SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
On the Differential (D) branch, Xn-1 is subtracted from Xn. The difference between the two most recent X values is then multiplied by Kd, giving a 24 bit signed number.
The D branch has an additional pole added after this multiplication, giving more control of the filter response to permit better matching to the power supply plant. This D alpha branch acts as an integrator on the output of the D stage. With each filter calculation, the 24 bit result of the previous D and D alpha calculation is multiplied by the 9 bit Kalpha (Kα). This gives a 32 bit signed result. To match the 24 bit data path, this must be rounded back down to 24 bits. This is the one rounding that actually takes place in the filter before the filter output.
There is also a simple clamp to prevent the K output from going outside a 24 bit signed number. Because of the integral element for the alpha stage, it is possible for it to exceed this range.