Output Impedance

Why is output impedance (zout) important?

A power supply is a source of current from a fixed voltage. Any impedance at the power supply output (zout) is in series with the load impedance being driven by the supply. The supply impedance and the load impedance form a voltage divider, so as the load requests current from the supply, the voltage at the load will change by the amount zout * iload.

Thus lower zout delivers fundamentally better voltage regulation. Watch the first three minutes of a video from Texas Instruments to learn more [choose Low distortion design (4) and topic External sources].

SPX Output Impedance vs. Frequency

What is Superpower's output impedance? Amazing! Here is a graph of Zout vs. frequency, showing a 50mΩ maximum impedance while delivering 10mA from 20Hz to 200KHz. This is measured on a typical production SPX78 with 12V out.

Zout vs. frequency

What are we showing you?

In this test, a 10mA AC current is pulled from the regulator output (top trace, at 10mA/div), as frequency of the current is swept from 20Hz to 200kHz. The AC voltage appearing at the output of the regulator (bottom trace at 50mΩ/div by using a gain of 1000 amplifier) represents the output impedance, where zout=vac/iac. Voltage is graphed on an oscilloscope where you can see frequency change across the horizontal axis at 20kHz per division.

SPX zout stays low across ten times the audio band. We dare you to find or build a better regulator! Take a look at our Zout comparison page to see how SPX compares to other available voltage regulators.

Superpower Output Impedance compared to other regulators

Belleson's newest SPX regulator was multiple years in development, and designed to have the lowest possible output impedance while keeping the circuit fast and stable.

How does Superpower compare with other voltage regulators? See these oscillograms and judge for yourself. Test notes are at the bottom of the page.

Why is low zout desirable?

Output impedance, or zout, is a small signal measurement of a regulator's ability to deliver current while vout remains constant. Any deviation from constant represents a voltage drop across the output impedance due to the AC excitation current from the output.

In this test, a 10mA AC current is pulled from the regulator, as frequency of the current is swept from 20Hz to 200kHz. The AC voltage appearing at the output of the regulator represents its output impedance, where zout=vac/iac.

If you look at a power supply as a source of current from a fixed voltage, any impedance at the power supply output is in series with the load impedance being driven by the supply. That load is, for audio, typically one or many amplification devices. The supply impedance and the load impedance form a voltage divider, so as the load requests current from the supply, the voltage at the load will change by the amount zout * iload.

Thus lower supply zout delivers fundamentally better supply voltage regulation. To learn more, watch the video from TI on this page. Find Topic "Low Distortion Design" and watch the first 3 minutes of "External sources" video.

SPX78

SPX78 10mA output impedance

Belleson's SPX78 has very low zout at audio frequencies and increases gradually to 50mΩ at 200kHz.


Sparkos Labs

Sparkos SS7812 10mA output impedance zout for the Sparkos SS7812 grows to 110mΩ at 110kHz then decreases slightly toward 200kHz.


LM7812

LM7812 10mA output impedance The LM7812 with high impedance around 10kHz and its noisy. zout is set mostly by the output capacitor.

LT3045

LT3045 10mA output impedance

The LT3045 from Analog Devices has a Zout that increases in a slight bow up to 60mΩ at 200kHz.


TPS7A4700

TPS7A4700 10mA output impedance The TPS7A4700 from TI has a bloom in the audio band, where zout is about 200mΩ at 10kHz.


Dexa/NewClassD LC7812 UWB Mk2

Dexa LC781  UWB Mk2 10mA output impedance Relatively high zout with a peak in the audio band up around 180mΩ , and visible noise.


Notes

Measurements are taken in the same test socket, with the same input stimulus and output sense for all devices. Measurements may differ from those you see in manufacturers' data sheets because of different setup, e.g. input or output capacitance, placement of sense device, wire lengths, etc.

Voltage is amplified by 500 and graphed on an oscilloscope. You can see frequency change across the horizontal axis at 20kHz per division.

The top trace is output current as measured with a Tektronix current probe amplifier set to 10mA per division. For some regulators the change in vout is too low to be useful so we fed it through our noise amplifier to give it a gain of 1000. Thus the bottom trace is 1000 x regulator Vout.

The LT3045 and TPS7A4700 are both surface mount monolithic devices that require a PCB to allow them to be plugged into a TO-220 style test socket. The tested devices were, when purchased, mounted on a PCB with MLCC capacitors already connected. Replacing the MLCCs with tantalum on the TPS7A4700 PCB significantly improved performance, and the measurements you see here are with 10µF tantalum.

It's possible that zout graphs of the lowest measured parts have some error due to feed–through in the test fixture. Thus zout could be lower than we measured.


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