JP2003152224A

JP2003152224A - High-efficiency led driving system

Info

Publication number: JP2003152224A
Application number: JP2001351464A
Authority: JP
Inventors: Takeshi Tanaka; 武田中
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 2001-11-16
Filing date: 2001-11-16
Publication date: 2003-05-23

Abstract

PROBLEM TO BE SOLVED: To improve the efficiency of an LED driving system by reducing the electric power corresponding to the product of a margin voltage and the current flowing to an LED by eliminating the need of setting the conventionally required output voltage having the standard margin of the forward voltage VF of the LED by controlling the output voltage of a boosting circuit so that the voltage may become a fixed value by detecting the current flowing to the LED at the time of driving a white LED, etc., by means of the boosting circuit. SOLUTION: A current detecting circuit is provided for the LED and the value of the current flowing to the LED is compared with a reference current value. The comparison output is controlled so that the output may become equal to the reference current value by inputting the output to the output voltage control circuit of the boosting circuit used for driving the LED. Although various circuits, such as the VCO, PWM, etc., can be used as the voltage control circuit of the boosting circuit, the basic idea is the same for all systems.

Description

【発明の詳細な説明】【０００１】【発明の属する技術分野】本発明はＬＥＤ駆動システム
に関するものである。【０００２】【従来の技術】現在携帯電話などの携帯機器が広く普及
しているが、待ち受け時間や連続使用時間の延長、ある
いは環境に対する配慮といった要求から、電源の高効率
な使用の取り組みがなされている。ＬＥＤなどの発光デ
バイスの高効率な駆動に関しても、様々な工夫がなされ
ている。【０００３】特に近年は、カラー表示の必要性から液晶
表示装置のバックライト光源として白色ＬＥＤが多用さ
れている。白色ＬＥＤは順方向電圧ＶＦが大きいため、
携帯電話のようなリチウムイオン電池などを１セルで使
用する機器においては昇圧回路を必要とする。昇圧回路
にはチャージポンプやインダクタを用いたＤＣ／ＤＣコ
ンバータなどがあるが、効率を重視してＤＣ／ＤＣコン
バータを使う場合が増えている。【０００４】以下、図３を用いて従来の白色ＬＥＤの駆
動回路を説明する。図３において、１が白色ＬＥＤであ
りこの図の場合では３個を直列に駆動しているが、並列
に駆動する場合もある。また、直列に駆動される個数は
特に３個に限るものではない。端子ＡがＤＣ／ＤＣコン
バータなどのＬＥＤ駆動昇圧回路４の出力であり、前記
白色ＬＥＤ１のアノードに接続されている。ＬＥＤ駆動
昇圧回路４は、白色ＬＥＤ１の順方向電圧ＶＦが大きい
ために、電池３電圧低下時に点灯できないことを防ぐた
めに必須となっている。ＶＦが小さいＬＥＤにおいて
も、直列接続をする場合には昇圧が必要である。２は定
電流源で、ＬＥＤ１のカソードに接続される。この電流
値は可変になっており、この電流値を電流値設定回路１
１によって調整することで、ＬＥＤ１の輝度を調整する
ことができる。【０００５】定電流駆動せずにＬＥＤ駆動を行なった場
合、ＬＥＤ駆動昇圧回路４の出力電圧が負荷変動等によ
り変化した場合に、連動して輝度変動するため、表示の
ちらつきなどを引き起こす。【０００６】また、電流源回路数を削減するためにＬＥ
Ｄを直列接続して駆動する方式がよく取られる。これに
より、すべてのＬＥＤに等しい電流が流れると同時に、
同一ロットのＬＥＤ素子を用いれば、素子ごとの相対的
な輝度ばらつきが大きくないため、ロットごとに電流値
を調整して、絶対的な輝度ばらつきを低減できる。【０００７】【発明が解決しようとする課題】しかしながら、図３に
示した上記従来の白色ＬＥＤ駆動回路の構成では、ＬＥ
Ｄ駆動昇圧回路４の出力電圧は直列に接続した全てのＬ
ＥＤにおけるＶＦ規格の最大値の和以上に設定しなけれ
ばならない。このＶＦ規格のマージンと実際のＶＦ値と
の差があるため、駆動電流によって電力損失が発生して
効率を落としていた。【０００８】本発明は、昇圧回路の出力電圧を制御する
ことで定電流駆動方式を実現し、最小の電力損失でＬＥ
Ｄを駆動することを目的とする。【０００９】【課題を解決するための手段】この目的を達成するため
に、本発明は、ＬＥＤに設定した定電流が流れるように
ＬＥＤ駆動昇圧回路の出力電圧を制御することを特徴と
するものである。【００１０】この構成によって、駆動システム全体での
電力損失を最小にすることができる。【００１１】【発明の実施の形態】本発明の基本概念について、図面
を参照しながら説明する。【００１２】図１は、図３で示した従来構成に対し、定
電流源２を電流検出回路９に置き換え、ＬＥＤ１に流れ
る電流値が一定となるようにＬＥＤ駆動昇圧回路４の出
力電圧値を制御する構成を示したものである。【００１３】電圧制御の方法はＤＣ／ＤＣ回路などの方
式にもよる。例えばＶＣＯの入力電圧を変化させ、ま
た、ＰＷＭ方式のパルス幅を変えるなどが考えられる。【００１４】より具体的な実施の形態を図２に示す。【００１５】図２において、ＬＥＤ駆動昇圧回路４とし
てＤＣ／ＤＣコンバータ１０を用いている。検出抵抗５
によりＬＥＤ１に流れる電流ＩＬＥＤが検出され、比較
器７によって、基準電圧源６に設定された値と比較され
る。検出抵抗５の電圧が基準電圧源６よりも低い場合、
出力電圧制御回路８のＶＣＯなどの入力電圧を変化させ
てＬＥＤ駆動昇圧回路４の出力を上昇させ、設定電圧よ
りも高い場合は逆に出力を下降させる。この結果、駆動
電圧におけるＬＥＤのＶＦが合計された値と、電流検出
抵抗に発生させる電圧の和の電圧が出力され、この回路
の効率はＬＥＤ駆動昇圧回路４の効率と電流検出回路に
よる損失のみで決定される。【００１６】具体例を挙げれば、代表的な白色ＬＥＤの
場合、ＶＦの標準値は３．６Ｖ程度である。しかしＶＦ
規格値は４Ｖ程度なので、昇圧回路の出力を一定電圧と
した場合は、仮に３灯のＬＥＤ駆動を行なう場合、定電
流駆動回路で必要な電圧＋３灯×４Ｖ以上の出力電圧を
必要とすることになる。よって、２０ｍＡ負荷の場合
（４Ｖ−３．６Ｖ）×３灯×２０ｍＡが損失となる。こ
れはＬＥＤ駆動に必要な電力の約１０％に相当する。【００１７】【発明の効果】以上のように本発明は、ＬＥＤを定電流
駆動する際に、昇圧回路出力電圧を制御することによっ
てＬＥＤのＶＦ規格によらず高効率駆動を実現でき、Ｖ
Ｆ値に応じた必要最小限の駆動電圧に昇圧回路の出力が
制御されるため、ＶＦのマージンによる損失は発生しな
い。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED driving system. 2. Description of the Related Art At present, portable devices such as portable telephones have been widely used. However, due to demands such as extension of standby time and continuous use time, and consideration for the environment, efforts have been made to use power supplies with high efficiency. ing. Various devices have been devised for highly efficient driving of light emitting devices such as LEDs. [0003] In recent years, in particular, white LEDs have been frequently used as backlight light sources for liquid crystal display devices because of the necessity of color display. Since the white LED has a large forward voltage VF,
Devices using a single cell such as a lithium-ion battery such as a mobile phone require a booster circuit. The booster circuit includes a charge pump, a DC / DC converter using an inductor, and the like, and the use of a DC / DC converter with emphasis on efficiency is increasing. A conventional white LED driving circuit will be described below with reference to FIG. In FIG. 3, reference numeral 1 denotes a white LED, and three LEDs are driven in series in this case, but may be driven in parallel. The number driven in series is not particularly limited to three. A terminal A is an output of the LED drive step-up circuit 4 such as a DC / DC converter, and is connected to the anode of the white LED 1. The LED drive step-up circuit 4 is indispensable in order to prevent that the white LED 1 cannot be turned on when the voltage of the battery 3 drops because the forward voltage VF of the white LED 1 is large. Even in the case of LEDs having a small VF, boosting is necessary in case of series connection. 2 is a constant current source, which is connected to the cathode of LED1. This current value is variable, and this current value is
By adjusting with 1, the brightness of the LED 1 can be adjusted. When the LED drive is performed without the constant current drive, when the output voltage of the LED drive step-up circuit 4 changes due to a load change or the like, the brightness fluctuates in conjunction therewith, thereby causing display flicker. In order to reduce the number of current source circuits, LE
A method of driving by connecting D in series is often used. This allows equal current to flow through all LEDs,
If the LED elements of the same lot are used, the relative luminance variation between the elements is not large, so that the current value can be adjusted for each lot to reduce the absolute luminance variation. However, in the configuration of the conventional white LED driving circuit shown in FIG.
The output voltage of the D drive step-up circuit 4 is the same as that of all the L connected in series.
It must be set to be equal to or greater than the sum of the maximum values of the VF standard in the ED. Since there is a difference between the margin of the VF standard and the actual VF value, a power loss occurs due to the drive current and the efficiency is reduced. The present invention realizes a constant current driving method by controlling the output voltage of a booster circuit, and realizes LE with a minimum power loss.
D is driven. In order to achieve this object, the present invention is characterized in that the output voltage of an LED drive booster circuit is controlled so that a constant current set to an LED flows. It is. [0010] With this configuration, power loss in the entire drive system can be minimized. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic concept of the present invention will be described with reference to the drawings. FIG. 1 is different from the conventional configuration shown in FIG. 3 in that the constant current source 2 is replaced by a current detection circuit 9 and the output voltage value of the LED drive step-up circuit 4 is changed so that the current value flowing through the LED 1 becomes constant. 3 shows a configuration for controlling. [0013] The voltage control method also depends on a system such as a DC / DC circuit. For example, it is conceivable to change the input voltage of the VCO and change the pulse width of the PWM method. FIG. 2 shows a more specific embodiment. In FIG. 2, a DC / DC converter 10 is used as the LED drive step-up circuit 4. Detection resistor 5
, The current ILED flowing through the LED 1 is detected by the comparator 7 and compared with the value set in the reference voltage source 6 by the comparator 7. When the voltage of the detection resistor 5 is lower than the reference voltage source 6,
The output of the LED drive step-up circuit 4 is raised by changing the input voltage of the VCO or the like of the output voltage control circuit 8, and when the input voltage is higher than the set voltage, the output is lowered. As a result, a voltage is output which is the sum of the sum of the VFs of the LEDs at the drive voltage and the voltage generated at the current detection resistor. The efficiency of this circuit is only the efficiency of the LED drive booster circuit 4 and the loss due to the current detection circuit. Is determined. As a specific example, in the case of a typical white LED, the standard value of VF is about 3.6V. But VF
Since the standard value is about 4V, if the output of the booster circuit is set to a constant voltage, if three LEDs are to be driven, an output voltage of at least the required voltage of the constant current drive circuit plus three lights x 4V is required. become. Therefore, in the case of a 20 mA load, (4V-3.6 V) × 3 lamps × 20 mA is a loss. This corresponds to about 10% of the power required for driving the LED. As described above, according to the present invention, when the LED is driven at a constant current, by controlling the output voltage of the booster circuit, high efficiency driving can be realized regardless of the VF standard of the LED.
Since the output of the booster circuit is controlled to the required minimum drive voltage according to the F value, no loss due to the VF margin occurs.

【図面の簡単な説明】【図１】本発明を示す概念図【図２】本発明の１つの実施形態を示す図【図３】従来の概念図【符号の説明】１ＬＥＤ２定電流駆動回路３電池４ＬＥＤ駆動昇圧回路５電流検出用抵抗６電流値設定用基準電圧源７比較器８出力電圧制御回路９電流検出回路１０ＤＣ／ＤＣコンバータ[Brief description of the drawings] FIG. 1 is a conceptual diagram illustrating the present invention. FIG. 2 illustrates one embodiment of the present invention. FIG. 3 is a conceptual diagram of the related art. [Explanation of symbols] 1 LED 2 Constant current drive circuit 3 batteries 4 LED drive booster circuit 5 Current detection resistor 6 Current setting reference voltage source 7 Comparator 8 Output voltage control circuit 9 Current detection circuit 10 DC / DC converter

Claims

【特許請求の範囲】【請求項１】ＬＥＤに設定した定電流が流れるように
昇圧回路出力電圧を制御することによって、駆動システ
ム全体での電力損失を最小にするＬＥＤ駆動回路。Claims: 1. An LED drive circuit that controls a booster circuit output voltage so that a constant current set to an LED flows, thereby minimizing power loss in the entire drive system.