it's really of marginal utility, though. It SOUNDS powerful, but you're really only safe from kobolds and other ~ 1d6 damage rolls. Once you're past lvl 2 or 3, you'll very rarely fight things that do under 2d6 or that don't have +3 or +4 str mods. After that, Heavy Armor Master might on average keep you alive for one more hit during a combat. Compare that to stat increases, where +2 str will give you +1 damage on every strike and improve skills. +2 CON would give you +1 hp/lvl up to your current level, and +1 hp every time you level up after that which will eventually equal or surpass the average effective HP increase you get from Heavy Armor Master. Earlier access to Polearm Master will give you a bonus attack each turn, or Sentinel could help you control the battlefield and help keep your allies safer by controlling choke points. Heck, even Observant would be fantastic, making it so you are extremely unlikely to be surprised or ambushed, or miss critical environmental clues and details.
If you're still super interested in Heavy Armor Master, I might sugguest you ask your DM if he will instead allow it to give DR equal to your Proficiency bonus. At least then it'll retain some value/utility as you level up.