Improving battery life: Use a capacitor ?

RE: Improving battery life: Use a capacitor ?

ShannonPahl — Fri, 20 Apr 2018 18:30:11 GMT

Thanks for the reply, I'll try those enhancements and retest.

RE: Improving battery life: Use a capacitor ?

AmbystomaLabs — Thu, 19 Apr 2018 14:02:11 GMT

Well the physics don't lie and this is how you reduce power supply ripple.

There are many things that can go wrong with your setup. The PPK uses analog switches (ie, FETs) for range control and with switches, your scope settings which may be good or bad, lots of local grounds, and two types of power supplies a lot can go wrong.

Also, your 1210 part is a poor choice for this design. As I said earlier we use an 0805 47uF along with distributed 0.1uF 0402's and 2.2uF 0402's. Our 0805 parts have an SRF of over 1MHz and the 2.2uF's have an SRF of over 5MHz. Your 1210 however has an SRF of only 575kHz. That's pretty low given the power amplifier in the nRF will come on in about 200nSec. I encourage you to do the spice simulation but that is a rather slow part for a fast turn on device. Plus since the modulation of BLE is 1MHz or 2MHz even the DC part of the modulation that shows up in the power supply will be well outside the range of your 1210 cap.

Since what you really care about is reducing the ripple of a button cell you should test that and only that. Get the PPK out of the equation and just test your DUT on a button cell then measure the voltage at the cell while under test. With only one ground and one power source the results will be more clear and you will see the lowering of the ripple of the button cell due to its internal losses as you put more fast (ie, over 1MHz) capacitance on the system.

I should add, if you really want to smooth out the ripple put a big inductor between the caps and battery. This way you end up with a second order filter. Again, I encourage you to simulate in spice, or equivalent, to see the results. You will have to do the calculations to come up with the appropriate uH's for the inductor and will need to balance ripple reduction with power loss from the DCR of the inductor.

RE: Improving battery life: Use a capacitor ?

ShannonPahl — Wed, 18 Apr 2018 00:39:34 GMT

I've put it after the wires that come out of the 'External DUT' connectors of the ppk and before connecting to the external board. I have also soldered it onto the board like it would be for real, and both give very similar results.

RE: Improving battery life: Use a capacitor ?

AmbystomaLabs — Tue, 17 Apr 2018 21:13:12 GMT

Where did you put the cap on the PPK?

You have to remember the PPK has a series resistance for current measurement. You have to put the cap after the resistance just ahead of the nRF.

RE: Improving battery life: Use a capacitor ?

ShannonPahl — Tue, 17 Apr 2018 20:57:58 GMT

Thanks for the links, I have been through those before. I have updated the question with some more details.

RE: Improving battery life: Use a capacitor ?

ShannonPahl — Tue, 17 Apr 2018 20:56:37 GMT

Thanks, I have updated the question with more details. I am using the ppk to do measurements, and with the ppk power selected to (reg) vs external (cr2450), there is some difference. I have seen the theory but the actual measured data is what I'm trying to use to dictate an approach.

RE: Improving battery life: Use a capacitor ?

hmolesworth — Tue, 17 Apr 2018 14:45:02 GMT

Have a look at this white paper, which dates back to when TI were trying to market their high-consumption BLE parts as coin-cell compatible; it's very useful: Coin cells and peak current draw swra349

RE: Improving battery life: Use a capacitor ?

AmbystomaLabs — Tue, 17 Apr 2018 13:42:31 GMT

You are correct in assuming reducing ripple caused by the coin cell will improve the battery life.

Your chart shows the input current to the circuit and not the battery voltage ripple. It is better to look at the battery voltage ripple but you will see a similar smoothing of the input current spikes.

The battery voltage ripple is caused by internal losses in the coin cell. If you look at any CR series battery spec you will see as the load resistance goes down the effective battery capacity goes down.

Capacitance smooths out the ripple (as it does on any power supply) by providing temporary power storage.

The problem you are seeing is caused by using the wrong cap. Caps have internal parasitic inductance and resistance. If either of these two are high enough the effect of the cap in the circuit is naught for high frequency short duration signals.

You should be using low esr chip caps and never tantalum or electrolytic. 0805 chip caps can be sourced with low internal inductance (ie, high frequency SRF) at 47uF. One should easily solve your ripple.

I would encourage you to take the time to calculate the change in ripple based on the load and a caps exponential model. You will appreciate the math behind how it works. The theory behind it can be found in any first year engineering textbook.

RE: Improving battery life: Use a capacitor ?

butch — Tue, 17 Apr 2018 13:19:11 GMT

Just thinking out loud, maybe not helpful... At first I thought it could be a sampling artifact: the spike was always there its just that without the larger capacitor, your scope was not sampling frequently enough to catch it. Then I thought: but the integral of the first advertisement current (which is power) looks larger in blue than in orange, so the larger capacitor really has changed the power used? Which is so curious that I think: you must have changed something else, because there is no other explanation?

RE: Improving battery life: Use a capacitor ?

Bjørn Kvaale — Tue, 17 Apr 2018 12:47:08 GMT

What kind of a Nordic device are you currently using? Is it a nrf52832? Are you using an SDK example?

I am unsure why there is a large spike at the start of the advertising event when you use a 100 uF capacitor. Normally, it is a good idea to check out the reference layout that we provide for each of our chips & use the capacitors that are provided in the reference layout (if it is a custom board).

It could also be a good idea to take a look at this devzone post. Even though it applies to the nrf51, a lot of it is still similar for the nrf52 device. These two links might also be useful (link 1, link 2).