The SN75480 (Figure 8-18) is a bipolar high-voltage 7-segment decoder/cathode driver for dc plasma displays. Off-state output voltage capability is 80 V minimum. It is designed to decode four lines of hexadecimal input and drive a seven-segment Panaplex II* type gas-filled (de plasma) display tube. The SN75480 employs a 112 bit read only memory to provide input decoding and output segment on or off control in accordance with the function table (Figure 8-19).
Segment drive outputs are constant-current sinks with adjustable operating levels. The current sink level is adjustable by connecting an external program resistor (RP) from VCC to the current programming input in accordance with the curve shown in Figure 8-18(c). Output current may be adjusted from nominally 0.2 mA up to 1.5 mA in order to drive various tube types and permit multiplex operation.
Output sink currents for other segments are proportional to the b-segment current to provide even illumination of all segments. Each sink output (Figure 8-18) is regulated to ensure a constant brightness level across the display even with fluctuating supply voltages. Typical on-state currents are within 1% for voltage changes from 3 to 50 V. Off-state voltages can vary from 3 to 80 V. The blanking input (pin 4) provides unconditional blanking of all outputs, while the A through D inputs (pins 1,2,6 and 7) and the ripple blanking input (pin 5) into the blanking circuit, allow simple leading or trailing zero blanking.
In the basic application circuit. Figure 8-20, a serial count from a controller or microprocessor is fed to the “A” input of an SN7493A binary counter. The SN7493A output is a BCD representation of the decimal number to be displayed. To hold the last input until time for an update, an SN7475 4-bit latch is used. The BCD input is decoded by the SN75480 to provide proper segment drive resulting in a displayed hexadecimal value in accordance with the function table (Figure 8-19).