TTL source/sink currents and LEDs (was Re: Some Altair questions, if I may..)

On Sun, Aug 24, 2014 at 2:06 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote: By which Tony means "can sink a **MUCH** larger current than it can source". If you look at the spec for the TI SN7404, the difference is a factor of 40: max -0.4 mA source (high), vs. max 16 mA sink (low). Generally speaking, 0.4mA is nowhere near enough to be usable for an LED, so driving the low side of the LED was the only practical approach with normal TTL. Even the max sink spec of 16mA wasn't enough to drive a typical mid-1970s red LED very brightly. For example, a Monsanto (later GI) MV5020 red LED at 20 mA would only give you 2.0 mcd typical (and 0.6 mcd minimum!). That very low source current spec (-0.4 mA) is about the same for all true TTL families, excepting some buffer parts that are spec'd able to source more current. Modern CMOS parts tend to have closer or even identical source and sink limits, so it has become somewhat more common to see high-side drive for LEDs, though many designers have stuck with low-side drive out of tradition. Many modern LEDs are also much more efficient than those from the 1970s, which is partially because different chemistries producing lower wavelengths are commonly used for red, e.g., 620-340 nm vs. 660 nm, and the human eye is significantly more sensitive to the lower wavelength. However, the modern chemistries are also more efficient in terms of absolute light output per current. A representative modern T1 3/4 LED with comparable spatial distribution to the MV5020 is the Cree C503B-RCS-CW0Z0AA1, which has a typical wavelength of 624 nm, and typical luminous intensity of 5100 mcd (minimum 3000 mcd) at 20 mA. When comparing LED specs, it is important to look at the spatial distribution. A narrow viewing angle LED will give higher luminous intensity but only on-axis. Eric