How to convert Float to Varchar in SQL

I am doing this

declare @num float = 7708369000

select  @num as [float], 
    convert(varchar, @num) as [varchar]

it gives me this

float                  varchar
---------------------- ------------------------------
7708369000             7.70837e+009

But I want this

float                  varchar
---------------------- ------------------------------
7708369000             7708369000

Please help.