Improving noise and thermal performance with simplified power architectures

The traditional setup for powering a clock, data converter or amplifier is to use a DC/DC converter (or module), followed by an LDO, followed by a ferrite-bead filter, as shown in Figure 9. This design approach minimizes both noise and ripple from the power supply and works well for load currents below approximately 2 A. As loads increase, however, power losses in the LDO introduce issues in efficiency and thermal management; for example, a post-regulation LDO can add 1.5 W of power loss in a typical analog front-end application.

The benefits of an LDO in a typical power architecture are to provide an accurate voltage rail while lowering switching noise in the high-frequency noise region with a high PSRR. The trade-off of using an LDO is an increase in thermals and power consumption. An effective way to ensure low noise while controlling power losses is to eliminate the LDO from the design altogether and use a low-noise DC/DC buck converter or module, as shown in Figure 10. This LDO-less design lowers power losses and improves thermals while achieving low noise.

The TPS62912 and TPS62913 family of low-noise buck converters, as well as the TPSM82912 and TPSM82913 modules, implement a noise-reduction/soft-start pin for connecting a capacitor, forming a low-pass resistor-capacitor filter using the integrated R_f and externally connected C_NR/SS, as shown in Figure 11. This implementation essentially mimics the behavior of the band-gap low-pass filter in an LDO, which allows for an output-voltage ripple below 10 μV_RMS. The TPS62913 can also achieve a low noise floor in the high-frequency region that is absent of the typical switching noise by taking advantage of the 2.2-MHz switching frequency and optional second-stage ferrite-bead inductor-capacitor filter.

The ADC12DJ5200RF is an RF-sampling ADC that samples from DC to 10 GHz with 4 W of power consumption. The PSRR attenuates any power-supply ripple and noise, but any residual ripple and noise will appear on the ADC output spectrum, which causes an error. The ADC12DJ5200RF has more sensitive power-supply requirements on the analog voltage rails, and therefore requires low noise. Using the TPS62912 for low-noise and high-power analog rails enables a simplified and efficient power architecture, while minimizing power losses compared to a DC/DC-plus-LDO combination.