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BQ24090: Differences between BQ24090 and BQ24074 for solar charging

Part Number: BQ24090
Other Parts Discussed in Thread: BQ24074,

Hello,

I found that the BQ24074 is used in this Adafruit module as a solar charger: learn.adafruit.com/.../design-notes. Note that one of the main reasons they use this module is because of the IN-DPM feature, where the device automatically reduces the input current if the voltage falls below the V(IN-DPM) threshold. This is useful for solar charging, because this feature *sort of* works as an MPPT (where the current is reduced if the voltage from the panel is dropping).

So, I have 2 questions:

  1. I have previously used the BQ24090 in one of my projects, and checking the datasheet again I found that it also has an IN-DPM feature. Are these features identical between the BQ24090 and the BQ24074? Namely, does the BQ24090 also gradually reduce the input current so that the input voltage does not fall below V(IN-DPM)? Or is it more of an on-off feature?
  2. How does the BQ24090 work when we have a load connected in parallel to the battery? When there is no load and the battery is connected, I understand that the current is allowed to taper to the termination threshold and then the charge current terminates. But, if I also have a load connected, how is this handled? Should I ensure that my load does not consume more current than the termination current I(TERM) (which is typically 10% of the charge current)? Because, as far as I understand, if my load consumes more than I(TERM), then the BQ24090 would never terminate charge?

Thank you in advance.

  • Hi Dan,

    It is an analog loop and not an ON/OFF type behavior. The working of the loop itself is similar. The BQ24074 is a  powerpath device so it will control the current into the system. The BQ24090 will control the battery charge current as a function of VIN voltage. 

    You are spot on with the termination current. In non-powerpath devices, the current into the battery or load connected cannot be independently controlled. So you will have to ensure the load is not as much as your termination current target. 

    Regards,

    Gautham

  • Thank you for your reply! Two follow up questions:

    1. Is it correct to assume that the charge current is approximately equal to the current into the system?
    2. Do you think using the BQ24090 instead of the BQ24074 is a good choice for the purposes of a solar charger?
  • Hi Dan,

    Is it correct to assume that the charge current is approximately equal to the current into the system?

    For the BQ24090, this is the case. For the BQ24074, the input current is best approximated as the sum of the charge current and the output current.

    Do you think using the BQ24090 instead of the BQ24074 is a good choice for the purposes of a solar charger?

    From a power consumption perspective, perhaps the BQ24074. VIN-DPM is available when configured for either of the USB modes. If the voltage does drop due to lower power output from the solar panel, the input current is also reduced.

    If this gets to the point where this input current is no longer sufficient enough to charge, the device can reduce the charge current to maintain the current load needed. This is known as DPPM/

    The device can also allow you to have the load current be higher than the termination to keep the battery fully charged. The highlight of powerpath devices is that both the path between battery and load are regulated mostly independently and VINDPM and DPPM kick in when needed. 

    As the BQ24090 does not have DPPM, the issue becomes that the battery does not completely terminate and the path for current is shared between load and battery if both are connected when charging is intended. 

    Best Regards,

    Anthony Pham

  • Hi Anthony,

    Thank you for your answers.

    If this gets to the point where this input current is no longer sufficient enough to charge, the device can reduce the charge current to maintain the current load needed. This is known as DPPM/

    In the case of the BQ24090, which does not have DPPM, if the input current is insufficient to charge (meaning the load is asking for more current than what is coming from the solar panel), then probably VINDPM would trigger and the battery would have to supplement the additional current. In my application, I expect this to happen because my load will consume about 50 mA for 15 seconds, and then 100 uA for 300 seconds. My 6 V panel will only produce about 20 mA, so during the high consumption 15 second phases, the battery will have to supplement the extra current. Then when the load goes to sleep mode, the battery can charge with most of what the panel produces. So, I understand the advantage of the BQ24074, it being a power path device, but since my currents are quite low and surely my panel cannot sustain the high consumption phases of the load (meaning DPPM isn't that useful, I think), don't you think the BQ24090 is a good enough choice for the application?

    the issue becomes that the battery does not completely terminate

    In this case, I would definitely ensure that the termination current threshold is set above my load current. So for example, I would set the termination current to 150 mA, so that when the battery is charging with a current of 100 mA (and the load is consuming 50 mA), the charger terminates. In this scenario, the battery would keep providing power to the system for a while, until it reaches the recharge threshold and charge initiates once more. Do you see any disadvantages of this sort of quick charge-discharge? I believe this would be the behaviour regardless of me choosing the BQ24090 or the BQ24074 (?)

  • Hi Dan,

    The BQ24074, which is more designed for charging and allowing a load in parallel may be more optimized but you are correct in that the BQ24090 would also work. 

    The issue I see with the quick charge to discharge is that there may be no device protection for the battery such as over current protection as the battery voltage becomes low. The BQ24074 will disable the output if, when in a battery supplement mode or battery only mode, drops below the battery voltage. It also has the advantage of keeping the output voltage to VDPPM regulation if the voltage at the output decides to drop. 

    Best Regards,

    Anthony Pham