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Drivers Light Emitting Displays Appnote
Dave Takagishi
purpose this application note provide some information integrated circuits presently available drive Light Emitting Diodes (LED) displays interface them various displays. Background displays come various sizes (0.1" 0.8"), colors (red, high-efficiency red, green, yellow blue), fonts (7/9/14/16 segment, dot-matrix, bargraph), types (common anode, common cathode, multi-digit). brightness essentially proportional current through each element within display should have same current brightness variation apparent. display subsystem made from several elements. Figure Display system
circuits using logic transistors, Figure
Figure Circuits transistor darlington transistor
Latch Main Data System Memory
Decoder
Driver
Display Transistor Darlington Transistor
partitioning these elements dependent drivers used; therefore display driver chosen dependent specifications display application. Also some types displays require using multiplexing technique because internal interconnections. This only applicable multi-digit displays. Typical Circuits Figure shows very basic circuit driving LED. series resistance easily calculated from following formula.
seen that (saturation voltage) driver going factor determining series limiting resistor. Therefore darlington versus single output transistor will have different current limiting resistor values maintain constant current through LED. Figure Common cathode display
Figure Basic circuit drive
Figure Common anode display
14-54
Figure Common cathode display with driver
Data Input
Figure Open collector type driver with common anode display
7448 Equivalent
7447 equivalent
Figure Common anode display with driver
Figure Open collector type driver with common cathode display
7447 Equivalent
7449 equivalent
Data Input
Selection factor choosing display and/or driver will whether display common cathode common anode type display. Another factor that different drivers high wired into different configurations. From Figures 6/7/8/9, appear obvious combine seven series resistors (Rs) into common resistor common line. However this should done because possible variation from segment segment. Variation cause variation current, resulting segment brightness differences. Table list some most common drivers available. Besides having different current drive capabilities, product have feature which make them easier particular application: Serial versus parallel input data Data latching type drivers Blanking Drive ripple blanking input (rbo) with pulse width modulation vary brightness Multi-digit drivers Constant current drivers Advantage constant current driver affect brightness, important with different color LEDs.
Multiplexing multiplex system, corresponding segment each digit bussed together driven from segment drive usual current limiting resistors. display data presented serially digit decoder driver together with appropriate digit signal (Figure 10). more information multiplexing, Appnote (Multiplexing Displays). simplify design procedure alphanumeric displays would consider Siemens Intelligent Displays. This device family incorporates necessary interface control with drivers memory built-in with display. This means designer need concerned about memory, multiplex circuitry, character generator, drivers these provided inside modular unit. More information these products available Siemens Optoelectronics Data Book. Circuits shown this Appnote reference only responsibility Siemens Optoelectronics. Products continually being improved vendors and/or obsoleted; therefore consult factory.
Appnote
14-55
Figure Block diagram-4 digit multiplexed display
Data Storage
Segment Decoder
Segment Driver
Data Selector Digit Selector
Digit Digit Digit Digit Driver Driver Driver Driver
Table Single Digit Decoder/Drivers Part Number 7447 74247 7446 Manufacturer Fairchild Hitachi Motorola National Signetics Teledyne Fairchild Hitachi Motorola National Signetics Fairchild Hitachi Motorola National Signetics National National Fairchild National Motorola National Signetics Signetics Fairchild Fairchild If/Segment Type Comments BCD-to-7 segment, open coll, ripple blnkng
7448 74248
BCD-to-7 segment, pull-up, ripple blnkng
7449 74249
BCD-to-7 segment, open coll, ripple blnkng
DS8857 DS8858 CD4511 4511 MC14511 DS8647 DS8648 NE587 NE589 CA3161E 9368 9374
BCD-to-7 segment, decoder, ripple blnkng BCD-to-7 segment, decoder, ripple blnkng BCD-to-7-segment, latched, blnkng
segment drivers BCD-to-7-segment, latched, blnkng, vari current BCD-to-7-segment, latched, blnkng, vari current BCD-to-7-segment, constant current drivers BCD-to-7-segment, ripple blnkng BCD-to-7-segment, ripple blnkng
Appnote
14-56
Table (continued) Multi-Digit Display Drivers Part Number SDA2014 SDA2131 MM5450 MM5451 MM74C912 MM74C911 MM74917 DS8669 CA3168E ICM7212 ICM7212A ICM7212M ICM7212AM ICM7218A ICM7218B ICM7218C ICM7218D ICM7218E TSC700A TSC7212A SAA1060 Other Drivers XR-2000 XR-2201 XR-2202 XR-2203 XR-2204 CA3081 CA3082 9665 9667 Exar Exar sink sink darlington transistors, MOS-to-LED darlington transistors, open collector w/diodes, TTL-to-LED, compatible Sprague (ULN-xxxx) Manufacturer Siemens Siemens National National National National National National Intersil If/Segment Type Comments digit, serial input element serial input segment serial input, brightness control segment serial input, brightness control digit, segment+decimal, decoder, output enble digit, segments controller/segment/driver digit, segments+decimal, decoder, output enble Dual BCD-to-7 segment decoder/driver Dual BCD-to-7 segment decoder/driver digit, latched, segment drivers, brightness cntl
Intersil Intersil Intersil Intersil Intersil Teledyne Teledyne Signetics
digit, segment (decoded/spcl), w/mem/drivers digit, segment (decoded/spcl), w/mem/drivers digit, segment (hex/bcd), w/mem/drivers digit, segment (hex/bcd), w/mem/drivers digit, segment (decoded/spcl), w/mem/drivers, cntls avble digit decoder/driver, parallel output, brightness cntl digit decoder/driver, parallel output, brightness cntl element serial in/parallel driver
Fairchild
sink source sink
common emitter transistor array common emitter transistor array common emitter darlington transistor array
Graph Drivers UAA180 LM3914 LM3915 Siemens National National 2-20 1-30 n.a. n.a. n.a. element driver element dot/bar linear output driver element dot/bar linear output driver
Appnote
14-57

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