Stripe subscription with auto-pay top-up recharges

Hello fellas, I'm stuck with something and can't get my head around it. Can you guys please help me with this???

Here's the summary –

At my company we are planning for a subscription based model where a user can buy a subscription plan in which he/she gets x number of credits. When those credits end before the renewal date we would want an auto-pay system where we charge user y amount for specified number of credits.

Any idea how we can use stripe for such an use case? Your help will be much appreciated.

Thanks again!!