I'm assuming the workflow is updating an item.  If this is the case, the modified column will change.  You can't stop that.  However there may be a different solution for your issue.  Can you give some more details around your scenario and why it is that you don't want the modified column to change?

Workflows in general perform item updates in the context of a user an thus will always modify the modified date.

However SharePoint also supports a SytemUpdate-Method for list items which will update items without changing the modified date. System updates are mostly performed in custom event handlers that need to be programmed but you could also use a PowerShell action like to do it.

If possible I would try to go without programming or using of PowerShell. Would it be an option to leave the modified date as it is but have another custom date field that will only be changed from workflows when you do it on your own? This way you get a date that will only change under circumstances you define.

Best regards

Enrico