range of 50 kHz to 3 MHz. When the inductor L is short-circuited, magnetic energy is stored in the inductor L. When the short circuit is released, a voltage obtained by combining the voltage across the inductor L and the input voltage _VIN is stored in the output capacitor C _OUT as a boosted voltage (output voltage) Vo via the Schottky barrier diode SBD. It is done. This boosted voltage (output voltage) Vo satisfies the forward voltage required for the series circuit of the first to fourth white light emitting diodes LED1 to LED4.

Note that the input voltage _VIN is generally in the range of 3V to 4.2V. On the other hand, the output voltage Vo is in the range of 15V to 16V.

Here, since the LED drive pin DRV is maintained at a predetermined voltage V _FB , a voltage of (Vo−V _FB ) is applied to the series circuit of the first to fourth white light emitting diodes LED1 to LED4. The predetermined voltage V _FB is, for example, 0.1V.

The LED driver 10 further includes a reference voltage generator 12, an error amplifier 13, and a PWM control circuit 14. The reference voltage generator 12, the input voltage V _IN through an input voltage pin VIN is supplied. The reference voltage generator 12 generates a reference voltage Vref. The error amplifier 13 compares the reference voltage Vref with a predetermined voltage _VFB and outputs an error signal. Based on the error signal, the PWM control circuit 14 controls switching (on / off) of the switching circuit 11.

  The reference voltage generator 12 and the PWM control circuit 14 are supplied with a PWM signal from a control circuit (not shown) via a shutdown pin SHDN. In the illustrated example, the repetition frequency of the PWM signal is 1 kHz, but may be in the range of 100 Hz to 1 kHz. When the PWM signal is at a logic high level, the PWM control circuit 14 turns on / off the switching circuit 11 at the switching frequency. On the other hand, when the PWM signal is at a logic low level, the PWM control circuit 14 turns off the switching circuit 11.

  That is, the PWM signal is for dimming the first to fourth white light emitting diodes LED1 to LED4 by changing the duty factor. For example, it is assumed that the initial setting LED current flowing through the first to fourth white light emitting diodes LED1 to LED4 is 20 mA, and the duty factor (ON duty) of the PWM signal is 50%. In this case, the average current flowing through the first to fourth white light emitting diodes LED1 to LED4 is 10 mA. This average current defines the brightness of the white light emitting diode. On the other hand, the initial setting LED current defines the color temperature of the white light emitting diode.

  In the conventional LED driver 10 shown in FIG. 1, all of the first to fourth white light emitting diodes LED1 to LED4 are adjusted by adjusting the on-duty (duty factor) of the PWN signal supplied from the shutdown pin SHDN. It is dimming at the same time.

US2004 / 0256625 (FIG. 1)

  As described above, there is a mobile phone including a main display and a sub display as a liquid crystal display device. In such a cellular phone, in order to control the main display and the sub display independently, it is necessary to use two LED drivers 10 as shown in FIG. Therefore, the cost becomes high. On the other hand, when only one LED driver 10 as shown in FIG. 1 is used, the main display and the sub display are driven at the same time.

  On the other hand, the charge pump type LED driver may be used as the LED driver. Since the charge pump type LED driver has a plurality of white light emitting diodes connected in parallel to the output capacitor, the main display and the sub display can be driven independently. However, there is a problem that the charge pump type LED driver is less efficient than the inductive type LED driver.

  Therefore, the subject of this invention is providing the inductive type LED driver which can control each light emitting diode independently.

According to the present invention, an input voltage (V _IN ) is supplied to one end of an inductor (L), and the other end of the inductor is grounded or opened by a switching circuit (11) that is turned on / off at a switching frequency. By repeating this, a voltage obtained by combining the input voltage and the voltage across the inductor is stored as an output voltage (Vo) in the output capacitor (C _OUT ), and the output voltage is connected in series to the first through In an inductive type LED driver (10A, 10B) for applying to Nth (N is an integer of 2 or more) light emitting diodes (LED1 to LED4), each of the first to Nth light emitting diodes is connected in parallel. First to Nth current control means (SW1 to SW4) for controlling currents flowing through the first to Nth light emitting diodes, respectively. An inductive type LED driver characterized by comprising I1 to I4).

  In the inductive type LED driver (10A), the first to Nth current control means may be constituted by first to Nth switches (SW1 to SW4), respectively. In this case, the first to Nth switches are controlled to be turned on / off by first to Nth control signals supplied from the outside. The inductive type LED driver (10A) includes a PWM control circuit (14) that controls driving and stopping of the on / off operation of the switching circuit by a PWN signal having a repetition frequency lower than the switching frequency. The repetition frequency of the first to Nth control signals is preferably higher than the repetition frequency of the PWN signal.

  In the inductive type LED driver (10A), the first to Nth current control means may include first to Nth current sources (I1 to I4), respectively. In this case, the current values of the first to Nth current sources are controlled according to the levels of the first to Nth control signals supplied from outside.

  In addition, the code | symbol in the said parenthesis is attached | subjected in order to make an understanding of this invention easy, and it is only an example, and of course is not limited to these.

  In the present invention, the current control means is connected in parallel to each of the plurality of light emitting diodes connected in series to control the current flowing through the plurality of light emitting diodes, so that the plurality of light emitting diodes can be controlled independently. it can.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  An LED driver 10A according to the first embodiment of the present invention will be described with reference to FIG. The illustrated LED driver 10A is shown in FIG. 1 except that the LED driver 10A further includes first to fourth switches SW1 to SW4 connected in parallel to the first to fourth white light emitting diodes LED1 to LED4, respectively. The LED driver 10 has the same configuration. Accordingly, components having the same functions as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted for the sake of simplicity.

  The LED driver 10A has first to fourth control input pins CTL1 to CTL4 that receive first to fourth control signals supplied from a control circuit (not shown). The first to fourth control signals are signals for controlling on / off of the first to fourth switches SW1 to SW4, respectively.

Each of the first to fourth control signals in this example is a digital signal that takes either a logic high level H or a logic low level L. When the first to fourth control signals are at the logic high level H, the first to fourth switches SW1 to SW4 are turned on, respectively. When the first to fourth control signals are at the logic low level L, the first to fourth switches SW1 to SW4 are respectively turned on. The fourth switches SW1 to SW4 are turned off. When the nth (1 ≦ n ≦ 4) switch SWn is turned on, the LED current I _LED flows through the nth switch SWn, bypassing the nth white light emitting diode LEDn.

  Accordingly, the first to fourth switches SW1 to SW4 are connected in parallel to the first to fourth light emitting diodes LED1 to LED4, respectively, and control currents flowing through the first to fourth light emitting diodes LED1 to LED4, respectively. The first to fourth current control means.

  As described above, the PWM control circuit 14 controls driving and stopping of the on / off operation of the switching circuit 11 by the PWN signal having a repetition frequency lower than the switching frequency. The repetition frequency of the first to fourth control signals is preferably higher than the repetition frequency of the PWN signal. For example, the repetition frequency of the first to fourth control signals may be twice the repetition frequency of the PWN signal. However, the repetition frequency of the first to fourth control signals may be lower than the repetition frequency of the PWN signal.

  Next, the operation of the LED driver 10A illustrated in FIG. 2 will be described with reference to FIGS. This example shows a case where the repetition frequency of the first to fourth control signals is twice the repetition frequency of the PWN signal. Further, it is assumed that the initial setting LED current is 20 mA.

  FIG. 3 shows that the duty factor (Duty) of the PWM signal supplied to the shutdown pin SHDN is 100% (see FIG. 3A), and the duty of the first control signal supplied to the first control input pin CTL1. An example in which the coefficient (Duty) is 50% (see FIG. 3B) is shown. Although not shown in FIG. 3, the second to fourth control input pins CTL2 to CTL4 are not supplied with the second to fourth control signals, and the duty of the second to fourth control signals is not shown. The coefficient (Duty) is 0%.

  In this case, as shown in FIG. 3C, the average current flowing through each of the second to fourth white light emitting diodes LED2 to LED4 is 20 mA equal to 100% of the initial setting LED current. On the other hand, since the duty factor (Duty) of the first control signal is 50%, the average current flowing through the first white light emitting diode LED1 is 10 mA, which is 50% of the initial setting LED current.

  FIG. 4 shows that the duty factor (Duty) of the PWM signal supplied to the shutdown pin SHDN is 50% (see FIG. 4A) and the duty of the first control signal supplied to the first control input pin CTL1. An example in which the coefficient (Duty) is 50% (see FIG. 4B) is shown. Although not shown in FIG. 4, the second to fourth control input pins CTL2 to CTL4 are not supplied with the second to fourth control signals, and the duty of the second to fourth control signals is not shown. The coefficient (Duty) is 0%.

  In this case, as shown in FIG. 4C, the average current flowing through each of the second to fourth white light emitting diodes LED2 to LED4 is the initial value because the duty factor (Duty) of the PWM signal is 50%. 10 mA, which is 50% of the set LED current. On the other hand, the average current flowing through the first white light emitting diode LED1 is that the duty factor (Duty) of the PWM signal is 50% and the duty factor (Duty) of the first control signal is 50%. This is 5 mA, which is 25% of the initial setting LED current.

  In this manner, in the LED driver 10A illustrated in FIG. 2, only the current flowing through the first white light emitting diode LED1 can be controlled independently of the second to fourth white light emitting diodes LED2 to LED4. . Obviously, any of the currents flowing through the second to fourth white light emitting diodes LED2 to LED4 can be controlled independently of the other white light emitting diodes in the same manner. Here, since the maximum value of the current flowing through the first to fourth white light emitting diodes LED1 to LED4 is equal to the initial setting LED current, the color temperature of the first to fourth white light emitting diodes LED1 to LED4 does not change.

  Further, since the inductive type LED driver 10A is used as compared with the charge pump type LED driver, each white light emitting diode can be lighted efficiently.

  Furthermore, in the case of a mobile phone having a main display and a sub display as a liquid crystal display device, in order to control the main display and the sub display independently, in the conventional case, an LED driver as shown in FIG. Since it was necessary to use two 10s, the cost was high. On the other hand, in the present embodiment, only one LED driver 10A as shown in FIG. 2 is used, and the main display and the sub display can be controlled independently. Therefore, compared with the conventional circuit, the number of parts can be reduced and the cost can be reduced.

  With reference to FIG. 5, an LED driver 10B according to a second embodiment of the present invention will be described. The illustrated LED driver 10B includes first to fourth current sources I1 to I4 in place of the first to fourth switches SW1 to SW4, and the first to fourth control input pins CTL1 to CTL1 to CTL1 to CTL1 to CTL1. The configuration is the same as that of the LED driver 10A shown in FIG. 2 except that the signal forms of the first to fourth control signals supplied to the CTL 4 are different. Accordingly, components having the same functions as those shown in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted for the sake of simplicity.

  In this example, the first to fourth control signals supplied to the first to fourth control input pins CTL1 to CTL4 respectively control the current values of the first to fourth current sources I1 to I4. It is a signal for. That is, each of the first to fourth control signals in this example is an analog signal that continuously changes in a range from a logic high level H (100%) to a logic low level L (0%).

When the first to fourth control signals are at the logic high level H (100%), the first to fourth current sources I1 to I4 do not flow current, that is, flow 0% current. On the other hand, when the first to fourth control signals are at the logic low level L (0%), the first to fourth current sources I1 to I4 respectively have the maximum current value that can be passed, that is, the current of 100%. Shed. On the other hand, when the first to fourth control signals are at an intermediate level (50%) between the logic high level H and the logic low level L, the first to fourth current sources I1 to I4 pass 50% current, respectively. . That is, the current value of the n-th current source In varies according to the level of the n-th (1 ≦ n ≦ 4) control signal, and the current flowing through the n-th white light emitting diode LEDn connected in parallel thereto is Can be controlled. Here, the current value of the combined current value of currents flowing current value of the current source In the n-th and the white light emitting diode LEDn The n-th note equal to the current value of the LED current I _LED is a constant value I want to be. In any case, the current values of the first to fourth current sources I1 to I4 are controlled by the first to fourth control signals supplied from the outside.

  Accordingly, the first to fourth current sources I1 to I4 are connected in parallel to the first to fourth light emitting diodes LED1 to LED4, respectively, and currents flowing through the first to fourth light emitting diodes LED1 to LED4 are respectively connected. It functions as first to fourth current control means for controlling.

  Next, the operation of the LED driver 10B illustrated in FIG. 5 will be described with reference to FIG. Here, the following is assumed. It is assumed that the duty factor (Duty) of the PWM signal supplied to the shutdown pin SHDN is 100%. Then, only the level of the first control signal supplied to the first control input pin CTL1 changes as shown in FIG. 6A and is supplied to the second to fourth control input pins CLT2 to CLT4. The levels of the second to fourth control signals to be set are assumed to be constant at a logic high level (100%). Further, it is assumed that the initial setting LED current is 20 mA.

  As shown in FIG. 6A, it is assumed that the first control signal continuously changes from a logic high level H (100%) to a logic low level L (0%) with a constant slope. In this case, as shown in FIG. 6B, the current value of the first current source I1 continuously changes from 0% to 100%. Since the levels of the second to fourth control signals are the logic high level H (100%), the second to fourth current sources I2 to I4 do not pass current, and their current values are 0%. is there.

  Therefore, the average current flowing through each of the second to fourth white light emitting diodes LED2 to LED4 is 20 mA equal to 100% of the initial setting LED current. On the other hand, the average current flowing through the first white light emitting diode LED1 is 100% of the initial setting LED current in inverse proportion to the current value of the first current source I1, as shown in FIG. 6C. It continuously changes from 20 mA which is 0 to 0 mA which is 0%.

  In this way, in the LED driver 10B illustrated in FIG. 5, only the current flowing through the first white light emitting diode LED1 can be controlled independently of the second to fourth white light emitting diodes LED2 to LED4. . Here, since the maximum value of the current flowing through the second to fourth white light emitting diodes LED2 to LED4 is equal to the initial setting LED current, the color temperature of the second to fourth white light emitting diodes LED2 to LED4 does not change. On the other hand, since the value of the current flowing through the first white light emitting diode LED1 changes, the color temperature of the first white light emitting diode LED1 changes. Obviously, any of the currents flowing through the second to fourth white light emitting diodes LED2 to LED4 can be controlled independently of the other white light emitting diodes in the same manner.

  Moreover, since the inductive type LED driver 10B is used compared with a charge pump type LED driver, each white light emitting diode can be lighted efficiently.

  Furthermore, in the case of a mobile phone having a main display and a sub display as a liquid crystal display device, in order to control the main display and the sub display independently, in the conventional case, an LED driver as shown in FIG. The cost was high because two 10s had to be used. On the other hand, in the present embodiment, only one LED driver 10B as shown in FIG. 5 is used, and the main display and the sub display can be controlled independently. Therefore, compared with the conventional circuit, the number of parts can be reduced and the cost can be reduced.

  In the present embodiment, since there is no sharp change in the output voltage Vo, there is an effect that ringing can be prevented.

  Although the present invention has been described above with reference to preferred embodiments, it is needless to say that the present invention is not limited to the above-described embodiments. For example, although the LED driver described in the above embodiment has been described only for the example of driving four light emitting diodes, the number N of light emitting diodes may be plural. Also, the first to Nth current control means connected in parallel to the first to Nth light emitting diodes are not limited to switches and current sources, and the currents flowing through the first to Nth light emitting diodes are respectively limited. Anything that can be controlled is acceptable.

It is a block diagram which shows the structure of the conventional inductive type LED driver. It is a block diagram which shows the structure of the inductive type LED driver which concerns on the 1st Embodiment of this invention. 3 is a time chart showing a first operation example of the inductive type LED driver shown in FIG. 2. 3 is a time chart showing a second operation example of the inductive type LED driver shown in FIG. 2. It is a block diagram which shows the structure of the inductive type LED driver which concerns on the 2nd Embodiment of this invention. 6 is a time chart showing an operation example of the inductive type LED driver shown in FIG. 5.

Explanation of symbols

10A, 10B Inductive type LED driver 11 Switching circuit 12 Reference voltage generator 13 Error amplifier 14 PWM control circuit 20 Battery CIN Input capacitor L Inductor SBD Schottky barrier diode COUT Output capacitor LED1-LED4 White light emitting diode R1 Resistor SW1-SW4 Switch I1 to I4 Current source SHDN Shutdown pin CTL1 to CTL4 Control input pin

Claims (4)

An input voltage is supplied to one end of the inductor, and the other end of the inductor is repeatedly grounded or opened by a switching circuit that is turned on / off at a switching frequency, whereby the input voltage and the voltage across the inductor are In an inductive type LED driver that accumulates a voltage in combination with the output capacitor as an output voltage and applies the output voltage to first to Nth (N is an integer of 2 or more) light emitting diodes connected in series.
The first to Nth current control means are respectively connected in parallel to the first to Nth light emitting diodes and control currents flowing through the first to Nth light emitting diodes. Inductive type LED driver.   Each of the first to Nth current control means includes first to Nth switches, and each of the first to Nth switches is supplied from the outside. 2. The inductive type LED driver according to claim 1, wherein on / off is controlled by a control signal. The inductive type LED driver includes a PWM control circuit that controls driving and stopping of the on / off operation of the switching circuit by a PWN signal having a repetition frequency lower than the switching frequency,
The inductive LED driver according to claim 2, wherein a repetition frequency of the first to Nth control signals is higher than a repetition frequency of the PWN signal. Each of the first to Nth current control means includes first to Nth current sources, and each of the first to Nth current sources is supplied from the outside. 2. The inductive type LED driver according to claim 1, wherein the current value is controlled by the level of N control signals.

Images