Transmission Burst Current Consumption - Page 1
Forum section: electronics / projects

author: trentt
author_rank: Contributor
timestamp: June 12, 2019, 08:57:13 am
content: Hi All,Long time lurker, first time poster. I'm hoping someone will be able to assist with a issue I am having. I'm currently working on a 4G LTE GPS Device and I'm having one pervasive issue.The device works as per normal when stationary but when moving, the module resets occasionally.As per the documentation, I provide a total of 1000uF capacitors but the issue persists.I now have a total of around 3.2mF and while the resetting has reduced by about 70%, I still have resets every so often.Typically the entire board is drawing under 150mA but these current surges are seemingly difficult to wrestle with.Is the trick just adding more capacitance? What should I look at doing?EDITI've also tried inserting power directly into the board, bypassing the LDO to verify that the regulator isn't the issue, but it seems to make little difference.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 12, 2019, 09:53:51 am
content: What battery?3.0V lower limit is quite high - most li-ion cells are OK with a lower cutoff, such as 2.5V. If you have a small or crappy battery with a lot of internal resistance, it may drop below 3.0 during higher current pulses, especially below 50% state-of-charge, which is OK for the battery, but not OK for your module.Note that 1mF of capacitance drops its terminal voltage by 1V when 1A is consumed for 1 ms. So when they specify 1mF is enough to deal with the 2A pulses while keeping drop below 300mV, this means their 2A pulse is supposedly 150 microseconds long.Even if you are able to bypass the largest and shortest (supposedly 2A for 150 us) peak with 1000uF of capacitance, it's likely there are longer bursts of something less than 2A, but possibly still more than your battery can supply. Such as the magical 0.6A "peak" number they give. Do they give the timescale for this 0.6A?Since electrolytic capacitors have energy density at least 4-5 orders of magnitude less than batteries, at some longer time scale (typically > a few milliseconds), it's just cheaper and smaller to use a larger (or better) battery instead of trying to bypass peak currents using capacitors.To rephrase, MLCCs are used for bursts in microsecond range, elcaps in millisecond range, after that, the battery must take over.A typical high-quality 18650 li-ion cell, for example, has its DC resistance at around 40mOhm, but if you combine all negative conditions (cold weather +100%, aged cell +100%, at low state-of-charge +100%), you may be near to 400mOhm. If we assume the "longer" peak current is 1A, the design is already marginal. The good news is, the 1kHz AC impedance of the batteries tends to be somewhat better (internally, the battery acts like an electrolytic capacitor), so this effect may be able to bridge some gap.TLDR: try a larger battery, combined with your massive capacitance to see if the problem goes away.

author: trentt
author_rank: Contributor
timestamp: June 12, 2019, 10:24:49 am
content: Hi Siwastaja,Thanks for your help.I've been trying to get away with 3 x AA as per clients requirements. The 2A burst is only on GSM frequency, whereas we are strictly using LTE which has the .6A burst.If we assume that we have a burst of 577us at .6A and the formula isF = Current I x time t / voltage dV, than the required capacitance should be 1731uF. Is this correct? I have more than double the capacitance in that case.The transmits burst is only given for GSM, but I assume it is similar in timescale for LTE ( This might be a bad assumption? ). If we are required to use this power source, is the only path forward more capacitance?I'll pick up a couple 18650s from Altronics on the weekend but ideally I'd like to stick with AA, even if it means more capacitors.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 12, 2019, 10:45:33 am
content: Hi,It's almost impossible to do properly with 3xAA. You need to tell this to your client.Their internal resistance is just too high. In addition, voltage is on the low side - if you design to a 1V/cell cutoff, so 3.0V for 3 cells, no voltage sag margin is available at all. If you have to do it with AA cells, you'd be better off with 4xAA and a buck converter capable of 100% duty cycle for low dropout (or a very low-drop-out LDO with 2A current rating). Still, it's very marginal and crappy.For 3xAA, practically you'd have to buy the best cells available, keep them warm, would only be able to use about 50% of their energy max, and still need quite massive capacitance.The datasheet excerpt you posted shows2Afor 577us, so given your formula, 3800uF is required. Note that extra drop comes from the ESR of the capacitors. Assuming 100mOhm for the 3800uF elcap, extra 0.2V sag is created at 2A, so you only have dV=100mV left in your formula - recalculate for capacitance, and you suddenly need 11500uF. Now such a massive cap has much less ESR, which again gives you more dV to play with, so if you iterate the formula once more, you are likely to hit somewhere between 5000uF and 10000uF needed. Note that the ESR of the elcap goes up in low temperature, so you need to oversize the capacitors.For an AA cell, the internal resistance is somewhere around 200mOhm for a FRESH battery, at room temp (http://data.energizer.com/pdfs/batteryir.pdf). For a partially used battery, at colder weather, it'smuchworse. Even assuming 200mOhm, 3 in series would total 600mOhm, which sags by 1.2V under the 2A load. So it's basically completely up to the capacitors alone to supply the peaks.Now if you look at the price and size of 10000uF, 6.3V, you'll see why they use batteries to do this instead. And, at the same time, you'll see why all the modern LTE gadgets and phones have relatively large li-ion batteries in them - it's not just to play Angry Birds.If it has to be with disposable, single use primary Alkaline cells, consider using the large D cells, and 4 in series to give headroom for voltage sagging. It's gonna suck in size and weight, and expense of use.

author: jbb
author_rank: Super Contributor
timestamp: June 12, 2019, 08:16:16 pm
content: Quote from: trentt on June 12, 2019, 08:57:13 amWell, making a tough power supply is certainly important, but not necessarily what's causing resets while moving. Do you have good status messages and logging on the module? If your host micro controller resetting as well?On the current pulse front, I think some companies cell supercapacitors intended for this application (e.g. 5V 0.22F). That might help but will cost more than 1x AA cell!What AA cells will be fitted? Will they be customer replaceable? If so, I suggest you go buy some 'supermarket' AA cells and arrange a demonstration. Set up a programmable load (or function generator, MOSFET and dummy resistor if all else fails) and see how long it can run before dipping below 3V (hint, set up an oscilloscope withNormal- not Auto - triggering and single shot mode to capture it). Then compare to 4x AA cells + LDO as Siwastaja suggests.

author: The Electrician
author_rank: Frequent Contributor
timestamp: June 14, 2019, 05:13:31 am
content: Given that the minimum voltage for proper operation is 3.0 V, what is the maximum allowable supply voltage?Wouldn't the capacitor you're going to need to use be about the same size as another AA cell, so that 4 cells plus a small cap would occupy about the same volume as 3 cells plus a large cap?