JPH0434752B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a display driving circuit using light emitting diodes (abbreviated as LED), etc., and more specifically, to a display driving circuit that uses a segment method to drive a display device that displays characters, numbers, etc. in a time-division manner. The present invention relates to a display driving circuit for display driving.

In general, a seven-segment numeric display device using LEDs has a configuration as shown in FIG. As a circuit for driving this display device, a circuit as shown in FIG. 1B is generally used. In the same figure, the display device 1 shown by broken lines is shown as an equivalent circuit (diode) by selecting only A to C of the segments shown in FIG. 1a and omitting the others. Now, if the transistor 3 that drives the drive terminal 2 of segment A is in a conductive state, and furthermore, the transistor 5 that drives the common terminal 4 that shares the current of each of the segments A to G is in a conductive state, Electric current flows and it emits light. Note that the resistor R is used to adjust the absolute value of the current.

Since the light emission intensity of the LED depends on the current flowing, it is desirable that the current flowing through each segment A to G be the same. Therefore, it was necessary to make the equivalent resistance value of the transistor 5, which drives the common terminal 4, extremely small when it is conductive. This is because the current flowing through the common terminal 4 changes greatly depending on the number of conductive segments, so if the equivalent resistance value of the transistor 5 is high, the voltage at the common terminal 4 changes greatly, and the light emission intensity of the LED changes. This is because it becomes In other words, the smaller the number of light-emitting segments, the brighter the light becomes, and the larger the number, the darker the light becomes.

Therefore, the equivalent resistance value of the transistor 5 for driving the common terminal is the voltage applied to each LED due to the difference in current between when one segment is conductive and when all seven segments are conductive. must be so low that it does not change much. For example, 1
Considering that the current flowing through one segment is approximately 10 mA, the equivalent resistance value of transistor 5 is 2~
Must be less than 3Ω.

As described above, the conventional configuration has a major problem in that the voltage fluctuates depending on the amount of current at the common terminal, and the light emission intensity of the segments changes.

The present invention aims to eliminate such inconveniences and provide a more rational display driving integrated circuit.

An embodiment of the present invention will be described below with reference to FIG. In FIG. 2, like FIG. 1b, only A to C of each segment are selected and the others are omitted, but all the circuits other than the LED display device 1 are composed of one integrated circuit. ing. Now, considering the case where only segment A is lit, the P channel transistor 3A for driving segment A and the N channel transistor 5A for driving the common terminal are electrically connected. Transistors 3A and 5A each have a constant equivalent resistance value, and these series resistances, the characteristics of segment A, and the power supply voltage V _DD
The current flowing through segment A is determined by . The current can be adjusted by setting the equivalent resistance values of transistors 3A and 5A to appropriate values.

Next, consider a case where three segments A, B, and C are lit at the same time. P-channel transistors 3A, 3B, and 3C for driving the segments are conductive, and N-channel transistors 5A, 5B, and 5C for driving the common terminal 4 are conductive. In this state, the three transistors connected in parallel on the common terminal 4 side are conducting at the same time, so if these characteristics are the same, the equivalent resistance value will be compared to the case where only segment A is lit. It is now 1/3. Segment drive transistor 3A,
The characteristics of 3B and 3C are the same, and segments A,
Assuming that the characteristics of B and C are also the same, the current flowing through the common terminal 4 will be tripled, and the voltage drop will be the same.

In other words, the voltage and current conditions applied to the segments are different from when lighting one segment and when lighting three segments.
It does not change when lighting one segment,
Therefore, the same brightness can be obtained.

Incidentally, the signal waveform applied to the gate of the common terminal driving transistor shown in FIG. 2 can be created from the signal applied to the gate of the segment driving transistor using a simple circuit as shown in FIG. The signal _Vc1 in FIG. 1 is similar to the signal Vc for driving the transistor 5 in FIG. 1, and corresponds to the digit signal. For example, if we assume that Figure 2 represents the first digit in a system that displays 8-digit numbers, the waveform of each signal in Figure 3 will be the 4th digit.
The result will be as shown in the figure. In other words, the segment drive signal (for example, V _SA ) is commonly used for each digit and changes depending on the number of each digit, but the drive signal for each transistor on the common terminal 4 side is only used when the digit signal V _c1 is present. It becomes effective. In this example, in the first digit, segments B and C are electrically connected, and the number "1" is displayed.

As described above, in this embodiment, as in the case of the conventional example shown in FIG. The voltage is
Set it to 2V. Assuming that the power supply voltage V _DD is 5V, in the configuration shown in FIG. 2, the sum of the voltage drop due to the segment drive transistor and the voltage drop due to the drive transistor on the common terminal side is 3V. Assuming that this is divided equally between the two, the voltage drop on one side will be 1.5V. In other words, the equivalent resistance value of one transistor is
It is 150Ω.

In the example in Figure 1, the equivalent resistance value of the driving transistor on the common terminal side had to be 2 to 3 Ω or less, but in the configuration in Figure 2, one of the driving transistors on the common terminal side had to have a resistance of 150 Ω. There are seven transistors with the same equivalent resistance value, so the effective resistance value is approximately 21Ω. Therefore, the total pattern area of the seven transistors is approximately 1/7 to 1/10 of the example shown in Figure 1.
This shows that it can be made much smaller.

According to the configuration of the present invention, the segment drive circuit
It can be reasonably realized using MOS integrated circuits.
Furthermore, the structure of the present invention can be used as a driving circuit for fluorescent display tubes, liquid crystals, electrochromic displays, etc., but even in these cases, conventionally, it has been difficult to integrate circuits due to the large pattern area. For things that are hot, it becomes easier to realize them.

Incidentally, in the present invention, the common terminal driving transistor is P
It is clear that it is also possible to configure the segment drive transistor with an N-channel MOS.

[Brief explanation of drawings]

FIG. 1a is a segment layout diagram showing a 7-segment LED display device, FIG. 1b is an example of a segment drive circuit, FIG. 2 is a circuit configuration diagram according to an embodiment of the present invention, and FIG. A circuit diagram for creating gate signals for each driving transistor on the common terminal side in the figure,
FIG. 4 is a drive waveform diagram of the circuit example of FIG. 2. 1... LED display device, 2... A segment drive terminal, 3A, 3B, 3C... segment side drive transistor, 4... Common terminal, 5A, 5
B, 5C...Common terminal side drive transistor.

Claims (1)

[Claims] 1 a plurality of display segments, a first transistor group individually connected to one of the current-carrying terminals of each of these display segments for selectively driving each display segment, and a first transistor group of each other of the plurality of display segments; are connected in common, and to this common connection terminal, a second group of transistors connected in parallel with the same number as the first group of transistors, and a group of transistors from the first group of transistors and the second group of transistors. A display driving circuit comprising drive signal forming sections that select the same number of driving signal forming sections.