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TPS62821: Two TPS62821 to work together

Terry Weng
Prodigy 50 points
Part Number: TPS62821
Other Parts Discussed in Thread: TPS61291, TPS62740, TPS62745, TPS61099

Can we use two TPS62821 to step down two primary lithium batteries from 3.6V to 1.6V and then add them up to have regulated 3.2V output? 

  • 0
    TI__Guru*** 142880 points
    Hi Terry,

    Do you mean to connect the output voltages in series? This would tie the GND of one IC to the output of the other. Since the outputs are not isolated, this will not work.

    The TPS61291 is one device used in your type of applications with primary cells. It would be operated in bypass mode until the battery voltage was below 3.3V, then it would be put into boost mode.

    The TPS62740 is also commonly used as a buck converter.

    There are many other solutions possible. Is 3.2V the only voltage you need? How much peak current do you need on it? What is the tolerance required on the 3.2V?
  • 0 in reply to Chris Glaser
    Prodigy 50 points

    Hi, 

    Thank you very much for the response. We had the TPS61291 evaluated last year. It won't be able to resolve the issue we have with the primary battery cells. Due to the lithium cell (open voltage ~ 3.6V) internal resistance issue (varies from a few ohm to a few 10s ohm even at new), when we need 60 mA pulses (~10ms) to read/write microsd on our device, the battery output voltage drops below 2.7V (that fails the usd memory). The boost converter doesn't help the high internal resistance + high current pulses issue. 

    Any other solution except to step down from two serial connected cells? Thanks.

    Terry.

  • 0 in reply to Terry Weng
    TI__Guru*** 142880 points
    Hi Terry,

    Yes, some customers would use 2 series cells with the TPS62745.

    Otherwise, you need to store enough energy in capacitors to ride through your pulse. Here's one way to do it: http://www.ti.com/tool/PMP9763

    Since your output power is pretty low, you could use a boost like TPS61099 to go to ~5V and have a few hundred uF cap there. Then, you would buck back down to the load voltage.
  • 0 in reply to Chris Glaser
    TI__Guru*** 142880 points
    Hi Terry,

    Were you able to find something to test out with what we discussed?