Power Integrity

Created; 5/01/2020Last update: 5/09/2020

Introduction

Power supply and its delivery path must met the load voltage ripple requirements in order for the product to operate reliably. Power integrity is an analysis on how well power supply deliver its power to the load in terms of voltage ripple, Load voltage regulation and target current path impedance.

Background

Measurement Analysis

Time Domain

  • Voltage Ripples
    • peak to peak
      • measures the peak to peak voltage ripples
    • Root mean square (RMS)
      • measures the RMS of voltage ripples
    • average
      • measures the average of voltage ripples
  • Load ripple
    • voltage ripple seen at the power supply input pin of the load/
      • Complex load such as, processor, memory, clock generation block, etc. has stringent voltage ripple requirements

Frequency Domain

  • Impedance vs. Frequency
    • This characterizes path impedance of power delivery network. Power delivery network is the path from output of voltage regulator to the power supply pin of the load.
    • We characterize the frequency from DC to MHz depending on the load requirement
  • Bathtub curve
    • Bathtub shaped impedance vs frequency curve is desired to power delivery network.

DCR

  • DC resistance of the power path

Power Distribution Network (PDN)

  • Current path from voltage regulator to IC load.
  • It is characterized by DC and AC PDN.
    • DC is the DC resistance path
    • AC includes target impedance from regulator output, bulk capacitance, decoupling capacitance, and plane capacitance.
      • It;'s an impedance vs frequency graph ranging from DC to 10s of MHZ for board level PDN. If IC packaging and on die capacitance are included, the up frequency range is in 100 MHZ.

Basic Power Supply Components

Passives

Bulk Capacitor

  • These capacitors are large in capacitance and size and is used to hold voltage steady at voltage regulator output
  • this type capacitor defines the low frequency roll off point of the bathtub curve

Decoupling Capacitor

  • These capacitors are small in capacitance and size and is used to hold voltage steady at the load power supply input.
  • This type of capacitor defines the high frequency rise point of the bathtub curve

Voltage Regulator

  • Switch Mode Power Supply (SMPS)
    • SMPS regulator is used for high load and high efficient voltage regulation.

Remote Sensing

  • A voltage regulator feedback technique that uses a pair of positive and negative trace that extends all the way to the power pins of the load. This sensing technique remote voltage droop due to IR voltage drop due to DCR.

Design overview

Voltage Regulator power integrity design

Reduce output ripple

  • A ripple injection circuit to reduce voltage ripples
    • Typically this is needed if required voltage ripple is very low < 20 mV peak to peak.
  • Use large bulk caps and conductor smooth SMPS output ripples.
  • Choose regulator with higher switching frequency
    • Note: increase frequency of a SMPS decreases its power efficiency. Only use this is voltage ripple can't be met.

Reduce DC resistance

  • Use remote sensing lines to reduce DCR voltage drop.
    • Some more advanced processor chip include on die sense pins that taps into on die power domain for more accurate load regulation.

Load Power Integrity Design

Reduce input ripple

  • Add a series of shunt ceramic decoupling capacitors with different values (e.g.0.1uF, 1uF, 2.2 uF) to extend the bathtub shaped impedance vs frequency to cover the main load frequency spectrum.
  • General Placement
    • Place decoupling capacitors right at the load power pins.
  • General Routing
    • Use 1 microvia every 0.5 A current density as a rule of thumb
    • Use 1 mm trace with for every 1 A current density as a rule of thumb
    • Route remote sensing trace pair tightly coupled between ground layers with ground guard rings.

Measurement Technique

Oscilloscope for time measurements

VNA for impedance vs frequency measure

Q&A

Does capacitor packaging affect impedance vs frequency curve?

Yes! A larger capacitor with the same capacitance compared to a smaller package has higher parasitic inductance due to longer leads. high parasitic inductance has higher impedance at higher frequency, lowering the rise up frequency point, resulting in width reduction of the bathtub curve. Hence the rule of thumb in design is use capacitor with smallest package with the same capacitance.

How do you lower DCR?

  • Use wide and short power trace.
  • Use multiple power planes to add parallelism of the plane to reduce effective DC path resistance.
  • Use large series component such as larger size inductor to reduce ESR of the passive

Summary & Conclusion

  • Power integrity is a requirement of a complex load such as a processor, memory, etc.
  • SMPS voltage regulator is good to deliver high current load with great efficiency (85 to 95 %)
  • Voltage ripple is used to characterize voltage ripple seen at the load.
  • Bathtub impedance vs. frequency analysis is used to meet power delivery network design for the load.
  • Bulk capacitor holds voltage regulator output steady
  • Decoupling capacitor holds load input voltage steady

Power integrity ensures voltage ripple swing and power supply capability are met for the high speed digital electronics (complex load) to operate properly in all functional states.