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TPS650250

David Zlater
Prodigy 100 points

In the TPS650250 datasheet 3,3uH is "required for output voltages >2.8V" but this is a bit confusing since w.c. ripple current is at VOUT = VIN/2, see analysis below. It is also a bit confusing that SLVU731A has a 3,3uH inductor for the 1,1V supply without explanation. In the two example diagrams in the datasheet, one has 2,2uH for all 3 DCDCs, and one has 3,3uh for the 3,3V reg.

If ripple current is the issue then there is a double fault in the texts mentioned. It should read: "Larger inductance might be a good idea the closer VOUT is to VIN/2".

Analysis: I_RIPPLE = VOUT*(1-VOUT/VIN)/f/L; dI_RIPPLE/dVOUT = (1-2*VOUT/VIN)/f/L; Maximum I_RIPPLE is when dI_RIPPLE/dVOUT = 0 i e VOUT = VIN/2. In our case we have VIN = 5V so w.c. in for VOUT = 2,5V (and max allowed VIn is 6V so it won't get much higher in any application).

(Note: This is a copy of a post I put in the analog forum. I was told to put it in Power Management.)

Edit: Added "= 0" after "dI_RIPPLE/dVOUT"

  • 0
    TI__Genius 16870 points

    David, 

    The increased inductance is required to limit the ripple when specifically needed during PFM mode of operation. The datasheet refers to this concept in the inductor selection of the design procedure on page 20. The converter is in PFM mode at only light loads. 

    Will you system draw light loads? (less than 300mA) The 300mA is not where the converter switches into PFM but, if you are above 300mA you are probably not in PFM. Equations 1 and 2 can help you out here. 

    However, I do agree with you that it is confusing why some of our documents have a 2.2uH for Vouts higher than 2.8V which is not what we recommend. This recommendation is a good starting point though and I always recommend getting an EVM to tryout you final component selection before going to your prototype. 

  • 0 in reply to Michael Green
    Prodigy 100 points

    Hi and thanks for the reply.

    Do you mean to say >2.2uH is of interest only in PFM mode? So that if we use forced PWM mode then we can always use 2,2uH?

    And why is voltage ripple amplitude higher i PFM mode? Don't they use the same current limit?

    And why is the criterion 2,8V? I think my analysis shows that worst case is when VOUT = VIN/2.

  • 0 in reply to David Zlater
    TI__Genius 16870 points

    David, 

    In PFM mode, the converter's switching frequency is lower. This is why you may want to change the inductor to a 3.3uF if your Vout is above 2.8V. If you force PWM mode then, you will lose efficiency at light loads. Thus, I would not recommend that either. What is you Vin, Vout, and Iout? How much do you plan on running the max Iout (in duty cycle %)?

    I am not really sure how our system engineer derived 2.8V but, I think it might have been empirical data. The equation 4 is only good for PWM mode for the frequency will change when you are in PFM mode. Otherwise, I do agree with you the highest Iout ripple is at Vout = VIN / 2.