Need ideas for a ice melee cantrip

[redwizard007]

I liked where CTurbo was going with helping others on the team, so here is my thought (copied much of CTurbo's):

Frozen Blade

Evocation Cantrip

Casting Time: 1 Action
Range: 5 Feet
Components: V, M (a weapon)
Duration: 1 Round

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.

On a hit, the target suffers the attack's normal effects, and it becomes sheathed in cold energy until the start of your next turn, during which the target's AC is lowered by your proficiency bonus until that creature is hit by an attack that targets its AC.

At Higher Levels. At 5th level, the melee attack deals an extra 1d8 cold damage to the target. The cold damage increases an extra 1d8 at 11th and 17th level.

This seems fine at low levels, but overpowered as hell at higher levels. I'd cap the AC penalty at -2 and reduce damage to 1d4 scaling.