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WEBENCH® Tools/LMZ31710: Usage of switching frequencies, other than the pre-sets provided.

Frank Schneider
Prodigy 60 points
Part Number: LMZ31710

Tool/software: WEBENCH® Design Tools

Hi all,

For a power supply solution, our customer like to have power modules used in the design. As the featured THT ones from the customer are not feasible for this particular design, your website recommended the LMZ31710 instead.

Now we have to use the synchronization fefature for the output voltages of 5V, 3.3V and 1.5V, each from one of the LMZ31710.

Unfortunately, there is some information missing in the datasheet, as well as options in the WEBench tools.

The input on all three will be: Vin = 12V ±5%

Output 1 will be Vout1 @2.5A = 5V
Output 2 will be Vout2 @2.5A = 3.3V
Output 3 will be Vout3 @3.5A = 1.5V

That said, the datasheet values will not allow to run the LMZ31710 with the same switching frequency on all three output voltages, as well as the WEBench tools.

But my calculation stated, that a minimum on-time of 210ns is within a safe operating area. From this, I calculated a worst case switching frequency and added some headroom.
With this, I landed on a switching frequency of 530 kHz, which results in an estimated R_rt = 165kOhm.

So, my question is, will this work as intended and will WEBench support using custom switching frequency on the LMZ31710 model?
The goal is to see the calculated operating parameters like peak efficiency, output ripple voltage, load step response, control loop bandwidth and all the values presented if I 'am using a regular (discrete) solution from TI.

BR

Frank

  • 0
    TI__Genius 14455 points

    Hi Frank,

        We will have the WEBENCH model enhanced to support the custom switching frequency. In the mean time you could change the Rt resistor by selecting "Select Alternate Part" and change the value of Rt manually. This should update the switching frequency appropriately. You can then run a transient simulation to check the behavior of the device as well. 

    We will get back to you with some more clarity on the datasheet table 7 shortly.

    regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    TI__Genius 17905 points

    Hi Frank,

    The max recommended Vout in Table 7 is limited by the positive current limit. Given that we have a fixed inductor value in the module, higher Vout and lower Fsw both increase ripple, which can push certain conditions to hit current limit based on the Vout and Fsw combination. However, Table 7 is based on the assumption that the device must run at full rated current of 10A. Your load currents are well below this, so there should not be an issue with synchronizing your 1.5V, 3.3V, and 5V rails all at 530kHz.

    We are planning updates to the datasheet to clarify the switching frequency limitations, and I will work with the WEBENCH team to make the switching frequency easier to customize in the tool. When I have an estimate on when the WEBENCH updates can be completed, I will let you know here.

    Kris