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RF harvesting application by using bq25505

Zheng Li43
Prodigy 30 points
Other Parts Discussed in Thread: BQ25505, TPS62736, BQ25570

Hi

I am a student with little experience on such project. I want to use a charge pump to boost up the recertified voltage - typical 0.4V with 0.6-1.5mA current to 2.4V. So, is this possible to use bq25505 for this application? and Is there some example case or suggestion for that ?

By the way, it looks there is no internal voltage monitor to lock its output on certain voltage which is higher than 1.9V (VBAT_UV ). Is that means it must set VBAT_OK_HYST to make VBAT_OK control external PFET to turn on/ off load ?

If let VBAT_OK connect to EN2 of TPS62736, will TPS62736 turn off the input from bq25505 when VBAT_OK is low ?

Many thanks

Zheng

  • 0
    TI__Mastermind 20240 points

    Yes you can use bq255xx for this. The bq255xx is essentially a boost charger that boosts up low input voltage to charge up a storage element.

    Yes, it is recommended to add an external PFET between the system load and VSTOR. An inverted VBAT_OK signal can be used to drive the gate of this system isolating external PFET.

    You can use VBAT_OK on BQ25505 to turn on and off the buck converter by connecting the signal to TPS62736 EN2 pin. When VBAT_OK is low on BQQ255505, this means battery voltage falls below the VBAT_OK threshold or has not reach above VBAT_OK_HYST threshold. It is an digital output signal of bq25505. 

  • 0 in reply to Jing Zou
    Prodigy 30 points

    Thanks for your suggestions. However, I tried BQ25505 and BQ25570 which not work.

    When I connect to a 1.2V input (with 50k load), it will pull it to 0.3V and output 0.3V.  I can't find what is wrong on design. Could you help me to find out the problem on my design? The VBAT and VOUT setting is listed below 

    Many thanks

    Zheng

  • 0 in reply to Zheng Li43
    TI__Mastermind 20240 points

    Please refer to datasheet page 17 section 7.4.2 Cold-Start Operation. When you connect a input that VIN > VIN(CS) and PIN > PIN(CS), and the voltage on VSTOR < VSTOR_CHGEN, the IC is operated in cold start. In the cold start mode, voltage on VIN_DC will be clamped to VIN(CS). I assume the output in your post means VOUT which is the output of the buck converter. Please refer to datasheet page 16 section 7.3.7. The buck converter is disabled when the voltage on VSTOR is below VBAT_UV condition. The buck converter continues to operate in pass (100% duty cycle) mode, passing the input voltage to the output, as long as VSTOR is greater than VBAT_UV and less than VOUT.

    You can try increase input current limit to allow VSTOR charge up to VSTOR_CHGEN to exit cold start.

  • 0 in reply to Jing Zou
    Prodigy 30 points

    I measured the VSTOR which was 0.8-1V and VBAT was 1.6V. It can't grow up even I connected it to 0.65V input voltage.

  • 0 in reply to Zheng Li43
    TI__Mastermind 20240 points

    What is your input current set to on your power supply? You might not getting enough input power to charge up the VSTOR cap.