Why do my phone screen goes blank one minute after turning on the phone?

i am having a sony Ericsson c 902 phone.When I turn on the phone the display is ok.After approx 1 min it goes of black.then i hav to restart the phone to see the screen. Menawhile if i am doing some thing like typing sms or anything with the phone ,display is there .if i am turning on the game ,phone will not go blank.If its kept Idle,it goes blank.why its so?