12VDC power management & lead acid battery management/charging

Thread Starter

spmaddox

Joined Nov 29, 2018
46
Hi everyone,

I'm working on a design for which I've completed all aspects except for the power input and power rails which i saved for last because it seemed logical to do the design starting with all of the things I am requiring the product to do and let these define the power requirements etc.

My design includes two power sources with the primary being a 12V/5A wall transformer and the secondary being a 12V/5aH (or higher aH) lead acid battery. My requirements here are;

  1. switch to secondary from primary when primary power is not present (ie: power goes out)
  2. switch to primary from secondary when primary power is present
  3. charge the battery from primary
  4. use battery, via relays, to drive an external device when required (keeps the load off the system power)

I'm not a HW engineer but I have designed and built working prototypes thanks to much of what i've learned here. The design includes op-amps, comparators, digipots, ADCs, a SOC, Ethernet and USB but after several days on the power I'm stuck and seeking some guidance.

The design includes 4 power rails (12V/2A, 5V/.5A, 3V3/3A and 1V8V/.5A) that need to be sequenced and the 3V3 rail actually has what I suppose amounts to 2 rails because I need one portion of the 3V3 to come up before the other due to a sequencing requirement of a chip in the design.

The attached crude illustration shows the concept and the sequencing. The rest of the design i've already done in CircuitStudio and I'm working with a PCB design engineer whose feeding my schematics into his tools to produce the circuit board design.

I've looked at the LTC4162-S but I cannot determine if it will output 12V/5A which I can feed into switching voltage regulators and I'm not positive if this is the "best" solution or what my other options might be. I've searched through the forum and not found something aligned with my ask, if i've missed something please point me to it and i'll dig into it.

thx - seanPower Source and Power Rails.jpg
 

Thread Starter

spmaddox

Joined Nov 29, 2018
46

Thread Starter

spmaddox

Joined Nov 29, 2018
46
I've solved this, updating for the benefit of others if they should encounter the same question.

Both the power path and the battery charge currents of the LTC4162 are adjustable, they are not fixed at 3.2A. The LTC4162 utilizes two external current sense resistors (referred to in the documentation as Rsnsi and Rsnsb) to determine the maximum charges. Rsnsi is the current sense resistor for the power path out, it is located between CLP and CLN pins. Rsnsb is the current sense resistor for the charge out, it is located between CSP and CSN pins.

For a current limit of 5A a 6.4 mOhm current sense resistor needs to be used, for a current limit of 3.2A a 10 mOhm current sense resistor needs to be used.

Because the LTC4162 utilizes external FETs you need to ensure the FETs you use will support the current you are setting with the current sense resistors.

thx - sean
 
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