I think it should always be above 0 as a philosophy. You are asking people to store money there. The convertible portion of the note can fluctuate and might lead to higher than a$1 value but the coupon should alsoways be above 0. Preferably it should be high enough that when the convert is valued at 0( in a selloff aka a busted convert) sbd stays above $1 just on the power of interest rates.