Advanced Low-Power Schottky TTL (74ALS/54ALS)
The basic ideas behind the development of the advanced low-power Schottky TTL (ALS-TTL) and advanced Schottky TTL (AS-TTL) discussed in Section 5.3.8 were further to improve both speed and power consumption performance of the low-power Schottky TTL and Schottky TTL families respectively. In the TTL subfamilies discussed so far, we have seen that different subfamilies achieved improved speed at the expense of increased power consumption, or vice versa. For example, the low- power TTL offered lower power consumption over standard TTL at the cost of reduced speed. The high-power TTL, on the other hand, offered improved speed over the standard TTL at the expense of increased power consumption. ALS-TTL and AS-TTL incorporate certain new circuit design features and fabrication technologies to achieve improvement of both parameters. Both ALS-TTL and AS-TTL offer an improvement in speed–power product respectively over LS-TTL and S-TTL by a factor of 4. Salient features of ALS-TTL and AS-TTL include the following:
1. All saturating transistors are clamped by using Schottky diodes. This virtually eliminates the storage of excessive base charge, thus significantly reducing the turn-off time of the transistors. Elimination of transistor storage time also provides stable switching times over the entire operational temperature range.
2. Inputs and outputs are clamped by Schottky diodes to limit the negative-going excursions.
3. Both ALS-TTL and AS-TTL use ion implantation rather than a diffusion process, which allows the use of small geometries leading to smaller parasitic capacitances and hence reduced switching times.
4. Both ALS-TTL and AS-TTL use oxide isolation rather than junction isolation between transistors. This leads to reduced epitaxial layer–substrate capacitance, which further reduces the switching times.
5. Both ALS-TTL and AS-TTL offer improved input threshold voltage and reduced low-level input current.
6. Both ALS-TTL and AS-TTL feature active turn-off of the LOW-level output transistor, producing a better HIGH-level output voltage and thus a higher HIGH-level noise immunity.
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