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High current problem in series connection uA78xx

Quinten Van den Brande
Prodigy 20 points

Hi everyone,

I have set up a power source circuit on a PCB of mine to provide it with different voltage levels (15V, 8V, 5V and 3.3V).

To accomplish this, I put a couple of uA78xx lineair voltage regulators in series. This way I feed the power circuit at 18V at the 7815, and feed the 15V output directly to the 7808 and so on. (See figure).

The problem is, this way the circuit uses a huge amount of current (460mA), resulting in the need for a cooling element on the 7808 voltage regulator. Because of subsequent problems with this situation, I replaced the 5V voltage source with a switched dc-dc regulator, directly getting its input voltage from the 18V. This resulted in a current drop of more than 200mA!


So now, I'm writing my master thesis and I would like to explain this phenomenon. However, I have no exact idea on how this could happen. If you could help me, it would be greatly appreciated (and a big thank you in my thesis preface).

  • 0
    TI__Genius 12410 points
    Quinten,

    I think what is happening that you are measuring the series load current when you had the LDO's in the circuit then when you connected the switching regulator in parallel you just checked the current coming out of the switching regulator. While LDO's and switching regulators are different and could have different input currents, the output load current should be the same no matter which one you use if you have the same load on it.

    The LDO will be less efficient than a switching regulator. Here is an app note about LDO's
    www.ti.com/.../slva079.pdf
    www.ti.com/.../slva072.pdf

    Best Regards,
    Cameron
  • 0 in reply to Cameron Phillips
    Prodigy 20 points
    Thanks for the quick reply.
    I should have said that I'm measuring the current with my power supply, so I'm in fact measuring the total current.

    Is it possible that the internal resistance of the ldo is temperature dependant, increasing the current needed to regulate the output voltage?
  • 0 in reply to Quinten Van den Brande
    TI__Genius 12410 points
    The switching regulator is more effiecent converting the power in into the power out. For the LDO whatever current you get in is what you'll get out and it gives the power off as heat. The switching regulator works to match the same power in as power out and not just dissipate it.

    So if you had 18V in and 5V with 200mA out using an "ideal" switching regulator your input current would be 55mA.
    With an LDO the input current would 200mA.