PPMT(rate,per,nper,pv[,fv,type])
PPMT calculates the amount of a payment of an annuity going towards principal.
Formula for it is:
PPMT(per) = PMT - IPMT(per)
where:
PMT = Payment received on annuity
IPMT(per) = amount of interest for period per
IPMT, PV, FV.