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Two LP38798 in parallel to get more current

Wiebren Draaijer
Prodigy 20 points
Other Parts Discussed in Thread: LP38798

Hi,

I would like to use use two LP38798 regulators in parallel to increase the output current. The regulators are going to be used as post regulators for a low noise linear PSU for an audio application.

Is if it is possible to use the regulators in parallel and does it effect the output noise? 

If the regulators can be used in parallel what would be the best electronic circuit to achieve this. My goal is to get a minimum output current of 1250mA, lowest possible output noise and the output has to be adjustable to 5/9/12/15/18V.

Uin = Uout +0.7V

Ccp = 10nF NP0

Cin = Os-Con 20V - 120uF 25mOhm + 10uF X7R (for each LDO)

Cout = OS-Con 20V-120uF 25mOhm + ??????

Regards, Wiebren

  • 0
    TI__Mastermind 22610 points

     Search this community (Linear Regulators) for "parallel".

    See the following : Application Report SLVA250 "Ballast Resistors Allow Load Sharing Between Two Paralleled DC/DC Converters" :

          http://www.ti.com/lit/an/slva250/slva250.pdf

    " ... Is if it is possible to use the regulators in parallel ..."

     

    Yes, it could be possible, but not certainly not very efficient and generally not recommended if any degree of precision output voltage is needed. Personally, I would not recommend multiple LP38798 be operated in parallel.

     

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Dear Donald,

    Thank you for your swift answer

    In the TPS7A45xx spec sheet, page 14 is an advised circuit for paralleling two TPS7A45xx LDO's. Is it perhaps possible to modify this circuit to increase the precision of the output voltage. The value of the 10mOhm resistors at the input of the regulators can be increased to 50 - 100mOhm. Maybe the type of op amp can improve the circuit also.

    For the output voltage a +/- 1% tolerance is acceptable. The output noise, line regulation and the load transient are important factors for the design. 

    Regards, Wiebren

       

  • 0 in reply to Wiebren Draaijer
    TI__Mastermind 22610 points

    Figure 31 in the TPS7A45xx datasheet appears to be a variation on an article published in Electronic Design on Dec 2004: Double Your Output Current With Parallel Voltage Regulators

    http://electronicdesign.com/power/double-your-output-current-parallel-voltage-regulators

    The point of the circuit is to compare the input currents via the IxR drop on the input sense resistors and force the controlled LDO to a higher/lower output voltage to facilitate current sharing.

    This is not likely to give satisfactory results with the LP38798 due to the device internal architecture. Disturbances at the LP38798 FB pin will cause the internal low pass filter to disengage during adjustments causing output noise to rise to levels that would be seen with a typical LDO. The output noise would return to a low filtered level when the output voltage stabilized. Unknown is whether or not the LP38798 reference voltage loop will always be stable with an active adjust circuit added.