Iin and Iout in 7805 Regulator IC

Hi,

Thanks for your reply. Actually we measured the output and i got almost same as input but for the confirmation i thread this query.

It is now clear to me and next we can calculate Power Dissipation and Heat sink calculation.

Thank you

Nandish

Your linear regulator is working exactly the way it is supposed to. The 78XX linear regulators are a robust series of parts. They are ideal for novice designers since they have a very good thermal shutdown circuit. There are a few quirks in this series that you will find out about once you stop simulating circuits and you actually build one. The first one you should notice is related to the power dissipation calculation.

It's roughly but not exactly equal. The ground-pin current of the 7805 is specified in the uA7800-series datasheet. It's 4 or 5mA typ and 8mA max. So if Iin=300mA then Iout = 295mA.

The diagram on page 3 of the datasheet shows the portion of the control circuit that needs this current. A significant thing to notice is that the base current of the NPN pass-element output transistors comes from the INPUT supply, and of course this goes out the emitter to the OUTPUT. If we're getting picky about it, the datasheet says there's a slight change of 1mA max due to input-voltage variation to 30 volts. There's another 0.5mA max change in the bias current, for output loads from 5mA to 1A. The latter would have to come from the NPN transistor's base current taken from the current mirror in the control loop.

This class of regulator requires a minimum INPUT-OUTPUT overhead, or dropout voltage, of 1.25 to 2 volts to operate properly. There's another class of regulator IC, called LDO or low-dropout regulators, that only require about 0.1 to 0.3 volts. These use a PNP transistor as the pass element, and the base current to operate this transistor comes from the ground pin. These ICs exhibit changes in ground current vs output current, for example up about 20mA for an LT1764 at 1A, so that Iin ≠ Iout. A 20mA current would imply the pass-element PNP has a beta of 50. While an LDO can have as much as 20 to 40x higher ground current than a 7800 type, at 1A, it's still on the order of no more than 2% to 3%, so one can say that Iin roughly = Iout.