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LM78M: Bipolar DC supply using two Positive Linear Regulators having isolated input supply

KPA
Prodigy 220 points
Part Number: LM78M
Other Parts Discussed in Thread: TPS7A33, TPS7A47, LM2941

Hi,

Is it advisable to generate Bi-polar DC supply using two Positive Linear regulator as depicted in below attached drawing?

I think it is possible technically as far as Regulator's Inputs are isolated.

But does it create any kind of stability problem for long hours of operation or any other kind of issue?

Regards,

  • 0
    TI__Mastermind 34665 points

    Hi KPA,

    This is completely fine, the linear regulator only requires that Vin > RTN in this case and you are satisfying that here.  There should not be a long term issue, the linear regulators should operate as expected.

    The challenge you have will be dissipating the thermal heat generated internally to the device under large load conditions and / or large Vin operating voltage.  The LM78M comes with a metal tab which is typically connected to GND to dissipate the heat.  Ground is typically a large slab of copper in most designs and offers the best heat spreading.  But in the case of the lower LDO, this tab is now -15V.  So if you choose this architecture then be sure to use a large -15V plane to dissipate the heat.

    Alternatively you can use a negative LDO.  If you wish to pursue this please provide some requirements and we can look at some options, or you can search for negative LDO's on TI.com.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 220 points

    Hello Stephen,

    Thank you very much for a very satisfying answer.

    Basically, I thought to go for this approach as it can reduce the BOM inventory item.

    Anyways, I can share my requirement for negative rail as below :

    My requirement 1 is : Vin = 17 to 18V; Vout = -15V;  Iout = 250mA

    My requirement 2 is : Vin = 17 to 18V; Vout = -15V; Iout = 800mA

    Any valuable suggestion from your side will be more than welcome.

    Regards

  • 0 in reply to KPA
    TI__Mastermind 34665 points

    Hi KPA,

    Please give me 2 business days to review and to offer the best solution.

    Thanks,

    - Stephen

  • 0 in reply to KPA
    TI__Mastermind 34665 points

    Hi KPA,

    For the negative LDO, the TPS7A33 would be a good choice.  For the positive LDO, there are plenty of options.  The TPS7A47 is excellent, the LM2940 and LM2941 are older but would work as well.

    Thanks,

    - Stephen