A zener diode datasheet will almost invariably specify the nominal zener current or the maximum zener current, rather than the minimum:
- Nominal (i.e. ideal) value may be specified as the zener test current, such as in the case of the 1n4740A: 25 mA, table on Page 2 of datasheet
- Maximum, as a function of the maximum power dissipation for the particular Zener diode family:
Izmax = Pdis / Vzener
Note that the second method depends on the maximum power dissipation, which needs to be derated for actual junction temperature, as per a derating value also typically provided by the datasheet.
The minimum current one can use is determined by how much precision is required. As the current is reduced, at one point the voltage graph for the device will stray outside the acceptable error. Therefore, for regulation purposes, the nominal value if available, or half the maximum value, would be good practices to follow.