Interest on late payment of interest for tds

Hi Friends,

We have received a Demand  Notice  under  Sec 156 of  the  Income Tax Act  that  they  charged  two  coloums of  Interest . i. e.   1) Amount of Interest payable  2 ) Amount of  Interest on Interest

The first one  is  1.5% calculated for default of month  with  Taxable amount.  but  second one,  on what basis did they charge such interest.

Anyone  pls  help and tell the calculation for second  one.

Selvam