SCHOTTKY TTL (74S/54S) BASIC INFORMATION

The Schottky TTL offers a speed that is about twice that offered by the high-power TTL for the same power consumption. Figure 5.19 shows the internal schematic of a Schottky TTL NAND gate.

The circuit shown is that of one of the four gates inside a quad two-input NAND (type 74S00 or 54S00). The circuit, as we can see, is nearly the same as that of the high-power TTL NAND gate.

The transistors used in the circuit are all Schottky transistors with the exception of Q5. A Schottky Q5 would serve no purpose, with Q4 being a Schottky transistor. A Schottky transistor is nothing but a conventional bipolar transistor with a Schottky diode connected between its base and collector terminals.

The Schottky diode with its metal–semiconductor junction not only is faster but also offers a lower forward voltage drop of 0.4V as against 0.7V for a P–N junction diode for the same value of forward current. The presence of a Schottky diode does not allow the transistor to go to deep saturation.

The moment the collector voltage of the transistor tends to go below about 0.3 V, the Schottky diode becomes forward biased and bypasses part of the base current through it. The collector voltage is thus not allowed to go to the saturation value of 0.1V and gets clamped around 0.3 V.

While the power consumption of a Schottky TTL gate is almost the same as that of a high-power TTL gate owing to nearly the same values of the resistors used in the circuit, the Schottky TTL offers a higher speed on account of the use of Schottky transistors.

Characteristic Features
Characteristic features of this family are summarized as follows: VIH = 2V; VIL =0.8 V; IIH =50 micro A;
IIL =2 mA; VOH=2.7 V; VOL =0.5 V; IOH =1 mA; IOL =20 mA; VCC =4.75–5.25V (74-series) and 4.5–5.5V (54-series); propagation delay (for a load resistance of 280 , a load capacitance of 15 pF, VCC =5V and an ambient temperature of 25 °C)=5 ns (max.) for LOW-to-HIGH and 4.5 ns (max.) for HIGH to-LOW output transitions; worst-case noise margin=0.3 V; fan-out=10; ICCH (for all four gates)=16 mA; ICCL (for all four gates)=36 mA; operating temperature range=0–70 °C (74- series) and −55 to +125 °C (54-series); speed–power product=57 pJ; maximum flip-flop toggle frequency=125 MHz.