linear regulator power dissipation - short pulses

[composition4]

Hi,

Just need 5VDC for powering the logic ICs + LEDs + latching relays in a circuit.  Need to obtain this from 9V AC.  Just trying to keep things clean and simple with linear 3 terminal regulators for now (2940CT5 - which is a low dropout version of 7805).  I have a switching regulator but would prefer not to use it.

Logic ICs + LEDs will draw 100-300mA constant, and when a footswitch is pressed (only occasionally) the current demand will increase to at most 700mA for a period of 30ms.

Now, I figure the 9VAC will rectify to around 11V DC including rectifier losses.  That means my linear regulator will need to drop 6V.  At 300mA (max constant current draw), this works out to 1.8 watts of power dissipated as heat.  Datasheet suggests that @ 50 deg celsius, regulator should handle about 2 watts - that's cutting it close so I will be looking at using a small heatsink of about 30 deg / watt - which should raise the max allowable dissipation to 4 watts.

When the current demand increases to 700mA, this increases the heat generated to 4.2 watts.  I know this is over the 4 watts that my heatsink will allow, but I'm thinking that because it's only for 30ms that the current draw is this high, it should be okay?

Please let me know if you think this is okay or not? It's just hard for me to say because all the datasheets state the dissipation figures as a constant figure, but none say the max allowable dissipation for short bursts...

Thanks
Jonathan

[head_spaz]

It's a minor surge of short duration. Run with it... you'll be fine.