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Re: Voltage sensing diode

 

I think this is an interesting design discussion, because it can have a long term impact on what we do...
I have not had time to research further, but the point about what it powers is relevant
In fact, it power several items, including the RTC, clock crystal, and backup registers. I don't see the NanoVNA using those yet. Not sure about that, though. If VBAT is not powered, as on my device, then they are not in use.
The voltage divider comment doesn't imply that ALL circuits downstream of the divider are supplied with VBAT/2. They /might/ be, but it may also be true that VBAT/2 is sent to the ADC, and VBAT goes straight to RTC, et al.

Is it possible on this device to lower the battery voltage, or is that set by the battery pack?
The power going to VBAT only powers the RTC et al. Battery power for the kit goes through the IP5303. So it seems that we are free to discuss this indepent of device power.

I don't have a suggestion...but it seems that there is much better info on the table now that can lead to better decisions. One of the obvious ones is just to install a diode with as small a drop as possible, and make the correction available to users. I lean toward a 3.8V <5% Zener or there abouts, but I understand that is more complicated.

Also, aside from gaining a better understanding of this device and its design, I keep in mind that this is really just a rough indicator of battery state...not sure that we need much more than estimates of .25 ,5 .75 1.0 Full charge.

~R~

On 2019-10-31 13:46:+0000, you wrote:

The Vbat does not "power" anything it just goes into a divider for monitoring
Actually, it DOES power the RTC in addition to being monitored.? What would Vbat-Vss give you? ...? since Vss is Gnd and at 0V and Vbat-Vss is 4V max as specified, you need the diode (or two) or zener so you can measure a 4.2V battery.
What I don't think is necessary is 3 decimal points for the voltage. 1 decimal is sufficient IMO.
...Larry

On Thursday, October 31, 2019, 9:31:29 a.m. GMT-4, Mike_nano <biounit.mike@...> wrote:

I know what it says elsewhere in the datasheet for maximums, but I think it's not that simple for Vbat. Snippet from datasheet -

3.10.3 VBAT battery voltage monitoring

This embedded hardware feature allows the application to measure the VBAT battery voltage using the internal ADC channel ADC_IN18. As the VBAT voltage may be higher than VDDA, and thus outside the ADC input range, the VBAT pin is internally connected to a bridge divider by 2. As a consequence, the converted digital value is half the VBAT voltage.

I think it's not going to hurt the chip because of this -

the VBAT pin is internally connected to a bridge divider by 2. As a consequence, the converted digital value is half the VBAT voltage.

The Vbat does not "power" anything it just goes into a divider for monitoring. I think it's just measuring half the external Vbat and then multiplying it back to the actual value, so it can handle higher voltages (makes sense). In any case, I've had my diode (the SD103AWS part) in two units unit for several weeks and had the things on for hours at a time and it still works the same. I do not think it's a problem. :-)
--
On the banks of the Piscataqua
Rich NE1EE
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