KL25Z Supply Current Relationship with VDD

[alexdean]

I've done some measurements on the KL25Z Freedom board and found that the MCU current doesn't drop with the supply voltage the way I would expect. Does the MCU use an internal voltage regulator or some other mechanism before powering its logic?

I power the board's P3V3 power rail directly with a variable power supply. I've cut the shorting trace on the back of J20, allowing diode D12 to isolate the linear voltage regulator U1. I measure the KL25Z MCU supply current by reading the voltage across J4 (with shorting resistor R73 removed) and dividing by 10 ohms. Here are my measurements.

Voltage on P3V3	KL25Z Current
3.3 V	10.88 mA
3.0	10.71
2.7	10.71
2.4	10.58
2.0	10.43

Thanks for any help!

Regards,

Alex

[Paul_Tian]

Hi, Alex

Yes, it should be as your testing result. When KL25 works above 1.71V, consume current depends on operation module. So as you find, the current will not heavily drop even your supply voltage is drop.

Hope my reply can help you.

Best Regards

Paul

[alexdean]

Thanks, Paul. Could you explain why the current doesn't change as the voltage changes above 1.71 V? I would like to understand the reason why - I can't think of an explanation and am quite curious, since this really affects the power/energy conservation options for a system designer.

Thanks for any help!

Alex

[eduardo_viramon · Accepted solution]

Alex, the device does in fact have a regulator, the core runs at 1.2 volts, this is what mostly accounts for very similar current across Vdd range.

You are probably not using a lot of MCU pins in your tests. Switching outputs at different Vdd values would show a more linear current drop.

I should also point out that the current is in fact dropping a little bit, with the voltage dropping, the overall power is lower.

[alexdean]

Thanks, Eduardo. Now it makes more sense. I'll have to evaluate the impact of I/O activity too.