Disable Button While Textbox Is Active

I have a userform with a textbox and a command button. I have used data validations on the textbox. Clicking the command button saves the data in the worksheet. It works well if the user hits tab, enter or down arrow key after entering the data. The problem is, the textbox validations fail to get executed if the focus stays on the textbox and command button is pressed. I want the command button to be disabled until the user has moved the focus away from the textbox -there by executing the validations!