Primordial black holes are a intriguing non-particle dark matter candidate. Their masses could in principle range over ~30 orders of magnitude, from about 10^15 - 10^45 g, but most of this parameter space has already been tightly constrained. We will focus on the lower end of this mass range and present an update on primordial black hole mass fraction constraints from Planck CMB anisotropy data. Compared to previous analyses assuming fixed cosmological parameters, we show that for monochromatic mass distributions, constraints are relaxed by up to an order of magnitude if other parameters are allowed to vary. However, for more realistic extended mass distributions constraints in the 10^16 -10^17 g mass range are significantly tightened, effectively closing this mass window for primordial black hole dark matter.
