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.
peak to peak
measures the peak to peak voltage ripples
Root mean square (RMS)
measures the RMS of voltage ripples
measures the average of voltage ripples
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
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 shaped impedance vs frequency curve is desired to power delivery network.
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
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
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
Switch Mode Power Supply (SMPS)
SMPS regulator is used for high load and high efficient voltage regulation.
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.
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.
Place decoupling capacitors right at the load power pins.
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.
Oscilloscope for time measurements
Please see System Validation for more detailed testing information.
VNA for impedance vs frequency measure
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.