EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: r4r on April 30, 2019, 08:29:17 am
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Hi!
For learning purposes, I'm about to build simple logger based on ATmega328, DS3231 RTC and microSD card, powered by 18650. With first two components there is no probles - wide range of Vin, I can power them directly from battarie. But microSD card requires more strict range ~2.8-3.4V.
To solve this problem, I've found several solutions (from best to worst IMHO):
1. TPS6303x High Efficiency Single Inductor Buck-Boost Converter. Unfortunatly, it has very small package 10pin VSON 2.5x2.5mm so with my soldering experiense it is impossible to mount on.
2. Module like this https://www.ebay.com/itm/DC-DC-Step-Up-Step-Down-2-5V-15V-to-3-3V-5V-Automatic-Buck-Boost-Module/173618472953?hash=item286c77a7f9:m:mq_DMDKKXIOHiOBqreWM5Mg (https://www.ebay.com/itm/DC-DC-Step-Up-Step-Down-2-5V-15V-to-3-3V-5V-Automatic-Buck-Boost-Module/173618472953?hash=item286c77a7f9:m:mq_DMDKKXIOHiOBqreWM5Mg) 75% efficiency (?)
3. Step down buck converter from 2x18650 in series to 3.3v. Probably №2 with 2x18650 in parallel will be more efficient.
4. Step up to 5V (atmega, RTC) and then step down to 3.3 as part of microSD "module".
Maybe another way? I couldn't find, please recommend the solution. 400-500mA max in peaks when writing to SD (once per 30 min).
Thanks!
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If processing speed is not an issue, you could sue Atmega at 3,3V it will run without problems at 8Mhz. All other things are fine with 3.3V anyway, so stepping up to 5V is not really good thing to do.
As for powering everything: you don't really need buck boost, as at around 3.3V you don't have much capacity left in li-ion cells. For longest runtime I would suggest deep sleep. You could even try disabling card's supply between readings with mosfet. Also that's a lot of current for a card.
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https://www.microchip.com/wwwproducts/en/ATmega328 (https://www.microchip.com/wwwproducts/en/ATmega328)
Datasheet 29.3 says Atmega 328 can work a little bit higher than 10MHz at around 3V.
You may use whole system at around 3V,
also DS3231 is working at 3.3V.
You may just use AMS1117 lineer regulator and
for the SD card write operations
just using a 100uF capacitor across Vdd Vss should be enough for SD card's peak current needs while write operations.
You may use MC34063 Switching regulator also,
it could easily supply 500mA at 3.3V output,
MC34063 old but cheap and well known easy to use, easy to find regulator.
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Atmega and DS3231 directly from the 18650 and the sdcard via an 1A schottky diode also from the 18650 (plus the decoupling caps etc..)??
The max voltage of the 18650 is 4.2V, afaik, 4.2-0.5=3.7V that is something the sdcard may survive.. Or use an 1N4001, 4.2-0.7=3.5V..
Btw atmega328 works fine 16MHz at 3.3V.
PS: you may consider a few_hundred ohms protection resistors for the SPI and CS signals from atmega to the sdcard in the case the Vcc voltages of the atmega and the sdcard differ.
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Make your life easy for the supply side and charging https://www.aliexpress.com/item/ESP32-ESP32S-For-Wemos-For-Raspberry-Pi-18650-Battery-Charge-Shield-Board-V3-Micro-USB-Port/32870411748.html?spm=a2g0s.9042311.0.0.49714c4d1bgPBL (https://www.aliexpress.com/item/ESP32-ESP32S-For-Wemos-For-Raspberry-Pi-18650-Battery-Charge-Shield-Board-V3-Micro-USB-Port/32870411748.html?spm=a2g0s.9042311.0.0.49714c4d1bgPBL)
(https://ae01.alicdn.com/kf/HTB1C_QyJ3aTBuNjSszfq6xgfpXaz.jpg)
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My memory is that cards draw significantly less current in SPI mode than their maximum rated. And older SD cards (up to 2GB) draw considerably less than SDHC. In any case, I don't think you would ever need anywhere near 500 ma. There's one manufacturer, SanDisk I think, that has a 100 ma maximum for all their cards. So I would urge you do some actual measurements with the cards you will be using before assuming that 400-500 ma will be needed.
Anyway, as has been suggested, it seems the best answer may be to run everything at 3.3V. Then you could step down from one 18650, and wouldn't need any level shifters on the SPI lines.
Some cards are supposed to go into idle mode when not being used, which draws little current, but I don't know if that's reliable, or used by all manufacturers. What I've seen is that SDHC cards draw about 30 ma just sitting there, which is no good for your battery, so if only writing twice an hour, I think the best practice is to switch off power to the card between logging events, probably with a mosfet.
And a big capacitor for the writes.
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Thanks for replies!
I think to use TPS73630 then. 75mV Dropout Voltage i.e. to get 3.3v I need roughly 3.4 (at minimum) on 18650. Right?
beanflying, interesting module, will order couple.
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Thanks for replies!
I think to use TPS73630 then. 75mV Dropout Voltage i.e. to get 3.3v I need roughly 3.4 (at minimum) on 18650. Right?
Did you mean TPS73633? Well it looks like either would work for the SD card.
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Did you mean TPS73633? Well it looks like either would work for the SD card.
Oh, sure 3.3v =)
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LDO is good choice here. Switching regulators are often more efficient than LDO but when you have small voltage change and low current draw LDO can be more efficient partly because of lower quiescent current. LDO is also usually smaller, easier and less noisy than switching.
Power loss in switching regulator ~= voltageOut * currentOut * ~(0.05 to 0.3)
Power loss in LDO ~= currentOut * (voltageIn - voltageOut)
Should probably use input values for these equations but I'm guessing you know your outputs better than your inputs so I used outputs. These are not exact equations, they just give an idea of the difference between switching and LDO.