• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 573 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25890H: Help Verifying inductor selection and capacitors

Eric Steimle
Expert 2290 points
Part Number: BQ25890H

I just want to make sure I've selected an appropriate inductor for my application.  I read the datasheet, did the math and looked at the reference design BOM.

For my application the specs are:

  • Input voltage 3.9V - 14.4V
  • Max battery charge current 2A
  • Max battery discharge current 4A
  • Max system input current 5A (system running and battery charging at same time).
  • Freq 1.5Mhz

If I choose L to be 1uH like it is in the reference design I get an Iripple of 2.4A based on the datasheet calculations  Then Ibat comes out to 1/2 of Iripple plus the worst case charger current.  I'm using 5A for that so I say Ibat is 6.2A.  The inductor in the datasheet IHLP2020BZER1R0M11 has a saturation current of 7A so it seems like this would still be an appropriate choice.

Does this make sense to you?

Also I tried to make sense of the capacitor selection advice in the following way.  

Buck Input capacitor

I'm assuming this capacitor is the one from Vbus to gnd, the datasheet recommends 25V and then talks about Ipmid which comes out to 2.5A in my calculations, but this will be a ceramic cap so ripple shouldn't be a big deal here.  I'm assuming a 1uF 25V capacitor will be sufficient here?

Sys output capacitor

Again going with some ceramic here, I didn't know what "s squared" was in the equations so I'm taking the advice in the later paragraph and going with 6V and 20uF (probably 40uF or more actually).  I'm assuming these are the caps after the inductor that lead to the system.

Boost Cap

The only thing I see for the boost cap is a 0.047uF capacitor in the reference design.  Are there some parameters I should use here for voltage?  The reference design uses a 25V ceramic.

Pmid Cap

The datasheet recommends an 8.2uF to 20uF capacitor depending on OTG output current.  The reference design uses 1000pF but looks like OTG is not supported.  What should I use here for voltage if I want to put in 20uF?  6V or more?

Sorry for the long request and thanks for looking at it.

  • 0
    TI__Guru**** 166106 points
    Hi Eric,

    Regarding inductor, the IHLP2020 with Isat=7A is a good choice for deltaILpk = 6.2A.

    Regarding buck input and PMID capacitor, technically the PMID capacitor is the buck input capacitor and OTG boost output capacitor due to the Q1 reverse blocking FET. The minimum recommended PMID capacitance is 8.2uF of ceramic capacitance in order to minimize input voltage ripple and boost mode output voltage ripple. In order to not violate USB power rules, the total capacitance on VBUS and PMID should be <=10uF. If your device will not be powered by USB then you can add more capacitance. The higher the capacitance on VBUS and PMID the lower the buck input and boost output voltage ripple/droop will be during normal operation/transients.

    Regarding the SYS output, the "s squared" is (2*pi*fSW)^2 where fSW=1.5HMhz. More capacitance at SYS means lower output switching ripple and less V(SYS) droop during a load transient. If you expect large (>2A) load transients, then you might want to increase the SYS output capacitance from the minimum of 20uF.

    Regarding the BTST "bootstrap" capacitor, this capacitor forms 1/2 of a charge pump to provide a voltage to drive Q1 and Q2 NFETs. It needs to be 0.047uF at least 16V rated.

    All ceramic capacitors lose capacitance when voltage is applied, some up to 80% when close to their rated voltage is applied. So, I recommend that you review the capacitor datasheet to confirm the actual capacitance value at the expected DC voltage on that capacitor. If not enough capacitance will be available, you can either add more of the same capacitors or chose another higher voltage rated capacitor.
  • 0 in reply to Jeff F
    Expert 2290 points
    Thank you, what's the rating on the PMID caps?
  • 0 in reply to Eric Steimle
    TI__Guru**** 166106 points
    PMID will see the same voltage as VBUS. So, the capacitor ratings need to be at least the maximum input voltage to charger. We usually recommended 25V rated capacitors.