In the last few blogs I have been spending a fair amount of time looking at various performance measurements for LDOs (see Measuring LDO Noise Spectral Density for a Novice, Measuring LDO Power Supply Rejection Ratio for a Novice, and Measuring LDO Line Regulation for a Novice). In this installment of this series I’ll explain how to collect load regulation for an LDO. It is quite similar to the topic of my last blog which was on measuring the line regulation of an LDO. Recall for the line regulation that the input voltage is varied and the output voltage is monitored. The change in output voltage divided by the change in input voltage is the line regulation. Taking the logarithm and multiplying by 20 results in the line regulation expressed in dB as the Typical Line Regulation (in dB) Plot of an LDO Under Test figure showed in my last blog. Now we will look at the change in output voltage in relation to the change in the output current.
Once again, the reason I have been spending some extended time with these measurements is due to the stringent space application requirements from customers. It is important to rigorously test a device going into space to make sure that it will perform to expectations in the harsh environment of space. This is not to say that commercial testing is not stringent, but it is merely an indication of how much more stringent testing is required for space applications. It is one thing to have some type of unexpected performance or failure in a commercial application, it is quite another to have an event in a space application. Once the device is up in space on a satellite there is no opportunity to do an onsite repair. While working on space product developments I see products like these where it is necessary to do additional testing to provide additional assurance that the device will be space worthy.
Let’s now take a look at the actual measurement and what is required. Much like the line regulation measurement, a DC power supply and a digital multimeter are required. Once again for this measurement the Keithley 2230-30-1 DC source meter and the Keysight 34461A digital multimeter are used. Any equivalent equipment may be used, but keep in mind that good source and measurement accuracy is required. As I mentioned in my previous blog the Keithley 2230-30-1 has 0.03% output voltage accuracy and 0.1% output current accuracy. The Keysight 34461A digital multimeter has 6 ½ digit output capability. These are both shown below in the following two figures.
Keithley 2230 DC Source Meter
Keysight (Agilent) 34461A Digital Multimeter
The measurement setup for this test is fairly straightforward. The Keithley 2230 DC power supply connects to the LDO input and the Keysight 34461A connects to the LDO output. The first step is to use the multimeter to accurately measure the two resistors used for the output load resistance RL. At least two different load resistors are required because the goal of the measurement is to find the change in output voltage versus the change in output load current. In order to change the output load current the output load resistance must be varied. To find the maximum change an open circuit (RL ≥ 1MΩ) is used first and then a resistance value that results in the maximum load current is used. This procedure provides the maximum and minimum output load currents. The input voltage in this case is held constant and the output voltage response is recorded for the two load currents. The output load resistance measurement is important because during the measurement the output voltage is measured on the digital multimeter and the resultant load current is calculated from output voltage divided by the load resistance. The resultant measurement provides ΔVOUT/ΔIOUT which is the load regulation for the LDO. Ideally there would be no change in the output voltage between the no load and maximum load conditions, however, every LDO will have some change in its output voltage when driven to it maximum load current.