I'm currently making a little electronic board on my own. On it, a microcontroller: an ATmega328p running with 5V. To communicate with a computer, I've planned to use an XBee, but for now I'm using simply an adapter TTL <-> USB based on an FTDI FT232RL chip (here).
The problem is, no matter if the power is plugged to my board or not, as soon as I plug this serial adapter to it, a little LED used to display whether my own board is alimented or not lights. After verification, the TX pin of the adapter is guilty: from it flow almost 11 mA.
The problem doesn't seem to come from my home-made electric board, since with an arduino, the same issue occurs. Moreover, the programmer (an USBasp clone) heats a bit, and I have the feeling my serial adapter is responsible...
How can I fix this ? Is this normal ? Couldn't it damage my microcontroller ?