is developed rapidly in the world in the late 80 s, new information display media which USES leds composed of lattice modules or pixel units, large display screen, with its high reliability, service life, strong environment adaptability, high cost performance, low use cost, in the field of information display have got the very extensive application.
LED display is mainly composed of light emitting diode array, drive circuit, control system and transmission interface and the corresponding application software, etc. , which drive the stand or fall of circuit design, display of LED display, the performance of the production cost and system plays a very important role.
So, design a kind of both can satisfy the requirement of control driver, at the same time, use less components, low cost control drive circuit is very necessary.
In this paper, the conventional type of drive circuit design make some analysis and put forward the methods of constant current drive circuit design.
2, design of conventional drive circuit of LED display, LED display driver circuit design, to match the control system used, usually divided into dynamic scanning type drive and static latch type drive two kinds big.
Type the following dynamic scan driver circuit design as an example for analysis: dynamic scanning drive mode refers to the type on the display lines 4, 8, 16 n row light-emitting diodes (leds) share a set of column driver registers, through the time-sharing work line drive pipe, make each row of the LED light to total time of 1 / n, as long as each refresh rate is more than 50 hz, the use of the human eye persistence of vision effect, people can see a full text or picture.
Conventional type of drive circuit design is generally use string into and out of the common integrated circuit chips such as 74 hc595 are needed or MC14094 as column data latches, small power N PN triode in the 8050 column, driven by darlington transistors TIP127 as line scan tube, such as the circuit is shown in figure 1.
As with single color dot matrix, line 16 x 64 listed as a basic unit, is 8 74 hc595 are needed, 64, 8050 and 16 line scan tube, its working principle is: will eight grade 74 hc595 are needed even, sharing a serial clock CLK and data latch signal STR.
When the first line need to display the data after 8 x 8 = 64 CLK clock will all out into a 74 hc595 are needed and produced a STR data latch signal at this time, the data level lock has 74 hc595 are needed after latch, compared with the corresponding output bit of 8050 will be a saturation conduction or cut-off;
At the same time signal generated by the line scan control circuit to make the first line scanning tube conduction, equivalent to the ends of the first line of the LED is cao bang, destroy the light of the first line of the LED tube is obviously depends on the 74 hc595 are needed in the latch signal;
At the same time of the first line of the LED tube light, in the 74 hc595 are needed in the second row to display the data, then the latch, and at the same time by the line scan control circuit will be the first line scanning tube closed on the second line, the second line of the LED tube light.
By analogy, when after the line scan 16 back to the first row, as long as the scanning speed is high enough, it can form a complete text or image, its job sequence as shown in figure 2.
Figure 2 working sequence diagram 3, the flaws of the conventional type of drive circuit (
When a line drive pipe effectively, the bank for all LED light-emitting diodes, light current will flow through the drive scanning tube, and the light of LED tube in the line of how many to display the text or graphics and constantly changing, so the current of line scan pipe flows has larger changes, would make the pipe pressure drop change;
How much light LED tubes caused by the current changes will also affect the power supply voltage fluctuations, thus will affect the first line of the LED tube on both ends of the voltage, make its display text or graphics with different fluctuations, affect the uniformity of the entire display brightness.
in this column constant current drive way to work, can do it no matter how much the number of a line of the LED tube light, the adults drive pipe of the pressure drop, though still have change, VCC power supply voltage can be changed, because each flows LED light-emitting diode current constant, so as to ensure that the LED display brightness are allowed.
Figure 4 column type constant current drive circuit.
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