In Interfacing to ADCs: Power Supplies, Part 5 we looked at the switching spurs and the PSRR of an LDO and an ADC when using a DC/DC converter (switching regulator) in combination with an LDO to drive the power supply inputs of the ADC. We have seen in this blog series that this method is more efficient than using just LDOs. In this blog we’ll take this one step further and look at driving the ADC power supplies directly from a DC/DC converter (shown in Figure 1.) The input supply voltage is 6.0 V, which is stepped down to 1.8 V for the ADC supply voltages. I have separately called out the LC filter that resides at the output of the DC/DC converter, as this is especially critical for this design to filter the switching transient spurs generated at the DC/DC converter switching frequency.
Once again, we will look at an example with the AD9683. In this example, we will use the ADP2442 DC/DC converter with 1 A output current capability. This is sufficient for the AD9683, which requires a maximum total current of 263 mA. In order to generate the proper application circuit for the ADP2442 to drive the AD9683 we will once again turn to the ADIsimPower tool, which is available for the ADP2442 at ADP2442 ADIsimPower Tool. The first step is to input the system parameters (shown in Figure 2).
As mentioned above, we are using a 6.0 V supply for the ADP2442 with an output voltage of 1.8V and an output current of 260 mA. I have used the maximum total current draw for the AD9683 and have set the temperature to 85o C since this is the maximum operating temperature. This will make sure that the design will operate up to the typical maximum temperature in most applications. I have chosen for the tool to design the application circuit for the lowest cost. The other available selections are least-part-amount, most-efficient, and smallest-size. I chose lowest-cost because one of the primary drivers in many applications today is for the least expensive solution. With these inputs the tool generates the solution circuit and values given in Figures 3 and 4.
The lowest-cost option gives us a total BOM (bill of materials) cost of $2.218. Based on the value of Rfreq we can see that the switching frequency is set to 1 MHz. In this case the efficiency (not pictured but generated by the design tool) is about 75%. The output inductor is a 3.3 µH Coilcraft component, and the output capacitance is a 10 µF Taiyo Yuden component. If we were to choose the option for most-efficient in the design tool, it would yield the BOM shown in Figure 5 below.
Choosing the most-efficient option yields a total BOM cost of $2.693, which is just a bit more than the previous example. This change results in an increase in the value of the output inductor from 3.3 µH up to 10 µH. As illustrated in the BOM cost, the output inductor is one of the largest cost adders in the system. This is a critical component for a DC/DC converter. The output inductor is typically chosen to have a low DC resistance (DCR), a high self-resonant frequency (SRF), and a high saturation current (ISAT ). In addition, the value of Rfreq is different and sets the switching frequency to 314 kHz instead of 1 MHz for the lowest-cost option. In this example, however, the overall efficiency is increased from 75% to 89%. The flexibility of the tool allows the user to choose the best solution for a particular system design.
Using this design tool for ADI power solutions has reminded me that ADI has several software design tools that can be used to prototype a system in advance of a physical design as well as evaluate product performance without hardware. Stay tuned as we take a look at ADIsimADC in the next blog.