Sorry for my English, thanks for your help but I believe that there is no power manager in LXDE mini.
This is the similar thing in menu > more applications > configurations > Screensaver:
but do not work, with 3 minutes and the blank screen only mode the system shutsdown completely're working or not
and these other options do nothing, do not work:
in short, the laptop does not have screensaver to put the screen in black, or may be suspended after some time of inactivity, and with xfce this is possible.