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which converter?

tim flippance
Prodigy 100 points
Other Parts Discussed in Thread: PMP20550, TPS82150

I need a DC/DC converter to convert 7.5vDC to 5v at about 100mA,  and about 15-0-15v at about 20mA. These two supplies must be totally isolated from each other. Can anyone recommend one?

  • 0
    Prodigy 5 points

    Hello Tim, thank you for posting.

    Here are 2 options.

    First; TI has many isolated modules, however  Vin range is 4.5-5.5 or 10.8-13.2V.

    There isn't a module that works at 7.5Vin.

    Step down 7.5V to 5V with a buck, then look at the isolated modules. 1.1W load , 300mA

    TPS82150  for 7.5Vin to 5Vout, 1A.

    DCH010505S for 5Vin to 5Vout isolated. 0.5W

    DCPA10512D for 5Vin to +-15V isolated,  0.6W

    Second option:
    This reference design is close to what you need.
    It makes an isolated 5/12V at 200/100mA

    The PMP20550 is a simple design using a SEPIC to produce 3 power rails in which 2 of those are isolated from the input supply. This design is made entirely with automotive qualified IC's and magnetics. The PMP20550 reference design has been designed with optional components to allow the user to adjust output voltage precision as desired.

    This is a discrete design.

    Your comments are welcome.

  • 0 in reply to Ed Walker
    Prodigy 100 points

    Thanks for the reply. Where the output voltage has a minimum and maximum value, how do you set it at the voltage you want?

  • 0 in reply to tim flippance
    Prodigy 5 points

    If you are referring to the modules, Tim, these have fixed conversion ratios.
    Meaning if your input is 5V the the output would be 5V.
    Vout follows Vin, so if Vin drops to 4.5V then Vout would tend to 4.5V.
    The modules are not adjustable.

    For the PMP20550 design, Vout2 Vout3 are set by the transformer turns ration and duty cycle, for the isolated rails.
    Vout1 output is set by the FB pin and the R5 R6 resistor divider.

  • 0 in reply to tim flippance
    Prodigy 100 points
    Can I just check one thing: with the TPS82150, the input voltage can vary between 3v and 17v and the output voltage remains fixed at 5v? or do you have to set it with resistors?
  • 0 in reply to tim flippance
    Prodigy 5 points

    Hello Tim,
    The TPS82150 is a stepdown synchronous buck.
    It will step down any voltage from 3-17V.

    From the datasheet for the device:
    Page 8.
    The TPS82150 offers a low input to output voltage differential by entering 100% duty cycle mode. In this mode,
    the high-side MOSFET switch is constantly turned on. This is particularly useful in battery powered applications
    to achieve longest operation time by taking full advantage of the whole battery voltage range. The minimum input
    voltage to maintain a minimum output voltage is given by:
    VIN(min) = VOUT(min) + IOUT * RDP

    So your input can drop to the above equation.

    To create 5V from 3Vin you need a boost mode.
    TI does make buck boost IC's that can do the job.
    The TPS630701 - Wide Input Voltage (2V-16V) Buck-Boost Converter could work here.
    www.ti.com/.../tps63070

  • 0 in reply to Ed Walker
    Prodigy 100 points

    Thanks Ed

    I only need 5v from a 7.4v battery, so its voltage wouldn't drop too low except when its almost totally discharged