SN75584A High Voltage 7 Segment Latch Decoder Cathode Driver Function & Application

The SN75584A is a BIDFET integrated circuit and has high voltage DMOS outputs. The SN75584A (Figure 8-24) is designed to decode four lines of BCD data and drive a seven segment (plus decimal point) gas display tube such as the Beckman and Panaplex II displays. Latches are provided to store the four data and decimal point inputs while the enable input is at a low logic level voltage.

This circuit employs a read only memory to provide output decoding for the BCD input digits 0 to 9. For input data codes greater than 9, the segment outputs are blanked. Each sink output is a constant current, regulated to ensure uniform brightness of the display even with fluctuating supply voltage. On-state output current is essentially constant over the output voltage range of 4 V to 100 V. Each current sink is normalized to the ‘b’ segment output current as required for even illumination of all segments. Output currents may be varied from 0.1 mA to 4 mA for driving various displays.

Figure 8-24. SN75584A DC Plasma Segment Driver

The output current is adjusted by connecting an external programming resistor (RP) from the current programming input to ground. Figure 8-25 shows the “b” segment output current versus programming resistance. IO(b) in mA = 3.6l6/RP in kΩ. The blanking input provides unconditional blanking of all segment outputs including the decimal point output. The enable input allows data to be stored internally while input data is changing. When enable is at a high level voltage, the outputs will reflect conditions on the A, B, C, D, and DP inputs in accordance with the function table in Figure 8-26. A transition from a high level voltage to a low level voltage at enable will cause the input data set up prior to the transition to be latched. In the latched state, the A, B, C, D, and DP inputs are in a high-impedance state to minimize input loading.

Thermal protection circuitry will blank the display, regardless of input conditions, whenever junction temperature exceeds approximately 150°C.

Figure 8-25. SN75584A Segment Output Current vs Program Resistance
Figure 8-26. SN75584A Function Table

SN75584A Application

A basic circuit (Figure 8-27) that automatically counts 0 through 9, resets and counts through again is used to demonstrate the SN75584A’s ease of operation and drive capabilities. The display is a 3-digit display with decimal points requiring a 180 V supply and typical segment operating currents of 1 mA.

The TLC555 provides clock pulses at about 1.6 Hz to an SN7493A 4-bit binary counter which in turn drives the A, B, C and D inputs of the SN75584A display driver in accordance with its count sequence. Sequencing through the binary input codes results in the output segment drivers cycling from 0 through 9. For all input codes over 9 all segment outputs are blanked off. Thus the display will be blanked off for about four seconds before the count sequence

starts over with zero. Blanking of all output drives (including decimal point) is accomplished by switching pin 5, the blanking input, to ground. If the latch enable input, pin 6, is connected to 5 V the SN75584A continues to sequence. Switching the latch enable to ground prevents further transferring of data to the 584A outputs and the segment outputs are held (latched) in the mode they were in just prior to switching. Selling pin 6 high again allows data present in the 584A memory to again control the outputs. The driver output may also be held by simply stopping the clock. This is accomplished by switching pin 5, control voltage input, of the TLC555 to ground. In this application, pin 4 of the SN75584A, (the current programming input) is connected through a 3.3 kΩ resistor to ground allowing about 1 mA of segment drive current. The decimal point input, pin 1, is grounded which leaves the decimal point on at all times except when blanked. A TTL logic high level at pin 1 could turn off the decimal point output.

Figure 8-27. Three-Digit Display Driver Application