Thingy 52 - Battery Voltage monitoring

[Sai Ram]

Hi,

I am trying to understand the design of the battery monitoring circuit of the Thingy52. I have few quiries:

1) The source resistor of the resistor divider network  is 1.5 M-ohm, but as described in the blog, source resistance can't be more than 800 k-ohms for acquisition time of 40 us. The same thing is mentioned in nRF52832_PS_v1.4 under the Acquisition Time sub-section of SAADC section. So is something happening in the software to rectify this?

2) What is the default minimum battery voltage level you are considering? Where to look in the code for this to change?

Thanks in advance!

[haakonsh]

We need more context here, what circuit and transistors are you talking about?

[Mishka]

The Thingy User Guide has the following one:

AFAIU with two FETs the VLI-Ion will be totally isolated. But why not use just one N-FET with BAT_MON_EN high for measure?

[Dmitry]

Hi,

output resistance of divider network is calculated as (R20*R21) / (R20+R21),  that is ~160k, much lower than 800k.

Mishka said:

But why not use just one N-FET with BAT_MON_EN high for measure?

Do you mean to break ground path? You would get permanent current leakage when the chip is not powered.

[Mishka]

Not quite, I'm thinking about something like that:

Dmitry said:

You would get permanent current leakage when the chip is not powered.

Via the BATTERY net, right?

[Dmitry]

Mishka said:

Not quite, I'm thinking about something like that

Hmm, it will work while VDD-Vadc exceeds VGS of Q1... I believe, a reference design, as well as SDK examples, should be quite clear and not over-optimized - you can always do an optimization in your schematic.