Topping the ball is a common fault amongst golfers that can have a variety of causes. You may have too much tension in your hands and arms, causing the arms to be bent at impact thus the club head cannot reach the ball. Your weight may shift to your heels during the downswing, moving your swing center too far from the ball. You may be swinging up through the ball in an attempt to lift the ball into the air and thus presenting the leading edge of the club to the ball rather than the clubface.
Possibly the most common fault though is a loss of spine angle during the swing and it’s this which is often misinterpreted as lifting your head. A loss of spine angle refers to you raising your upper body so that the angle that you create between upper body and hips at address is changed during the swing. This results in the swing center, center of your chest, rising up and thus away from the ball, making it difficult to get the club back to the ball consistently.
Too often we see players who have taken keeping their heads down to amazing levels. So much so that the ball has flown a good 50 yards and they are still staring at the divot they have made on the ground. This places excessive pressure on the back, does not allow the body to turn freely through the shot and decreases the transfer of energy to the ball. There are some players on the Professional Tours who never see the club head hit the ball and are still extremely successful players. Annika Sorenstam, David Duval and Robert Allenby are all in this category with their head rotated down the target line when the club strikes the ball.
If you want to stop topping the ball, check that you are maintaining your spine angle throughout the swing. A great drill to help with this is to hit balls while resting against a chair. Take your address position with a 7 iron. Then place the back of a chair up against your rear, being sure not to rest your weight on the chair. It is important that your weight stays on the balls of your feet. Take some practice swings and be sure to keep you rear turning along the chair throughout the swing only allowing you to leave the chair to complete your finish position.
As you do this drill you may feel that your chest is staying more over the balls of your feet than you are used to feeling. But this is the move you need if you want to keep your swing as simple and thus consistent as possible. After doing this drill several times, place a ball on a tee and hit a few half shots, ensuring that you maintain your spine angle as you did during the practice swings.