Did you explore where negative values occour in your resulting image? generally negative values will be associated with borders (of entire landsat scenes), very small original DN values, presence of NA values.

Bellow some links to discussion on this topic.

http://surfaceheat.sites.yale.edu/sites/default/files/Negative%20Reflectance%20Values.pdf

http://gis.stackexchange.com/questions/166072/landsat8-dn-to-toa-reflectance-minus-values

http://gis.stackexchange.com/questions/193025/negative-reflectance-values-after-topographic-corrections-cosine-correction-and

http://grasswiki.osgeo.org/wiki/Atmospheric_correction

Hi Alexandre.  Try to check this document.  This might help explain the negative TOA values.

Cheers!

http://surfaceheat.sites.yale.edu/sites/default/files/Negative%20Reflectance%20Values.pdf

Thank you very much Eric, Ariadne and Paulo. This papers and tips help me!!

Best wishes,