standby mode resets the psp?

[frugalman]

scummvm version: 0.11.1
psp fw: 3.90 M33

I'm running some games in scummvm such as monkey island and i put it on standby mode by pushing the power switch up.

if i resume the psp fairly quickly like 1 minute later, it's fine. but if i wait like 1 hour later and resume the psp, the system shuts off and I have to restart the psp again.

any ideas?