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TMS37157: How to charge a 3.7V Li-Polymer battery

Weiguang Zhou
Prodigy 130 points
Part Number: TMS37157
Other Parts Discussed in Thread: TMS3705, , BQ24202

Hi,

I've made it to communicate with TMS3705 based RFID Reader. I wanna use energy derived from RF field directly to charge the battery.  

I took reference to this:

I use a UC27424D to enhance RF field on RFID Reader side , but the voltage of Charge Capactitor on responder side reaches 2V at most during a long time charge phase, which is expected to be near 6V to make normal communication possible. I guess too much current is drawn from RF field.

Is there any other way to charge a 3.7V Li-Polymer battery? 

  • 0
    TI__Guru*** 128095 points
    Hello,

    For one, the Battery Charge feature is limited to outputting ~3.4V w/ a max of 3.55V, so it won't ever reach 3.7V. For two, the maximum current it can source is 2mA. So you would need to work within that current limit. Is your application going to be able to function around these limitations?

    As far as power supplied over RF, I have to ask the product team about that.
  • 0 in reply to Ralph Jacobi
    TI__Guru*** 128095 points
    Hello,

    I need to revise my prior comment, I hadn't realized VBAT was an input only when I made that comment after reading DS specs.

    After consulting the product team, there is a lot more information needed to help provide more feedback.

    1) Explanation of requirements for voltage/current for battery charging to achieve application goal
    2) Explanation of where the requirements are coming from (we don't understand the need for 6V to charge a 3.7V battery currently...)
    3) Your schematic with the battery charger device (unclear what connections are needed to it, what device is being used, whether the SHT21 is being used, etc.)

    Also regarding getting higher voltages out of the RF field, you may find some of the info in this app note useful: www.ti.com/.../slaa480.pdf
  • 0 in reply to Ralph Jacobi
    Prodigy 130 points
    Hi~

    Thank you for your help. Information you needed is as below:

    1) 15 mA charge current is needed in my apllication.
    2) 6V is the VCL-voltage required to perform normal operation according to datasheet of TMS37157. When a battery charger IC( for example ,bq24202)is connected to VCL point to get charge current from RF field , the voltage goes down to around 2V as too much current drawn from RF field. In this case, normal operation of TMS37157 is not possible.
    3)As I mentioned above, battery charger device is something like bq24202. In my application , VBAT is not connected to TMS37157 & MSP430(there is no SHT21). I use a dc-dc to provide 3V power out of battery.

    By now, I've given up the idea to charge while communicating. But I still want to know, ignoring communication issues, how much charge current can RF field provide to directly charge a 3.7V Li-Polymer battery if I use a TMS3705 based Reader Station with 5V power? The distance is better reader and responder is less than 10mm.

    Best regards
  • 0 in reply to Weiguang Zhou
    Prodigy 130 points
    Correction for my last sentence: The distance "between" reader and responder is less than 10mm.
  • 0 in reply to Weiguang Zhou
    TI__Guru*** 128095 points

    Hello,

    I am attaching a document which has charge current statistics based on some measurements done with the TMS37157 based on range: PaLFI_current_supply_110318_KA.PDF

    On the PaLFI Target board, both the standards antenna on the board and also a larger antenna from Neosid were used.