JPS6365956B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a driving circuit for n light emitting diodes (LEDs) connected in series, and more particularly to a circuit driven by a constant current. Recently, it has become popular to connect n LEDs in series and use this as a level meter for audio products. It has been proposed to use a constant current source to make the intensity of light emitted by the LED constant, and one example of this is known as the configuration shown in FIG. In the figure, LED1~n are connected in series.
It is an LED and is driven by a constant current source 1. Also,
PNP transistors TR1 _~o are connected in parallel to each LED as a switching means.
The lighting state of each LED is controlled according to the conduction state of TRs ₁ to TR o. That is, for example, when TR ₁ becomes conductive, the current flowing to LED ₁ bypasses TR ₁ , resulting in a non-lighting state. The control signals for TR _{1 to o} are
Control signal supply terminal connected to the base of TR _1~o
Supplied to CONT _1~o . Now, since a constant current source is used in the drive circuit with this configuration, if this constant current value is I _f , then when the LED _o is not lit, the base current I _Bo of TR _o is I _Bo = I _f / It is necessary to flow a current of β...(1). Here, β is the grounded emitter current amplification factor of TR _o . Therefore, the emitter current I _Eo of TR _o is I _Eo = I _Bo + _If = If / _β + _If = (1 + 1 / β) _If (2). This current will flow through LED _(o-1) .
Similarly, if LED _o and LED _(o-1) are not lit
The emitter current I _{E( o-1) of TR (o-1)} is: I _E(o-1) = I _Eo + I _Eo /β = (1+1/β) I _Eo = (1+1/β) ² I _f … …(3) is shown. Therefore, if only LED ₁ is lit,
A current of I=(1+1/β) ^n-1 I _f ...(4) will flow through this LED ₁ . In this configuration, as is clear from comparing equations (2) and (4), the current flowing through the LED will differ depending on the lighting state of the LED, and the intensity of the LED's light emission will vary. . Normally, in order to ensure saturation of each switch element TR, when β is 100, the base current is 1/10 of the collector current I _C , that is, I _B = I _C /10...(5 ) is flowing. Therefore, the influence of the base current cannot be ignored, and the series connection
As the number of LEDs increases, the effect becomes greater. The present invention was made in view of the above drawbacks, and in an LED drive circuit using constant current, by connecting a switch element between the cathode electrode of each of n LEDs connected in series and a constant current source,
This is intended to prevent the phenomenon in which the light emission intensity of the LED changes depending on how the control signal is applied. The present invention will be described below with reference to drawings showing one embodiment. FIG. 2 shows the present invention in a block diagram, and shows the case where four LEDs (LEDs 1 to 4) are connected in series. A switch element is connected between the cathode electrode of each LED and the constant current source I.
SW _1~4 are connected. A control signal is supplied from control signal generating means C to each switch element. Now, FIG. 3 shows a specific example of the circuit, and parts corresponding to those in FIG. 2 are given the same reference numerals. That is, LEDs 1 to 4 are series-connected LEDs that constitute, for example, a level meter, and each cathode electrode receives a constant current I ₃ through NPN transistors Q _{19 to 22} that constitute switch elements SW1 to SW4.
It is connected to the output terminal I ₀ of a constant current source I that supplies . Further, the collectors of NPN transistors _Q15 to _Q18 forming the gate circuit G are connected to the bases of the transistors Q19 to _Q22 . and,
Data 1 to 4 from a control signal generating means (described later) are supplied to the bases of these transistors _{Q15 to Q18} . On the other hand, the constant current source I includes a reference voltage generation circuit made up of _Q1 to _Q4 and a voltage-current conversion circuit made up of _Q6 to _Q13 . In other words, the reference voltage generated by the reference voltage circuit
Vref is supplied to the CC terminal by NPN transistors Q ₅ and Q ₆ . Therefore, the voltage V _(CC) at the CC terminal is expressed as V _(CC) = Vref + V _{BE (Q5)} - V _{BE (Q6)} ... (6), and assuming that the V _BE of Q ₅ and Q ₆ are equal, V _{( CC)}
= Vref. Here, V _BE indicates the voltage between the base and emitter of the transistor. Therefore, the current I ₁ flowing through the CC terminal is expressed as I ₁ =Vref/R ₆ ...(7), and this current flows through the PNP transistors Q _{7 to 10} .
After passing through the first current mirror circuit consisting of
It flows through the collector of the NPN transistor Q13 on the output side of the second current mirror composed _of NPN transistors _Q11-13 . Here, for example, the emitter area ratio of Q ₁₁ and Q ₁₃ is 1:10, and a current 10 times the collector current of transistor Q ₁₁ flows as constant current I ₃ . Furthermore, in order to reduce the temperature coefficient of this constant current source, the resistor _R6 connected to the CC terminal is an external resistor made of, for example, a carbon film resistor, and is adjusted to match the temperature coefficient of Vref. Further, _D1-4 constitute a bias circuit, and supply bias voltages to the respective NPN transistors _Q19-22 . Now, in this configuration, the relationship between the lighting state of the LED and the data (Data _{1 to 4} ) input is as shown in Table 1.

[Table] In the table, the data input column is defined by voltage logic, and the output terminal column indicates 1 when the output current is flowing. It also lights up
In the LED column, 1 indicates a lit state and 0 indicates a non-lit state. The input data of the data input terminal is as shown in the table above.
There are only five states: either one of Data1 to Data4 is 0, or all data is 1. Data1="0" and all other inputs are 1
In this case, the output current flows only to the _O1 output, and no current flows to the other outputs. In this case
Only LED1 lights up. When Data2="0" and all other inputs are 1, the output current flows only to _O2 , but this time the current flows to LED1 and LED2, causing the two LED1 and LED2 to light up. Data3=
When it is "0" and the other inputs are 1, the output current flows only to _O3 , and the three LEDs LED1, LED2, and LED3 light up. When Data4="0", an output current flows to _O4 and four LEDs LED1 to LED4 light up. When Data1 to Data4 are all 1, no current is output from _O1 to _O4 , so the LED does not light up at all. In this way, which Data among Data 1 to 4 is “0”
The lighting status of LEDs 1 to 4 can be controlled by turning on the LED.
1. LED1, LED2, etc. are connected in series and are controlled in a connected lighting state, which is extremely convenient for level meters. Now, if you want to light up only LED1,
NPN transistor _Q19 needs to be in a conductive state, but in this case the current flowing through LED1, i.e.
The collector current I _C19 of Q ₁₉ is: I ₃ = I _B19 + I _C19 = 1/βI _C19 + I _C19 = (1+1/β) I _C19 ……(8) ∴ I _C19 = I ₃ /1+β ……(9) Become. On the other hand, to light up all LEDs 1 to 4,
It is necessary to make the NPN transistor Q ₂₂ conductive and make the other Q _{19 to 21} non-conductive, but the current flowing through the LED in this case, that is, the collector current I _C22 of Q ₂₂ is I ₃ = I _B22 + I _C22 = (1 + β ) I _C22 ……(10) ∴ I _C22 = I ₃ /1+β ……(11) Therefore, as is clear from a comparison of equations (9) and (11), in the configuration of the LED drive circuit according to the present invention, the drive current of the LED is constant regardless of the lighting state of the LED. Note that this is obviously not limited to the series connection of four. FIG. 4 shows another embodiment of the LED drive circuit according to the present invention, showing an improved configuration of the output circuit section. That is, the switch elements SW1 to SW4 have a Darlington connection structure of transistors. For example, SW1 is composed of two stages of transistors Q ₂₃ and Q ₁₉ . Therefore, compared to FIG. 3, even if the resistance value of R ₉ is made larger, for example 10 times, and the base current of Q ₂₃ is made 1/10 times, Q ₁₉ can be sufficiently driven. Power consumption by _R9 can be reduced. Also, the control signal from Data1 is G1,
G2 supplies Q ₁₅ and Q ₁₉ respectively. Therefore, for example, when Data1 is at the "1" level, Q ₁₅ becomes conductive and operates to make Q ₁₉ non-conductive, but in this case, one of the outputs of G1 connected to the base of Q ₁₉ The output operates to absorb leakage current when _Q19 is non-conducting, ensuring reliable switch operation. Note that G1 to G8 are, for example, I ² L
(Integrated Injection Logic) circuit is used. The generation circuit for Data 1 to 4 consists of a counter section that counts pulse signals input as serial data, and a latch circuit section that latches the contents of the counter section as parallel data. FIG. 5 shows an example. That is, a serial pulse signal is input from the SP terminal, and the frequency is divided and counted by four stages of flip-flops. The outputs of Q _F1 to _{Q F4} of these flip-flops constitute a latch circuit.
It is input to a flip-flop having input terminals _DA1 to _DA4 . And these latch outputs are
Q _D1 , _D1 ... Output as Q _D4 , _D4 , 4-stage
Supplied to the NAND gate circuit. The input terminal of the NAND gate circuit N1 that outputs Data1 is
Similarly to Q _D1 , _D2 , _D3 , _D4 , N2 has _D1 , Q _D2 ,
Q _D3 , _D4 , N3 have Q _D1 , Q _D2 , _D3 , _D4 , N
4 is connected to _D1 , _D2 , Q _D3 , and _D4 .
In other words, it is configured to output Data 1 to 4 shown in Table 1 according to the SP input, and when the input pulse is 0, all the LEDs are off and one by one lights up as the pulse increases. The number of LEDs increases. Note that RP is a reset pulse that resets the flip-flops from T ₁ to T ₄ , and LP is a latch pulse that changes the contents of T ₁ to _{T 4} to D ₁ to
Give D ₄ the timing to latch. In other words, as shown in Figure 6, the pulses input from the SP terminal within a certain period of time are counted using RP pulses and LP pulses, and the LEDs are sequentially turned on according to this number.
lights up. By repeating this operation, continuous display will be performed. As explained above, in the present invention, a switch element is connected between the cathode electrode of n LEDs connected in series and a constant current, and the lighting state of the LED is controlled by the signal supplied to this switch element. It was configured to control. Therefore, it is possible to prevent the current flowing through the LED from changing depending on the control signal supplied to the switch element and the emission intensity from changing depending on the lighting state of the LED. Therefore, since the LED can be driven with a constant emission intensity, the original purpose of using a constant current source can be achieved, and its effect can be further increased.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an example of a conventional LED driving circuit, Fig. 2 is a block diagram showing an embodiment of the present invention, and Figs. 3 and 4 are diagrams showing a specific example of the present invention. FIG. 5 is a diagram showing an embodiment of the control signal generating means, and FIG. 6 is a diagram showing the relationship between input pulses. LED1-4...Light emitting diode, SW1-4...Switch element, C...Control signal generation means, I...Constant current source.

Claims (1)

[Claims] 1 n light emitting diodes (LEDs) connected in series
In an LED drive circuit that drives with a constant current supplied by a constant current source, one terminal is connected to the cathode electrode of each of the light emitting diodes, and the other terminal is connected to the constant current source. An LED driving circuit comprising n switching means and control signal generating means for supplying a switching control signal to each of the switching means.