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Tool/software:
Team,
My customer, Eric Zuber at Collins Aerospace, has the following questions:
1) for the 2 different switching regs inside the TPS65219 PMIC, is there an equivalent discrete reg that would have more complete design tools?
Or for the PMIC is the design really just use the Cin/L/Cout values recommended and move on?
2) the datasheet indicate the I-sat of the inductor needs to be roughly 2x the rated current of the switcher…..3.5A,7.4A for #1 and 2A,5.4A for the #2/3. Is the peak current mostly set by the Iout being used – so we could reduce the I-sat spec proportionally for a design where we are using lower than the max Iout? Or is it a bit more complicated than that….just the LC switching current is rather large, even for low I-outs? Along the lines of Buck1, the switching current when operating requires a 3.9A overhead, then the actual DC load current adds to that.
did some work with ripple current estimation….is the large ripple current on this part mainly due to operation in the PFM mode? Seems to imply switch frequency could drop to ~500k in PFM – if we set in forced PWM mode, we should be able to reduce that ripple current overhead significantly?
3) it looks like TI now has a card to program PMIC EEPROM? Is that the only available method for programming, or does that use I2C such that the process could be performed in-system if desired? Is there somewhere where EEPROM contents and writes to EEPROM are documented?
Regards,
Aaron
Thank you Sathish, here are a few more questions from the customer:
Question1: For LDO outputs that are enabled in the ’08 PMIC but not used in our design – do we need Cin / Cout caps on them at all? I plan to disable them via I2C after the AM625 is up, but need the PMIC power-up sequence to complete for that to happen…
Question2: Checking out the NVM programming guide…. I do see where voltage ripple is specified for PFM and PWM modes, but as far as ripple current, I only see the the BuckN current ratings in the spec table - for example
And the design guidelines down in section 8
Inductor Selection - Buck1, Buck2, Buck3
Internal parameters for the buck converters are optimized for 0.47uH inductor. DCR must be 50 mO or lower. Ensure that the selected inductor is rated to support saturation current of at least 7.4A for Buck1 and 5.4A for Buck2/Buck3.
The target is a space constrained application – so trying to identify the smallest package size inductor that we can get by with for the Vin/Vout and expected Iout current in our application. The inductor Isat in section 8 gives a bit of margin above from the peak current limit from the specification tables, but I believe that peak current limit could be quite a bit lower in this application with a 3.7V Vin, Iout quite a bit lower than Iout-max and using the forced PWM setting.
As an example, using TI’s ripple current guide, I get a nominal ripple current of ~0.8A for forced PWM mode (with 2.3 MHz Fsw) compared to what looks like roughly a 3.4A max ripple current from the spec table….I can get to much smaller size inductor packages if I can justify taking advantage of lower I-sat requirements based on our usage – trying to verify that the process we use to reduce the max values from the data sheet are valid.
