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LMR34206-Q1: Recommended inductance difference between "typical application", "equation (3)" and "webench"

Yoshiki Ochi
Genius 9455 points
Part Number: LMR34206-Q1

Tool/software:

Hi team, 

*I copied the question in extenal e2e thread w/o any confidential information.

For 5V/150mA output, 6V~18V input, and 2.1MHz fsw,  there are several inductance value TI shows.  

  • Datasheet typical application shows "10uH" but with ouptut voltage is 3.3V. Do you recommend using 10uH even for 5V/150mA output? 
    • Equation (3), it includes "IOUTmax". Do you recommend using IOUTmax as 600mA instead of 150mA in this case?
  • On the other hand, when we select K=0.3, VOUT=5V, and IOUTmax=150mA, L=31uH is calculated by equation (3). 
  • In addition, Webench shows 5.6uH as example. 

There are three different recommended inducctance above. Which one do you recommend and why? 

Regrads,
Ochi

  • 0
    TI__Mastermind 22280 points

    Hi Ochi,

    For this device Ioutmax is the full load capability of the device (600mA), not the intended load of the application. L=31uH would be a very large inductance to use.

    The 10uH example is calculated using K=0.3, a Voutmax of 18V steady state and Ioutmax=600mA. For your application, this would be the recommended inductance for 30% current ripple.

    The 5.6uH example is calculated for a maximum input of 12V steady state with 40% current ripple. If you change the maximum input voltage in the Webench settings you will see the recommendation change.

    Generally when sizing inductors for peak current mode devices, it is recommended to target 20%-40% of the maximum load current of the device. 

    The 10uH inductor will be sufficient for your design with Vin = 18V maximum.

    Best regards,

    Ridge