The picture below is on page 7 of the datasheet of TL497, it is a switching regulator that I use as an inverting regulator (it has an inverting mode) to generate -24V from 12V in input, in the first designing equation on this page I have 2 unknowns "I(pk)" and "Io max" I don't know how can I find their values ?? In the datasheet it is wrote that I(pk) maximum value is 500mA so if I(pk) in the equation means 500mA why they didn't write 500mA simply unless of I(pk) also I(pk) max value is 500mA it can be lower than that and if 500mA means the "Io max" then I(pk) value will be 3A for VI=12V and |Vo|=24V which is impossible because I(pk) max =500mA. I hope I explained well. datasheet:http://www.mouser.com/ds/2/405/tl497a-405896.pdf
IO max is your maximum load current - how much current you need to run your load at the output voltage. Calculate I(PK) using the equation you have circled and as @scorpdaddy says, if its less than 500 ma you are ok without the external transistor. Otherwise add the transistor.
Choose Rcl to give you the current sense you need, and choose a transistor with a collector current in excess of your calculated I(pk). Choose appropriate base resistor divider values low enough for sufficient base current.