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BQ25570: Reg: Connecting storage element to VBAT

Yoganand Biradavolu
Prodigy 20 points
Part Number: BQ25570
Other Parts Discussed in Thread: BQ25505, BQ25504

Hello,

I am working on creating a tag that can harvest energy using RF signals and use the microcontroller to perform basic operations. In my circuit, I have 10 stage voltage doubling circuit and I store the final voltage on a capacitor.

I want to connect the output of my voltage doubling circuit to VIN pin of BQ25570 and use it as a boost charger to provide minimum voltage required for the MCU.

The question here is where should I connect the final capacitor to store the output voltage from boost charger? Also, is it okay to not connect any storage element to VBAT?

  • 0
    TI__Genius 13905 points

    Hi Yoganand,

    The storage element, in this case your final capacitor, should be connected to the VBAT pin. The capacitor that you use for storage should exceed at least 100uF. You can connect your system to the LBUCK as a stepped-down load or to VSTOR directly. Your storage element will be charged until VBAT_OV and then will discharge to the system load at VSTOR.

    Best Regards,

    Juan Ospina

  • 0 in reply to Juan Ospina
    Prodigy 20 points

    Thank you very much for your reply. Just to clarify on few connections, the final storage capacitor should be connected to VBAT and VSTOR should be connected to the microcontroller's VIN? 

    Is it VSTOR or VBAT that gets charge first? From what I understand is that it takes at least 600mV for the circuit to exit cold start and at that time VSTOR would have charged to 1.8V and then continues to charge the storage element even at low incident powers. Once the storage element reaches VBAT_OV, it discharges to system load at VSTOR. Please confirm this as it will help me in designing the circuit. 

  • 0 in reply to Yoganand Biradavolu
    TI__Genius 13905 points

    Hi Yoganand,

    Yoganand Biradavolu said:
    the final storage capacitor should be connected to VBAT and VSTOR should be connected to the microcontroller's VIN? 

    That's correct. Your system load should source its power from VSTOR or VOUT (the VOUT buck sources its power from VSTOR internally). VBAT should be connected to your energy storage.

    Yoganand Biradavolu said:
    Is it VSTOR or VBAT that gets charge first?

    VSTOR gets charged first as the flow of power from the input source, goes through the inductor into LBOOST, through the high-side boost FET into VSTOR. From there it flows through the FET that is between VSTOR and VBAT to charge VBAT. With this flow VSTOR gets charged first as it must be above VSTOR(CHGEN) to begin normal charging.

    Best Regards,

    Juan Ospina

  • 0 in reply to Juan Ospina
    Prodigy 20 points

    Thanks again for your reply. I understand.

    After VSTOR reaches VSTOR(CHGEN) (1.8 V from the datasheet), normal charging begins. During this normal charging, will VSTOR hold that 1.8V until VBAT reaches VBAT_OV?

  • +1 in reply to Yoganand Biradavolu
    TI__Genius 13905 points

    After VSTOR reaches 1.8V it will continue to hold that until VBAT rises above 1.8V. Then VSTOR = VBAT and both will continue to increase until VBAT_OV.

    Best Regards,

    Juan Ospina

  • 0 in reply to Juan Ospina
    Prodigy 20 points

    Thank you for your reply. I understand the circuit much better now.

    1. In my case the RF source is only on for few seconds. In this case how much time it takes for VSTOR to reach VSTOR(CHGEN) meaning how long the input should stay above 600mV.

    2. Is there any other Power Management IC from TI that can exit cold start at voltage lower than 600mV.

    3. As my source is only ON for few seconds (max of 5 sec), will setting connecting MPPT 80% or 50% have an impact in my case? Because datasheet mentions that MPPT circuit obtains a new reference voltage every 16 s (typical) by periodically disabling the charger for 256 ms (typical) and sampling a fraction of the open-circuit voltage (VOC).

    Thanks in advance for your time

  • 0 in reply to Yoganand Biradavolu
    TI__Mastermind 35335 points

    Someone will get back to you by Monday.

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    TI__Mastermind 35335 points

    Hello,

    1. The time for VSTOR to reach VSTOR(CGHEN) will depend on VSTOR's capacitor as well as the provided input power.

    2. The BQ25504 and BQ25505 are examples with cold start at 600 mV.

    3. Please configure the MPPT according to what the input voltage should be regulated to for the application.

    Regards,

    Mike Emanuel