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Voltage Step up IC for solar cell

John Stefanopoulos
Prodigy 60 points

Other Parts Discussed in Thread: TPS61200, TPS61020, BQ25504, TPS61202

Hi I have two solar cells that which are rated at 1.5A @ .58v open circuit. Ill most likely but them in series so ill have 1.5a @ 1.2v and will need a regulated output of 5v at around .4 amps. 

 

This will be a straight solar cell to load application (no battery) so what is the most effiencent and simplest chip I can use for this? 

  • 0
    TI__Guru**** 152110 points

    The TPS61200 was designed for this and has an MPPT app note: http://www.ti.com/lit/an/slva345b/slva345b.pdf

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    This chip does not seem very efficient in the 1.2 volt range and an output current over 300ma these two cells will provide. My only option would be to add a third or fourth cell to get the voltage up to 1.8-2.4v

  • 0 in reply to John Stefanopoulos
    TI__Guru**** 152110 points

    Yes, it is very difficult to make an IC that has great performance across such a wide operating range.  The beauty of the TPS61200 is that it will work down to 0.3V in, so for a lower power single cell application that gets plenty of shade.

    Another option that is more efficient is the TPS61020 family.

    Lastly, I should mention our ultra low power energy harvester that we released today: bq25504.  You might find it useful for some smaller power applications.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    I went ahead and sampled the TPS61202 chip, so this is the fixed 5v output version. 

     

    Checking the data sheet the typical application circuit is show for the adjustable output version. Since I have the fixed version whats the minimum connections I need?

     

    Do i need just the input capacitor, inductor, and output capacitor, and the FB pin connected directly to the VOUT pin?

     

    What about the EN PS and UVLO and VAUX pins? Can I leave these not connected to anything? I don't need power save mode or under voltage protection. 

     

     

  • 0 in reply to John Stefanopoulos
    TI__Guru**** 152110 points

    I would highly recommend that you save yourself the effort of designing a PCB and order the EVM for the device.  This provides a known good layout.

    Page 4 of the datasheet explains how to connect the pins.  The digital inputs must be terminated high or low.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    why is there a resistor on the input line on the evm? Wouldn't that severely limit the current going into the chip? This makes no sense.

  • 0 in reply to John Stefanopoulos
    TI__Guru**** 152110 points

    R1 is on the Vin input as an RC filter.  This is a signal input to the IC.  The power path is through the inductor and L pin, not the Vin pin.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    Thanks, why does the schematic on the data sheet not include this resistor or the second capacitor after the resistor?

    Another question, I just noticed how small the chip is, so it would be almost impossible to solder the connection manually so it can be used on a prototype bread board. Is there an IC socket that will fit this chip? I haven't found one. 

     

  • 0 in reply to John Stefanopoulos
    TI__Guru**** 152110 points

    The RC filter on Vin is optional but does provide a cleaner supply to the IC's Vin pin which is the input for the IC's sensitive control circuitry.   It is not required but good analog design practice to include one.

    The only way that I have found to solder these leadless thermal pad packages is with hot air.  I would not recommend soldering by hand as it is very easy to damage the IC.  A socket is also not recommended as the extra trace length and inductance sometimes gives poor performance and failure as voltages overshoot and ring more, etc.  For these reasons we make EVMs for you to evaluate our ICs on.  There is no point in making your own board for a simple evalaution.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    The EVM would be great, but it has preset inductor resistor and capacitor values that need to be changed /tweeked for our application. We also need to add more circuitry in places that are in between the input and outputs which the eval board won't allow.