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TLV61220: Switching Frequency, Inductor Size, and Current Limitations

JP
Expert 3791 points
Part Number: TLV61220
Other Parts Discussed in Thread: TPS61071

Hello Team,

I am working with a customer who is looking for a DC/DC Boost Converter for their current application. Below are the requirements for the boost converter:

Input Voltage: 2.4V typical

Output Voltage: ~5V 

Output Current: ~300uA (very low)

Initially, the customer is looking into the TLV61220 boost converter, and had a question on the inductor size limitations. 

After reviewing equation (2) in section 11.2.2.2 of the datasheet (seen below), I calculated Fsw to be roughly 1.35MHz when the Iout is 200mA and L = 4.7uH (well above 75kHz). 

When I modified Iout to equal 300uA, and L to be 2.2uH (lowest recommended value) the Fsw turned out to be 1,925 MHz. Considering that the switching frequency limits of this device are 500kHz < Fsw < 2,000kHz, this solution is not a viable option. Is there a specific reason why we cannot reduce the inductor below the 2.2uH? I don't foresee that reducing the inductor will be able to put us where we need to be, but I was just curious on the reasoning behind the limitation. 

Best Regards,

Javier Palomo

  • 0
    TI__Mastermind 46905 points
    for your application, i would suggest to select typical 4.7uH inductor. smaller inductance does not help to improve the performance.
    this device is based on current hysteresis control topology, and fast inductor current changing rate may impact the normal operation. so the inductor can't be lower than 2.2uH.
  • 0 in reply to Jasper Li
    TI__Expert 3791 points

    Hello Jasper,

    Do you have any recommendations of being able to take a 2.4V input voltage to drive a 5V output with 300uA of current output? 

    The switching frequency of the TLV61220, would not support a current output that low or is there something that I am missing?

    Best Regards,

    Javier Palomo

  • 0 in reply to JP
    TI__Guru**** 191445 points
    I put your inputs into our on line Webench tool. Webench returned 31 possible solutions including TLV61220. webench recommends ysing 4.7 uH inductor for your circuit.

    www.ti.com/.../overview.page
  • 0 in reply to JP
    TI__Mastermind 46905 points
    the TLV61220 can support this application. the switching frequency will be low at light load condition. it can operates at PFM mode. as suggested by John, you can simulate the device with Webench.
  • 0 in reply to Jasper Li
    TI__Expert 3791 points

    Hello Jasper,

    Understood, and I was able to run the simulations on the webench. The only problem I encountered is that the switching frequency needs to be above 75kHz (customer requirements), and the simulation reports a Fsw of 16.1kHz.

    My apologies for not highlighting the Fsw requirements earlier, but this is why I found limitations with the TLV61220. Is there another potential solution where I will be able to meet that requirements?

    I was able to find other solutions on webench that will meet the switching freq requirement, but the efficiency is significantly reduced.

    Best Regards,

    Javier Palomo

  • 0 in reply to JP
    TI__Mastermind 46905 points
    the TPS61071 can meet the frequency requirement, but the efficiency would be not good.
    the frequency change with the load condition if you need high efficiency at light load, which mean the frequency is uncontrollable. i don't have a good solution to meeting both high efficiency and frequency higher than 75Khz.
  • 0 in reply to Jasper Li
    TI__Expert 3791 points

    Hello Jasper,

    Thank you for looking into that for me.

    Best Regards,

    Javier Palomo