Tool/software:
Hi Folks,
Currently working on a power supply design that I could use a bit of advice on...
The device in question will be hard-wired to power in the +12 VDC to +24 VDC range. We would like the device to operate for up to 6 hours or so on battery power if main power fails. The device requires +5 VDC at up to 5A (although typically less). As well as a higher voltage in the +9 VDC to +30 VDC range at about 100 mA for an alarm. And, finally, ultra-low noise +3.3 VDC and +1.8 VDC supplies for running a couple high-performance TI ADC and associated analog sensing parts. The device will be utilizing a Raspberry Pi CM5 that can attach to the I2C bus. But ideally I would like the entire power supply chain to operate out-of-the-box without the involvement of the CM5. Other than to be notified when running off battery power to allow it to monitor the battery voltage and shutdown cleanly before completely losing power.
Given these general guidelines, I've been looking at using 4 x 18650 Li-Ion batteries along with a BQ25798. I've read through the BQ25798 data sheet and a bunch of the forum posts. But I'm still getting a bit hung up on the choice between using PMID or SYS to power things.
Q1: Am I correct in understanding that the voltage at PMID will either be VBUS when power is available - in my case whatever the hard-wired power is providing - and it will be the VOTG register setting (+5 VDC by default) if I enable backup power supply mode and switch-over occurs?
Q2: Am I also correct in understanding that the voltage at SYS will vary between VSYSMIN and the voltage of the fully-charged batteries?
Since my hard-wired input voltage is variable it would seem like I'll need an extra buck converter regardless of whether I use PMID or SYS to power the system. Consequently, I'm thinking that it makes more sense for me to run off SYS instead. Probably with a 4s Li-Ion battery configuration. That gives me a SYS that varies from a VSYSMIN of +12 VDC to VREG of +16.8 VDC. I can use that to directly power the alarm since it doesn't mind the variable voltage. Maybe use something like a TLVM13660 buck power module to produce the +5 VDC 5A supply from SYS. And, finally, a pair of TPS7A4700 LDO to produce my +3.3 VDC and +1.8 VDC from the +5 VDC.
Q3: Does that seem like a generally reasonable idea?
Q4: Am I correct in thinking that I can run in "VBUS Input Only" mode in this case? As described in section 9.3.5.2 of the data sheet?
Q5: The CM5 should be able to monitor the ~INT pin which, combined with I2C queries, should allow me to determine when "Good input source removed" occurs. And to monitor the battery voltage. Correct?
Q6: It doesn't seem like the CM5 should need to do anything else other than what is outlined in Q5. Also correct?
And, finally:
Q7: Given that I wouldn't be using the OTG/UPS functionality, or the dual inputs, or the solar cell charging capability, is there a cheaper BQ part that would get the job done as well?
Thanks in advance for your assistance!
Best regards,
William