TPS53355: How to calculate TPS53355 output accuracy with RSS method

Steve Chen94 

Root Sum of Squares (RSS) is a method for adding up a series of independent random tolerances that contribute to a final tolerance.  The formula is fairly straight forward:

Tolerance (RSS) = Square Root [ (1+Tolerance 1)^2 + (1+Tolerance 2)^2 + 1+Tolerance 3)^2 ]

The only aspects that are specific to the TPS53355 are what each of the tolerances for output voltage accuracy are:

1) Reference Voltage Tolerance:  This is given in the datasheet electrical specification table.  It is +/- 0.5% at 25C and +/- 1% over the -40 to +85C temperature range.

2) Resistor Divider Tolerance: This is based on the tolerance of the tolerance of the resistor divider, it is approximately +/- (Tolerance Rtop + Tolerance Rbot).  For RSS tolerance, I would recommend considering the Top and Bot resistors independent tolerance variables.

3) RCC Injection Ramp Tolerance: If external RCC ripple injection is used, there is some tolerance to the output voltage due to variations in the injected ramp amplitude.   The offset voltage at the FB voltage from the RCC injection is 1/2 VOUT/VIN * (VIN-VOUT) * 1/Fsw * 1/RC.  The tolerance of the RCC injection, as a percentage of the output will 83.3 *{ [ VOUT/VIN(min) * (VIN(min)-VOUT) * (1/Fsw) * (1/RC(min)) ] - [ VOUT/VIN(min) * (VIN(min) - VOUT) * 1/Fsw * (1/RC(max) ]  %

The dominant factor in this tolerance is typically the variation in VIN or the capacitor tolerance, which is why I have highlighted those variations above.