Looking to efficiently reduce 22v 2A DC power to small usb power

[soaringchicken]

I'm building a custom portable sound system that runs on 6 li-ion batteries totalling in a 22v 2A dc power supply that goes straight into the amp. But I wanted to add bluetooth capability, and most bluetooth circuits run on usb power. I'm curious to know how people might suggest I draw off this small usb power (5V .7A ?) from the bigger load in an efficient way that wont burn out my little usb circuit whenever the amp takes big draws.

Thanks!

 

[alexan_e]

You can use a switching regulator like LM2596 https://www.ti.com/product/lm2596

You can get a module from ebay for about a dollar