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Drivers For Light Emitting Displays Appnote 24

Latch Main Data or System Memory

Drivers For Light Emitting Displays Appnote 24
The purpose of this application note is to provide some information on the integrated circuits presently available to drive Light Emitting Diodes (LED) displays and how to interface them to the various displays. Background LED displays come in various sizes (0.1" to 0.8"), colors (red, high-efficiency red, green, yellow and blue), fonts (7 / 9 / 14 / 16 segment, dot-matrix, or bargraph), and types (common anode, common cathode, multi-digit). The brightness is essentially proportional to the current through an LED and each element within a display should have the same current or a brightness variation may be apparent. A display subsystem can be made up from several elements. Figure 1. Display system
Latch Main Data or System Memory
Decoder
Driver
Display
TTL or Transistor
Darlington Transistor
It can be seen that Vce (saturation voltage) for the driver is going to be a factor in determining the series limiting resistor. Therefore a darlington versus a single output transistor will have different current limiting resistor values to maintain a constant current through the LED. Figure 4. Common cathode display
Figure 5. Common anode display
2000 Infineon Technologies Corp. · Optoelectronics Division · San Jose, CA www.infineon.com / opto · 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG · Regensburg, Germany www.osram-os.com · +49-941-202-7178
May 31, 2000-13
Figure 6. Common cathode display with driver
Data Input V cc
Figure 8. Open collector type driver with common anode display
7448 or Equivalent A RS B RS C RS D RS E RS F RS G RS
7447 or equivalent
Figure 7. Common anode display with driver
Figure 9. Open collector type driver with common cathode display
VCC RS RS RS
7447 or Equivalent 1 2 3 4
7449 or equivalent
Data Input
Selection One factor in choosing the display and / or driver will be whether the display is a common cathode or common anode type display. Another factor is that different drivers go low or high or can be wired into different configurations. From Figures 6 / 7 / 8 / 9, it may appear obvious to combine the seven series resistors (Rs) into one common resistor in the common line. However this should not be done because of the possible variation in Vf from segment to segment. Variation in Vf can cause a variation in current, resulting in segment brightness differences. Table 1 is list of some of the most common LED drivers available. Besides having different current drive capabilities, one product may have a feature which may make them easier to use in a particular application: - - - - - - - Serial versus parallel input data Data latching type drivers Blanking Drive ripple blanking input (rbo) with pulse width modulation to vary brightness Multi-digit drivers Constant current drivers Advantage of a constant current driver as Vf may not affect brightness, important with different color LEDs.
Multiplexing ln a multiplex system, the corresponding segment of each digit is bussed together and driven from one segment drive via the usual current limiting resistors. The display data is presented serially by digit to the decoder driver together with the appropriate digit signal (Figure 10). For more information multiplexing, see Appnote 3 (Multiplexing LED Displays). One way to simplify the design procedure for alphanumeric displays would be to consider OSRAM Intelligent Displays. This device family incorporates all necessary interface control with drivers and memory built-in with the display. This means the designer need not be concerned about the memory, multiplex circuitry, character generator, or drivers for these are provided inside a modular unit. More information on these products is available in OSRAM Optoelectronics Data Book. Circuits shown in this Appnote are for reference only and are not the responsibility of OSRAM Optoelectronics. Products are continually being improved by vendors and / or are obsoleted therefore consult the factory.
2000 Infineon Technologies Corp. · Optoelectronics Division · San Jose, CA www.infineon.com / opto · 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG · Regensburg, Germany www.osram-os.com · +49-941-202-7178
Appnote 24
May 31, 2000-13
Figure 10. Block diagram-4 digit multiplexed display
Data Storage
Segment Decoder
Segment Driver
Data Selector OSC Digit Selector
Digit Digit Digit Digit Driver Driver Driver Driver
2000 Infineon Technologies Corp. · Optoelectronics Division · San Jose, CA www.infineon.com / opto · 1-888-Infineon (1-888-463-4636) OSRAM Opto Semiconductors GmbH & Co. OHG · Regensburg, Germany www.osram-os.com · +49-941-202-7178
Appnote 24
May 31, 2000-13