The minimum input voltage for the IC to maintain line regulation is around 18V. Keeping this in mind and the DC rectified voltage of around 20V, we get the minimum negative peak of ripple voltage to be around 2V. The ripple voltage is double the minimum peak and is around 4V. The value of filter capacitor can be calculated keeping in mind the ripple voltage, ripple frequency, quiescent current drawn by the regulator and the maximum load current. This can be established mathematically as: