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BQ24075EVM: Solar charger design inquiry

Jonathan Navor
Genius 9125 points
Part Number: BQ24075EVM
Other Parts Discussed in Thread: BQ24075, TPS61092, BQ24074,

Hi Team,

Good day! This is posted in behalf of the customer. Please see whole inquiry below.

I am trying to build a solar charger using BQ24075 EVM. I am connecting a 6V/10W solar panel at the input, a 3.7 V/1100 mAh LiPoly battery at the battery terminal and a 5 V USB device (Amazon Kindle Fire) at the output.

I have set EN1 and EN2 such that the input current is limited to 500 mA. I have disabled the timers.

With these connections, I was expecting most of the energy to be diverted to the USB device on priority and only residual energy to be used for battery charging. What I see is exactly opposite. I see the RED LED ON meaning battery is charging and the USB device shows it is not charging at all. I monitored OUT = 4.5 V and V_iset = 0.65 V.

Next, I connected CEz to Vss to disable battery charging. In this case, RED LED went OFF and the USB device started charging - but very very slowly.

I was wondering if it is something to do with the output capability of the solar panel. So I removed it and connected a wall adaptor with 5V/3A rating. The observation with CEz = HI was exactly same. The USB device charged a bit faster when CEz = VSS.

If I remove the input source, I see OUT =  BAT = 3.5V and the USB device does not charge 

What could be the issue?

I was suspecting if the output voltage in each case was too small to charge a USB device?

For my use case, is BQ24074 (with VOUT = 4.4 V) followed by a boost like TPS61092 a better option?

Thank you in advance! 

Best regards,

Jonathan

  • 0
    TI__Mastermind 23170 points

    Hi Jonathan,

    There's a lot of information that's still needed to narrow down the reasoning behind what the customer is seeing.

    Jonathan Navor said:
    I have set EN1 and EN2 such that the input current is limited to 500 mA. I have disabled the timers.
    Jonathan Navor said:
    With these connections, I was expecting most of the energy to be diverted to the USB device on priority and only residual energy to be used for battery charging. What I see is exactly opposite. I see the RED LED ON meaning battery is charging and the USB device shows it is not charging at all. I monitored OUT = 4.5 V and V_iset = 0.65 V.

    This information is good but without knowing the charge current, I'm not able to use the V_iset value to see if the 3.7V BAT pin is getting the expected charge. Also without knowing the voltage and current the solar panel is showing at the time, I can't say if OUT = 4.5V is because of the solar panel being 4.5V or if the BQ24075 is in a regulation loop that would cause it to drop (input current limit for example). 

    Jonathan Navor said:
    USB device shows it is not charging at all

    It'd be good to know how much current the Amazon Kindle Fire is attempting to pull to charge its own battery. It most likely has its own charging IC within the device that has its own charging loops similar to the BQ24075 that could be preventing it from charging. If the manufacturer recommends 5V, it's good to have it the OUT voltage on the BQ24075 be 5V whether it be with VIN = 5V (as this device operates in passthrough below VOUT = 5.5V) or with a boost to ensure 5V.

    Jonathan Navor said:
    Next, I connected CEz to Vss to disable battery charging

    /CE on the BQ24075EVM connected to VSS should be enable charging to the battery. 

    EVM settings that would be helpful:

    - RISET (measured when VIN is off)

    - SYSOFF 

    Best Regards,

    Anthony Pham