5 Voltage Regulation Accuracy

As the process technology advances, the FPGAs, micro-controllers, and ASICs require tighter voltage accuracy and lower operating voltages for their core rails. The processor’s datasheet may specify the voltage tolerance as a percentage or as a value in mV, which includes DC, AC and ripple variations over the entire operating temperature range. Any voltage outside of this range is not recommended and the processor can behave unexpectedly. Designers much also consider the tolerance of the resistor divider used by the DC/DC converter, the routing and trace losses of the circuit board, and also the variations of the application, like the input voltage variations, temperature swings, and fast changes in the load. All of these contribute to the accuracy of the DC/DC converter. Many designers will want head-room or margin to make sure the solution is always within the tolerance expectation of the processor.

It is important to check the initial feedback voltage accuracy of the DC/DC converter in the data sheet rather than the front page. Table 5-1 shows the regulated feedback voltage specification of the TPS54218, which is a 2.95 to 6-V, 2-A converter, and shows that the reference accuracy is ±8 mV or ±1% over input voltage and temperature variations. The total output voltage accuracy is improved by choosing tighter tolerance resistors. If more headroom is needed, designers can choose 0.1% or 0.5% resistors1, even though they may cost a little bit more. The additional headroom will allow the total ±3% or ±5% output voltage variation to be met with less bulk and bypass capacitance.

It is always wise to place the DC/DC converter as close to the load as possible. Often times layout constraints, connectors, and board density requirements interfere