Phone Fastboot problem in XPERIA Sola phone

Fastboot problem in XPERIA Sola phone

hello ..
i have sony xperia sola mobile and i can not open fastboot screen.
i press Volume up button at the same time as i connect the device to the computer with a USB-cable and see blue led light but at 10sec go to charging screen .. please tell me how to get fastboot screen.

u know my device is unlocked and tern on debugging

please help