My design involves 2 micros that operate at 3.3v. I have output line drivers that operate at 5v. One of the micros is a wifi device so I want to stay away of switching power supplies.
5v or 12v can be applied to the POWER IN to the board (Vin). I have a P-Fet providing reverse battery protection (90 milliohms ).
I need to get to 5.0v ... from there I have a 3.3v LDO reg supplying the micros.
My question is the best way to implement the circuit between the Rev-Bat P-Fet (either 5v or 12) ... and get to 5v.
I would normally use a boost-buck regulator but not with the radio being so near.
I need about 400 milliamps ... so I'm designing for 1A.
I can get several 1 or 2 amp 5v linear reg (under $3) that have an enable control. The main micro can come up and keep current low until it decides how to configure the vreg control.
So - I can have the 5v linear on by default (assuming 12v is applied) and measure the input voltage. If I determine Vin is actually 5v -- I can turn off the 5V vreg and enable a bypass switch to route the Vin of 5v to the input of the 3.3 LDO.
So ... after all of that --- what is the best circuit design for the bypass that can be controlled by the micro? More than likely another P-Fet -- I'm just not sure of the actual circuit design.
Another question ... I've been out of hardware design for 45 years -- now software. At 400ma and 7v being dropped across the 5v Vreg when Vin is 12v -- that almost 3W the regulator has to dissipate. How hot is that? How hot with a finned heat sink?