Difference between Asset Impairment and Unplanned depreciation

Hi Guys,

Need some clarity on the following.

What is the main difference b/w unplanned depreciation and Impairment? they both reduce NBV of an asset but in Impairment,  effect on depreciation will be in the next period where as in unplanned depreciation it is available in same month.

Can I use unplanned depreciation instead of Impairment for an asset, Are there things that I need to consider before I choose this route?

regards