Site icon Pallav Aggarwal

IQ of Power Supply in Low power Embedded System Design

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.

Three power supply modules used in the experiment

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

TPS62840 (60nA)

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.

MAX38640AELT+T (330nA)

Test Results:

Quiescent Current

no load output voltage1.845V   
no load IQ @ Vin0.66uA @ 4.2V0.53uA @3.6V0.50uA @ 3.3V0.45uA @ 2.7V

Quite close to what datasheet claims.

Efficiency:

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.

 InputOutput
V4.1591.836
mA0.91.83
mW3.7433.359
efficiency89.76 
V4.1751.838
uA10.0618
uW42.00033.084
efficiency  78.77 

TLV62568DBVT (35uA)

Test Results:

Quiescent Current

no load output voltage1.803V   
no load IQ @ Vin124uA @ 2.7V143uA @ 3.3V153uA @3.6V173uA @ 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.

I hope you found this post useful.

Do share your feedback in the comment section.


You can read my other interesting blogs & articles on embedded system design here.

I help embedded engineers and start-ups build reliable and successful embedded products. If you need any help please let me know here.

Happy Learning to you!


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