A 5 V regulated DC supply (100 mA maximum output). The resulting T1 output is full-wave rectified via bridge rectifier BR1 and converted into a reasonably smooth DC voltage via electrolytic capacitor C1.įIGURE 1. In each case, the AC input voltage is stepped down to a value in the 9-12 V range via transformer T1 (which has a VA power rating at least double that of the final power supply’s DC output). These are typical TTL PSU designs, and all work basically the same way. Three power-supply circuits are shown in Figures 1 to 3. The PCB’s supply rails should be liberally sprinkled with 4.7 ♟ tantalum electrolytic capacitors (at least one for every 10 ICs) to enhance low-frequency decoupling, and with 10 nF disk ceramics (at least one for every four ICs, fitted as close as possible between an IC’s supply pins) to enhance high-frequency decoupling. Connections and interconnections should be as short and direct as possible. In general, the TTL circuit’s PCB’s +5 V and 0 V supply rail tracks must be as wide as possible (ideally, the 0 V track should take the form of a ground plane). Consequently, practical TTL circuits should always be powered from a low-impedance, well-regulated supply such as one of those shown in Figures 1 to 3, and must be used with a PCB (printed circuit board) that is very carefully designed to give excellent high-frequency supply decoupling to each TTL IC.
The 74-series TTL ICs are designed to be used over a very limited supply voltage range (4.75-5.25 V), and - because they generate very fast pulse edges and have relatively low noise-margin values - must be used with supplies with very low output impedance values (typically less than 0.1 Ω). Assuming that the matter of fan-in and fan-out has already been taken care of (as described in Part 2 of this mini-series), four other basic usage themes remain, and these are described under the headings Power Supplies, Input Signals, Unused Inputs, and Interfacing. It is usually a fairly simple matter to design logic circuitry using “74-series” TTL ICs, provided that a set of TTL basic usage rules are observed.
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