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?
Best Answer
A solution to keep the circuit simple would be to choose a buck regulator that is able to handle 100% duty cycle. I found two of them with a bit of searching : LTC3864 and NCV8852. There are probably more.
When the input is 12V, it will efficiently drop the voltage down, without heatsinking required. When the input is 5V, it will maintain the PFET on, thus having only the sense resistor, the PFET, and the inductor that will drop some voltage from the DC jack to the powered circuit. You can manage to have a total resistance for this (Rsense+PFET+inductor) in the range of 150mOhm, which would drop 60mV with a 400mA current. This is certainly acceptable.
LTC3864 can be simulated with LTSpice, by the way.
And you shouldn't worry much about the Wifi and switching regulator interference. Keep them as far as possible on the board, and follow closely the recommendations for the layout of the regulator (there are design references for both chips I mentioned), and you'll be OK.