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Interfacing to ADCs: Power Supplies, Part 6

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.

Figure 1

Driving ADC Power Supply Inputs with a DC/DC Converter and LDO

Driving ADC Power Supply Inputs with a DC/DC Converter and LDO

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).

Figure 2

ADP2442/AD9683 Design Parameters

ADP2442/AD9683 Design Parameters

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.

Figure 3

ADP2442/AD9683 Application Circuit Schematic

ADP2442/AD9683 Application Circuit Schematic

Figure 4

ADP2442/AD9683 Application Circuit Component Values (Lowest Cost)

ADP2442/AD9683 Application Circuit Component Values (Lowest Cost)

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.

Figure 5

ADP2442/AD9683 Application Circuit Component Values (Most Efficient)

ADP2442/AD9683 Application Circuit Component Values (Most Efficient)

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.

15 comments on “Interfacing to ADCs: Power Supplies, Part 6

  1. samicksha
    November 27, 2014

    One of the important factor i see in tools is comparing your build design with efficient and available designs keeping accountable keys in list such as controllers, supervisory elements.

  2. uchiha
    November 27, 2014

    @samicksha: How to differentiate what tool is better on such a scenario ? You cannot test all because of the cost isn't it ?

  3. chirshadblog
    November 27, 2014

    @Uchiha: How about word of mouth ? It plays a big role in making decisions

  4. dassa.an
    November 28, 2014

    @Uchiha: Well testing can be done. All you have to do is to request for a sample or a test run (demo version).

  5. samicksha
    November 28, 2014

    @Uchiha: It is not much about differentiate, but about learning and and adopting string points from other available circuits.

  6. yalanand
    November 29, 2014

    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.

    @Jonathan, thanks for the post. I totally agree with you that such design tools helps us evaluate product performance without hardware. I am curious to know how accurate these predictions or do we need to take care about some parameters which these design tools overlook ?

  7. yalanand
    November 29, 2014

    Well testing can be done. All you have to do is to request for a sample or a test run (demo version).

    @dassa.an, thats a good idea but I am not sure if demo version will have all the features enabled.

  8. yalanand
    November 29, 2014

    How to differentiate what tool is better on such a scenario ?

    @uchiha, thats a valid point. I agree with you that it wont be possible to test with all the tools. I think we first have to understand the strength and weakness of the tools and choose the best tool accordingly.

  9. yalanand
    November 29, 2014

    How about word of mouth ? It plays a big role in making decisions

    @chirshadblog, word of mouth definitely plays role. I think we can read past reviews about the tools in design communities before choosing any particular tool for the analysis.

  10. dassa.an
    November 29, 2014

    @yalanand: Yes there are many options available for us, its just a matter of time in picking the right one

  11. jonharris0
    December 1, 2014

    Wow, what a lot of great comments posted on this blog.  I am glad that it sparked a lot of interest from readers out there.  Thanks for the great dialog! 

    @yanaland, I'll be looking at more of the tools that ADI offers..admittedly with a bit of an ADC bias since I'll be looking at mostly components that would be used to interface to the ADC.  We will look more specifically at the capabilities of the tools in future blogs.  I'll at least start though by saying the ADIsimADC tool will give you a very good representation of what the ADC is capable of.  You'll see performance nearly identical to what you'd see on an evaluation board.  It does have some limitations…keep in mind that modeling interactions with other components can be difficult.  We'll look more closely at this question in the coming blogs.  Thanks for the great question!!

  12. Davidled
    December 3, 2014

    As figure 4 shows multiple voltage output control,it could be possible that there is the limit of current rating for each voltage output. Therefore, current limit circuit might be required between filter and each analog/digital/driver supply in order to protect the master chip, ADP2442 and other circuit. 

  13. jonharris0
    December 4, 2014

    Hi DaeJ, thanks for the comment.  The ADP2442 does have an overcurrent-limit protection circuit built in that will limit the output current.  The details can be found in the ADP2442 data sheet at: http://www.analog.com/static/imported-files/data_sheets/ADP2442.pdf

  14. Davidled
    December 4, 2014

    In the data sheet, page 11 describes the internal block diagram with only one current limit comparator, current sense amplifier and state machine gate control logic. Instead of using this chip alone, designer might need to monitor this chip to make sure that all voltage and current are feeding into output load, even though this chip has any protection circuit intenally.

  15. ue2014
    December 31, 2014

    @ yalanand >  word of mouth definitely plays role. >>>>>>>>

    The importance of WOM is that it gives you thoughts and feedback on actual succes stories and failures taken place based upon experienced people had using the same tool. Therefore, it allows you to make a better selection. 

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