making sense of HCT ACT cmos "addional supply current"
HCT/ACT devices have a modified input stage that is compatible with TTL levels, but is still composed of MOSFETs. Since these MOSFETs will be in their ohmic region with a TTL high, there will be a high-resistance path from VCC to GND. The "additional supply current" represents this extra drain when the TTL high (specifically VCC-2.1V in this case) is presented to the input.