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LM7800:

Prodigy 30 points

Replies: 4

Views: 71

Part Number: LM7800

1) Dual output voltage circuits with 79xx and 78xx are common and stable circuits. Somemanufacturers also suggest  negative output voltage circuits with 78xx.

2) Is it possible do develop stable and effective dual output circuits by using two 78xx one with positive output settings other with negative output settings

e.g. 24 V DC input, voltage divider with two 100K resistors between +24 V and grd. 

one 7812 with possitive settings and other 7812 wth negative settings.

  • HI Rafet,

    There are some issues with trying to do something like this.

    First: There is a drop across any LDO or Linear regulator. In this example it is 2V (LM7800). So with a 24V different at the input, your best case outputs could be +/-10V.

    Second: Because IQ will vary between the devices over load, the above circuit is prone to become unbalanced. There are a number of methods of creating a virtual ground that can better accommodate this type of approach and the best approach will depend on you application needs.

    I hope this answers your question.

    John Cummings 蔣康明
    Texas Instruments 德洲儀器
    Johncummings@ti.com

    Please click the "Resolved" link at the bottom of this post if I have answered your question.

    Thank you,

  • In reply to JCHK:

    Thank you for your answer.

    As I understand it is not possible to develop a stable design with possive regulator negative out settings.

    I want to develop a stable settings for an instrumentational amplifier. First I connect two power supplies in series [+12------[Com--+12]------Com ] to optain dual output source. Use it to supply voltage regulators. But this setting have problem in high voltage tests.

    Then I work on a 24V power supply with a voltage dvider consist of two 8 ohms / 50W for virtual ground. 


    Third option 24V power supply with DC-DC converters.

    What should be the best way to develo virtual ground for an instrumentational amplifer  (+9V, -9V)?

  • In reply to Rafet IRMAK:

    HI Rafet,

    Here is a link to a few ideas on how to implement a virtual GND:  

    Virtual GND

    The above is not a TI site, so be sure to verify these circuits work well for your application.

    With a proper virtual GND established, As long as the LDO on the + side of the virtual GND is sourcing and the LDO on the - side of the virtual GND is sinking, this concept should be able to work.

    Not sure how much current you really need but we do have a simple dual +/- LDO the TPS7A39 which also incorporates startup tracking when ensure both rails start up symmetrically.

    I hope this helps.

    John Cummings 蔣康明
    Texas Instruments 德洲儀器
    Johncummings@ti.com

    Please click the "Resolved" link at the bottom of this post if I have answered your question.

    Thank you,

  • In reply to JCHK:

    Thank you for your help.

    The link link includes detailed explanation.

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