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TPS76350

Paul Leong
Prodigy 220 points
Other Parts Discussed in Thread: LP2985, TPS763

Hi,

I want my device (5V) to take input voltage from 5V to 10V.  How do I handle the case when the input is 5V?

Also, is there an equivalent chip to do input voltage up to 15V?

Thanks!!!

  • 0
    TI__Expert 8890 points

    Hi Paul,

    When the Vin-Vout voltage difference is less than the "dropout" of the regulator, for a given load current, the regulator will no longer regulate the output voltage. This "dropout" condition will mean that the output will now follow the input voltage minus the dropout voltage.

    An alternative to the TPS763 might be the LP2985.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 220 points

    So if the input is 3 V then I'll see 3V-dropout voltage even though I am using a 5V regulator?

  • 0 in reply to Bill Stokes
    Prodigy 220 points

    Hi Bill,

     

    Can you confirm the following?  Do you mean that if the voltage going into the 5V regulator is say 2.5Volts, then at the output I'll see 2.5 volts minus the dropout voltage (which is around just 100 mV or so)?

     

    Thanks,

    Paul

  • 0 in reply to Paul Leong
    TI__Guru**** 150870 points

    Yes, this is the case.  LDOs can only reduce your input voltage.  They cannot boost it.

    What power specs (Vin, Vout, Iout) are you trying to meet?  There is a power quick search at power.ti.com that can help you find an acceptable solution.

  • 0 in reply to Paul Leong
    TI__Expert 8890 points

    Paul,

    Yes, for the case you describe where Vin=2.5V, the internal pass-fet will be fully ON and so this fet will be acting like a resistor. The dropout voltage is simply the output current drop across this resistance (which is around 100mV or so as you say).

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 220 points

    HI Bill,

    Is this the same with LP2985 that you suggested?  I actually have a min Vin of 4.8V to 5.0V and I am concerned if this voltage will pass through to the Vout.  I wanted it to pass through.  So will this Vout be 4.7V to 4.9V (considering the dropout voltage) in this minimum Vin case for the LP2985 as well?

    Thanks,

    Paul

  • 0 in reply to Paul Leong
    TI__Expert 8890 points

    Hi Paul,

    Yes, your thinking is correct, depending on the fully-on resistance of the pass-transistor, normally referred to as Rds_on,  all linear regulators work the same. The LP2985 is no different.  This is a very typical problem that many engineers run into. The only way around it is to use a much more expensive buck-boost switching regulator.  Or maybe use a voltage supervisor to turn on a very low resistance fet - to bypass the linear regulator at low voltages (I hear this works).

    Bill

  • 0 in reply to Bill Stokes
    Prodigy 220 points

    Hi Bill,

    For the TPS76350, the spec says that the max current output is 150mA.  What if I draw up to 180mA? I see that there is more dropout on the graph in the spec sheet.  But can it actually draw more than 150mA?

     

    Thanks

    Paul

  • 0 in reply to Paul Leong
    TI__Expert 8890 points

    Hello Paul,

    We typically tell our customers that the specifications are only guaranteed up to the rating of the part, which is in this case 150mA. The part was only characterized up to 150mA when we created the Electrical Characteristics table.  The metalization on the die and the wire bonds were also designed with the 150mA rating in mind. However the part will likely work just fine at 180mA assuming that the thermal junctions are kept below 125degC but for some reduction in reliability - no guarantees. Notice that the part will eventually  limit the current but this current limit is much higher than 180mA.