Design analysis of constant current drive circuit for LED display

LED display is a new type of information display media that developed rapidly in the world in the late 1980s. It uses dot matrix modules or pixel units composed of light-emitting diodes to form a large-area display screen. It has high reliability, service life and strong environmental adaptability. , high performance-price ratio, low cost of use, etc., has been widely used in the field of information display.

The LED display mainly includes an array composed of light-emitting diodes, a driving circuit, a control system, a transmission interface and corresponding application software, etc. The quality of the driving circuit design affects the display effect, production cost and system performance of the LED display. plays a very important role. Therefore, it is necessary to design a control and drive circuit that can meet the requirements of control and drive, and at the same time uses few components and low cost. This paper makes some analysis on the design of the conventional drive circuit and proposes the design method of the constant current drive circuit.

1. Design of conventional drive circuit for LED display

The design of the LED display drive circuit, in conjunction with the control system used, is usually divided into two categories: dynamic scanning drive and static latch drive. The following is an example of the design of the dynamic scanning drive circuit for analysis:

The dynamic scanning driving mode means that the n-row LEDs on the display screen share a set of column drive registers. 1/n of the time, as long as the refresh rate of each line is greater than 50Hz, people can see a complete text or picture by using the visual persistence effect of the human eye.

The design of conventional drive circuits generally uses serial-in and parallel-out general integrated circuit chips such as 74HC595 or MC14094 as column data latches, 8050 and other low-power NPN transistors as column drivers, and Darlington transistors TIP127 as row. scan tube,

If a single-color dot matrix and 16 lines × 64 are used as a basic unit, 8 pieces of 74HC595, 64 pieces of 8050 and 16 line scan tubes are required. Clock CLK and data latch signal STR. When the data to be displayed in the first row will be moved into the 74HC595 after 8×8=64 CLK clocks, a data latch signal STR is generated at this time, so that the data is latched in the post-stage latch of the 74HC595, then it is The 8050 corresponding to the output bit will be in saturation on or off;
At the same time, a signal is generated by the line scanning control circuit to turn on the first line of scanning tubes, which is equivalent to the positive end of the first line of LEDs being connected to a high level. Obviously, the lighting of the first line of LED tubes depends on the latched signal in the 74HC595;

When the first row of LED tubes are lit, the data to be displayed in the second row is shifted into the 74HC595, and then latched, and at the same time, the row scan control circuit turns off the first row of scan tubes and turns on the second row. Light up the second row of LED tubes. By analogy, when the sixteenth line is scanned and then back to the first line, as long as the scanning speed is high enough, a complete text or image can be formed.

2. Defects in conventional drive circuits

(1) When a row drive tube is valid, the lighting current of all LED light-emitting diodes corresponding to the row will flow through the drive scan tube of the row, and the number of LED tubes lit in a row depends on the text to be displayed or The graph changes continuously, so the current flowing in the line scanning tube has a large change, which will change the voltage drop of the tube;

(2) The change of the current caused by the number of lit LED tubes will also affect the fluctuation of the power supply voltage value, which will affect the voltage across the first row of LED tubes, causing it to fluctuate continuously with different displayed text or graphics. It affects the uniformity of the brightness of the entire display.

3. Design of LED display constant current drive circuit

Figure 3 shows the relationship between the relative brightness of the light-emitting diode and the current flowing in it. It can be seen from the curve that: within a certain forward current operating range, its luminous brightness is approximately proportional to the current flowing in it , which are current-driven devices. Therefore, as long as the current flowing through each LED light-emitting tube can be guaranteed to be a constant, the brightness can be guaranteed to be consistent.

From the working principle of the conventional drive circuit, it can be seen that because the row and column drive tubes work in a saturated state, the current cannot be controlled, so the fluctuation of the external power supply voltage and the change of the voltage drop of the row scan drive tube etc., it directly affects the current flowing in the LED light-emitting tube, that is, changes its display brightness. If the column drive tube is changed from a saturated state to a linear amplification state and becomes a constant current type drive, the uneven brightness of the display screen caused by the above factors can be eliminated.

When the power supply voltage VDD is stable, the high-level output voltage V of the 74HC595 is also very stable. For example, when the power supply voltage VDD is 6V, V = 5.9V. So when a certain bit output of 74HC595 is high level, the LED of its corresponding column will be lit, and the current flowing in it is approximately:

As long as the values of R1, R2, and R3 are selected reasonably, the current flowing in the LED can be guaranteed to be stable and unchanged, and the LED light-emitting diode can be operated in the best state of forward current and corresponding luminous brightness.

With this column constant current driving method, no matter how many LED tubes are lit in a row, although the tube voltage drop of the row driver tube still changes, the power supply voltage VCC can also be changed. The current flowing in the light-emitting diode is constant, thus ensuring the uniformity of the brightness of the LED display.

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