Voltage range of power supply

AchimKraus

According Specifications and Requirements - Power supply

3.3V Buck with < 1uA of quiesent current. 750 mA ouput max
Operating range 3.0 to 6.5V

In my experience, if the supply voltage goes under 3.3V, the buck switches off. I guess, that’s a “left over” from an older design. With that, does the 6.5V applies to the feather in v5 ?

(I currently plan to use 4× 1.5V batteries in order to have about 5V (+/- 1V direct from battery) for some sensors. The sensor I2C bus still supports the 3.3V I/O so it’s only about the power supply of that sensor.)

jaredwolff

AchimKraus In my experience, if the supply voltage goes under 3.3V, the buck switches off. I guess, that’s a “left over” from an older design. With that, does the 6.5V applies to the feather in v5 ?

If there are dips in the power supply that go below the UVLO of the switching supply then yes, you may see some brown outs.

It is though capable of 100% duty cycle. So anything below 3.3V is pass through.

AchimKraus With that, does the 6.5V applies to the feather in v5 ?

Yup, the TPS62840 supports <= 6.5V

AchimKraus

Using a PPK II in source mode connected to the JP2 plug, my result is (still):

Works with 3305 mV.
Doesn’t work with 3223 mV.
Below the 3.3V the modem frequently restarts.

jaredwolff

Ahh, could be the power rail watchdog in the nRF91 itself.

AchimKraus

nRF91 itself.

I’ve retested it with the PPK II and a nRF9160-DK v1.1.0:
Results: works with 3200mV and 3100mV.

jaredwolff

Hmm interesting. So maybe its power supply related?

AchimKraus

The part of my question with upper voltage limit was the important one for me :-). The other part, with to lower limit was more for completeness because I observed that. I think, that requires more investigation, but for now I’m too busy with other stuff.

AchimKraus

So anything below 3.3V is pass through.

I didn’t find that in the “TPS62840 - SLVSEC6D – JUNE 2019 – REVISED MARCH 2020”. Do you have the reference at hand? Looking at the scheme, I consider, that even if the TPS62840 switches SW permanently on, the inductor L2 will prevent the nRF9160 from working below 3.3V BAT.

jaredwolff

I believe it was in the data sheet in the summary paragraph. It’s commonly advertised as capable of 100% duty cycle.

AchimKraus

Great! Thanks for the answer. It’s Section 8.4.3 100% Mode Operation, page 15:

V IN min = V OUT + I OUT × (R DS(on) max + R L )
where

I OUT = output current
R DS(on) max = maximum P-channel switch R DS(on)
R L = DC resistance of the inductor

Considering I OUT of 150mA and R DS(on) of 430mOhm (high side) gives a drop of 0,06V. I measure 0.6 Ohm at L, so that results in 0.15 * (0.43+0.6) = 0.15V.
I guess, that’s why the MCU starts (lower I), but once the Modem switches on, this gap and the delay caused by L will shut it down again.

AchimKraus

Just as latest update:
Using olmboard-tps62840 and a nRF9160 MIKROE works with an battery input of 3.1V.

jaredwolff

And the nRF9160 Feather does not @AchimKraus?

AchimKraus

At least in my experiment back in November (see above), the feather stops working pretty short below 3.3V.