Have you ever worked on low power embedded system design?
Do you know what is Quiescent current?
I did not pay attention to this term before about 2 years ago when I started working on a project that needed a power backup of 6 months with a single charge of battery.
I will not be able to get into details of what that project was all about but that project taught me a great lesson which I will never forget i.e “when you are working on a low power embedded system design, you need to find every uA consumption and try to reduce it”.
After completion of the project, I wrote an article with lots of details on how one can reduce the power consumption of their embedded product.
What is quiescent current?
Quiescent current of a power supply is the amount of current which it consumes at zero or no load.
Have you ever measured how much your power supply is consuming when there is no load or a very low load?
I would suggest you go back to your lab and test before you read further. It will be fun.
I did a small experiment to measure what different power supplies consume as no-load current and I was blown away with the result so wanted to share it with you so that you can be careful.
I tested a few readily available power supply modules and an LM1117-3.3V LDO circuit on one of my boards.
My test setup was quite simple I used a series resistance to measure the current, using voltage drop across the resistor using my Fluke 179 True RMS meter. I used a precision power supply with 7-15V DC input and set all dc-dc power supply output to 5.0V DC.
Here are my results:
Power Supply using LM2576
The no load current consumption was around 10-11mA. Matches with the datasheet.
Power Supply Using MPS2307
The no load current consumption was around 6-7mA. Doesn’t match with the datasheet, so probably they are using a duplicate or clone dc-dc converter IC 🙂
Power Supply using TPS563201
As the quiescent current was quite low, I had to use a different series resistance in this power supply. The no load current consumption was around 0.47 – 0.5mA which matches with the datasheet.
So, what do you understand, even if your MCU is consuming zero uA, your power supply may be consuming a lot of currents and draining your battery.
Always remember, if you are working on a low power embedded system design where battery back is critical, use a low Iq power supply 🙂 and the power supply has very high efficiency at the current range your circuit will be consuming.
Example of Low Quiescent Current Buck Converters
You can watch this video for the test results I got.
Recently I have designed an ultra low power personal tracking device for a US based client and got the opportunity to test both of these ICs.
I cannot share much details about this board but I will only share details about the buck converter testing I did using this board.
|no load output voltage||1.845V|
|no load IQ @ Vin||0.66uA @ 4.2V||0.53uA @3.6V||0.50uA @ 3.3V||0.45uA @ 2.7V|
Quite close to what datasheet claims.
I have used Nordic Semiconductor’s Power Profiler Kit 2 for the testing there will be some error in the measurement also but it gives very good indication of what we can expect.
|no load output voltage||1.803V|
|no load IQ @ Vin||124uA @ 2.7V||143uA @ 3.3V||153uA @3.6V||173uA @ 4.2V|
Datasheet claims 35uA at no load but the results are way off, I have contacted Texas Instruments to figure out what could be the issue. I have used the recommended application circuit only. I will share once I know more.
TPS78223 LDO (500nA IQ)
I have created a custom board for testing TPS78223 LDO. Enable pin is pulled up by 100K resistor.
- Vin is 4.996V
- Output Voltage is 2.3V
IQ measured using is around 550 – 590nA (Nordic Semiconductor Power Profiler Kit 2)
Which is quite close to what datasheet claims.
TPS62841 DC DC (560nA IQ)
On the same board shown above I have assembled the TPS62840 circuit, as TPS62840 ICs were not available I have ordered TPS62841 and soldered to test the circuit. Here are the results
- Input voltage: 4.99V
- Output Voltage: 1.512V (VSET resistor used is 100K, Enable pin is pulled up by 100K)
- Measured IQ: 180 – 220nA (using Nordic Semiconductors PPK 2)
Datasheet claims ~60nA, so, may be there is a measurement accuracy issue.
I did check again with NanoRanger is I was able to se 55-60nA as IQ. Check the video where I have show.
I hope you found this post useful.
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Happy Learning to you!