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Evaluation boards are very helpful. Manufacturers of complex circuits such as DC-DC converters provide boards with those circuits ready to try out, saving us time and effort to design the printed circuit board around them. Evaluation boards are supposed to help us to understand the capabilities of the device. But with the many potential user applications, what should a particular user expect and look for in an evaluation board? We need to know how to properly evaluate an evaluation board.
My February 2013 Quiet Power column featured an LTM4604 evaluation board. In that column the purpose was to discuss different measurement techniques; the subject was not the regulator itself. In this column we look at an LM20143 evaluation board to explain what may matter during the evaluation.
The LM20143 is an adjustable-frequency synchronous buck regulator with current-mode control loop . The input voltage can be anywhere in the 2.95 to 5.5V range, the maximum continuous output current is 3A. The switching frequency is adjustable in the 500 kHz to 1500 kHz range. The default output voltage setting of the evaluation board is 1.2V. The integrated circuit includes the output switching devices. Figure 1 shows the top view of the evaluation board with no cable attached. To make the board work, all we have to do is connect a voltage source to the input terminals and pull the enable pin (labeled ‘EN’ on the board) to logic high.
The first rule in every test and measurement (also true in simulations, by the way) is “Know what to expect.” We measure something because we may want to validate a design or we measure something because we are not sure exactly how the circuit behaves. This latter case, however, is no excuse to ignore the rule: we still should have some idea what we expect as a result. If we don’t, it becomes a full-fledged exploration and we need to be extremely careful to make sure that accidental mistakes or measurement errors don’t mask the correct signature that we are after. In an evaluation board of a DC-DC converter, we can test many different aspects of operation. There are items that require only DC voltage and current meters. This way, for instance, we can check the line and load regulations and efficiency at different input and output voltages and load currents. To test for dynamic parameters, we can use an oscilloscope and transient current source. In the frequency domain, with a frequency response analyzer or vector network analyzer we can test the gain-phase curve or output impedance. These measurements can be done with small-signal excitation or large-signal excitation.
To read this entire article, which appeared in the September 2016 issue of The PCB Design Magazine, click here.