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Original question:

BQ25756: Voltage rating of MOSFETs

BQ25858B-Q1: Replacing BQ25858/BQ25858B with BQ25856

Lary Cotten
Prodigy 130 points
Part Number: BQ25858B-Q1
Other Parts Discussed in Thread: BQ25858-Q1, BQ25758A, BQ25758

I was recommended by TI to use the BQ25858 instead of the BQ25856 a few months ago. I'm getting ready to have the prototype boards made, and the BQ25858B/BQ25858 doesnt seem to actually exist. I'm looking for specific notes/guidence on using the BQ25856 as a last minute alternate. Specifically how should I adjust the design to cleanly allow this. It seems to mainly be about the FB, FBG, TS and MODE pins. 

The IIN and IOUT pins seem to simply be renamed as ILIM_HIZ and ICHG, but are programmed the same, so I'd like to confirm that these can stay as is. 

This design is simply meant as a wide range input buck/boot power supply with some management ability. I'm not using the "reverse" mode of the BQ25856 in this design.

Schematic page attached:

 BQ25858B.pdf 

  • 0
    TI__Genius 9290 points

    If you still need to use the BQ25856 instead, here are the answers to the pin changes:

    • FB: This is a reference pin that needs a voltage divider to set the output voltage
    • FBG: This is the ground return for the FB pin. It goes to high impedance when there is only a battery connected to reduce IQ. This pin can be connected directly to ground if needed. The full feedback network is the output voltage sense to RTOP to FB to RBOT to FBG.
    • TS: This is a thermistor connection for battery temperature monitoring. TS pin can be disabled via I2C if necessary. Otherwise, a resistor divider of REGN to 5kOhm to 7.25kOhm to ground should be sufficient.
    • MODE: does not exist on BQ25856. This pin is repurposed to PGND.
    • IIN: Same as ILIM_HIZ
    • IOUT: Same as ICHG

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 130 points

    I really meant physical availability. The part has been "new" long enough that I expected inventory by now. Worldwide inventory still seems to be zero. I have my distributor looking as well. I contacted TI and they just gave me the circular answer of using the product availability tool on the TI website. I've found no information to know if this product will be available at any time. If I can alter the design to work with either part, then maybe I can move forward with a prototype build (15-20 units), though I'd rather build with the actual part. As it happens, I'm using the BQ25856 on a different board. 

  • 0 in reply to Michael Bradbourne
    Prodigy 130 points

    Thanks, That helps. 

  • 0 in reply to Lary Cotten
    TI__Genius 9290 points

    Hi Lary,

    That is interesting about the inventory. I thought the part was readily available too.

    I understand wanting to build with the actual part. The 2 devices are compensated differently. The BQ25856 is compensated for batteries so it will not behave identical to the BQ25858. If you do build with the BQ25856, set EN_TERM=0, EN_CHG_TMR=0, and EN_PRECHG=0. You might also need more capacitance to reduce the voltage ripple.

  • 0 in reply to Michael Bradbourne
    Prodigy 130 points

    I've set the FB/FBG feedback resistors for the default output voltage I need from the BQ25856. That raised a question about the BQ25858. Is there no output voltage until a register gets written? Without a feedback divider, there's nothing to set the initial output voltage. 

  • 0 in reply to Lary Cotten
    TI__Genius 9290 points

    Hi Lary,

    The default value is 5V until overwritten by I2C. This is because it was primarily intended for USB-C PD applications.

    Also, I realized this after the last reply, but if the BQ25758/BQ25758A is available, this would be very similar to the BQ25858-Q1 for a proto build. The main difference is that it is not automotive qualified.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 130 points

    So, I'm trying to send my board out for fab and assembly, hopefully by a week or two ago. The '856 would require a few resistor changes and additions.  Is the BQ25758 going to be a drop in ( other than Automotive qualification/ratings of course). I ask because you said "similar" and "main difference" and not simply "the non automotive version". Is there anything I should add to the schematic/board to accommodate it. Or hardware values I need to adjust? I'd rather find out now what the other differences are, and if they're significant for my purposes. 

  • 0 in reply to Lary Cotten
    TI__Genius 9290 points

    Hi Lary,

    BQ25758 will be pin-to-pin and almost register to register with the BQ25858-Q1. There are some register bit differences based on different features, but the register map is the same for the most part. For example, the BQ25758 will not have the dithering feature that the BQ25858-Q1 has.

    Best regards,
    Michael