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Measuring LDO Line Regulation for a Novice

I hope that you have enjoyed looking at these various measurements on LDOs thus far. In this installment I will be discussing the line regulation measurement. This is actually a much more simple measurement than the NSD and PSRR measurements that I discussed in the last two blogs (see Measuring LDO Noise Spectral Density for a Novice and Measuring LDO Power Supply Rejection Ratio for a Novice). Isn’t everything much simpler at DC? Well, as we saw in the last two blogs the NSD and PSRR measurements are not quite as simple at DC since we have to have equipment capable of performing the measurement at such low frequencies. However, in the case of line regulation the measurement is much less complex.

Recall that the reason I have been spending some extended time with these measurements is due to the stringent space application requirements from customers. A device going into space must be much more rigorously evaluated than a device going into a commercial product. Hence the need to do a good deal more characterization and ‘proving out’ of the device to make sure that it will endure the harsh environment in space as well as the long lifetimes required.

The line regulation measurement simply requires a DC input source and a digital multimeter. It is best to have as much accuracy in both the DC input source and digital multimeter as possible. For the DC input source I chose the Keithley 2230-30-1 triple channel DC power supply. It has 0.03% output voltage accuracy and 0.1% output current accuracy. For the digital multimeter I chose the recently released Keysight 34461A digital multimeter with 6 1/2 digit output capability that replaces the 34401A digital multimeter from Keysight (Agilent). This unit replaces the trusty 34401A unit from Keysight (Agilent) that has been in service for a number of years.

Keithley 2230 DC Source Meter

Keithley 2230 DC Source Meter

Keithley 2230 DC Source Meter

Keysight (Agilent) 34461A Digital Multimeter

Keysight (Agilent) 34461A Digital Multimeter

Keysight (Agilent) 34461A Digital Multimeter

The measurement setup for this test is pretty simple. The Keithley 2230 DC power supply connects to the LDO input and the Keysight 34461A connects to the LDO output. The input voltage is varied and the output voltage response is recorded. The line regulation is simply a measurement of ΔVOUT /ΔVIN . The goal of the measurement is to see how much the output voltage changes when the input voltage changes at a given load current. The load resistance RL is set such that typically the maximum current is sourced from the LDO. Ideally there would be no change in the output voltage and it would remain at a constant level, however, in reality this is not possible. The better LDOs on the market will have a very small change in the output voltage for a given change in the input voltage.

Line Regulation Measurement Setup

Line Regulation Measurement Setup

Line Regulation Measurement Setup

One can simply adjust the voltage input to the LDO from the Keithley 2230 DC power supply and manually record the output voltage from the LDO from the Keysight 34461A multimeter. An alternative is to use some ingenuity and create some sort of script to vary the input voltage and record the output voltage to a file. In this case, I have had some limited experience with Python so I requested some assistance from a coworker (who is quite adept with Python) to assist with creating a Python script to automate this measurement and create an output file. Personally, I have not really ever been very good with generating any type of program from scratch, but have a decent knack for modifying a program that already exists. I had some assistance generating the program (aka script) to perform this measurement and later modified it to perform the load regulation measurement which I’ll be discussing in my next blog. The script adjusts the input voltage to the LDO in 10mV steps and records the input and output voltage into a .csv file. Once I had completed the measurement I used Microsoft Excel to post process the data to convert the line regulation into decibels in order to generate the plot below.

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Typical Line Regulation (in dB) Plot of an LDO Under Test

Typical Line Regulation (in dB) Plot of an LDO Under Test

For the device under test the default output voltage is 5.0V and the input voltage range is up to 5.5V. The headroom required by the LDO is 300mV. Based on these criteria the test range was set from 5.3V up to 5.5V. On average in this input voltage range for this device the line regulation is approximately -40dB. This means that for every 100mV change in the input voltage the output voltage only changes by 1mV. That is pretty good performance.

There are a few lessons to be learned here. One is that the concept for line regulation is pretty simple and straightforward. Another is the beauty of using programs (scripts) to automate measurements where possible. Notice that the NSD and PSRR measurements did not lend themselves well to automation so the measurements were much more manual in nature. Being able to automate a measurement greatly speeds up the process and allows for quicker measurements. Making quicker measurements using programs as engineers allows us to not only get the measurement made quicker but test a larger number of devices. There is some upfront work in putting the program together but the benefits in throughput outweigh the initial effort up front. Stay tuned as we look at measuring the load regulation for an LDO in my next blog.

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