I was looking into undervolting my laptop CPU. I could only find guides on phc that are said to be outdated. Is there a tool for that in the new version? Is it possible with TLP?
Yes, with TLP you can scale you CPU frequency (Look at this post: TLP don't change CPU frequency). You can specify to reduce the CPU frequency.
More info here: http://linrunner.de/en/tlp/docs/tlp-configuration.html
However I found another article about CPU limiting
Although, it isn't about limit CPU frequency, you could reduce your CPU load/consumption with an application like cpulimit (https://github.com/opsengine/cpulimit)
Hope this helps.
You need a PHC module to start undevolting, see this forum: http://www.linux-phc.org/forum/index.php.
However, this is not compatible with newer intel processors that use intel p-states, instead of the old acpi-cpufreq (google " Undervolting Ivy bridge core i7 laptop with Ubuntu 16.04 - TLP & PHC not suitable"). It appears that we might never be able to undervolt in newer PCs using linux unless somebody with coding experience steps up to the task....
Unless you are using an older laptop, then you should find your answer in the first link ;) and a compatible PHC module here: http://www.linux-phc.org/forum/viewtopic.php?f=7&t=267. The tutorial suggested by FabricioFCarv should be fine (have not tested myself).