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LM7800: Linear regulators

Karthik A
Prodigy 90 points
Part Number: LM7800

Tool/software:

Hi.,

Could you help me with the design calculation of C1 and C2(input and output)of LM7805.

  • 0
    TI__Guru 50025 points

    Hi Karthik,

    This device does not require any output capacitor for stability, so there is no minimum requirement and in regards to transient performance more is better. Of course high output capacitance leads to high inrush current so there is a tradeoff. Do you have a target accuracy spec to design to? 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 90 points

    Hi Nick,

    Thanks for response.

    the target accuracy spec is +/- 2% accuracy with <50mV ripple

  • 0 in reply to Karthik A
    TI__Guru 50025 points

    Hi Karthik,

    This device is spec'd for 5% over line, load, and temperature, and 4% at 25C over just load for the 5-V device. A 2% accuracy spec is not achievable. You will need to move to a better device for this, especially with the 50mV requirement (if my assumption that "ripple" includes load transients). 

    If you can provide me with more of your application conditions (e.g. VIN, IMAX, any other key considerations or features) I can help find a device that can fit your requirements. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 90 points

    Hi Nick.,

    I do need reference design calculation for LM7800 family regulator.,

    if you could help with any reference calculation for LM7805 with Vin 12V Imax 500mA .,

    if you could provide any reference documents or calculations with respect to linear regulator selection and design parameter, it is helpful.

  • +1 in reply to Karthik A
    TI__Guru 50025 points

    Hi Karthik,

    Since this is an old legacy device with limited documentation and specifications, the best we can do is linearly interpolate the line/load regulation specs to see how much we can get them down. The load regulation spec is 50mV (max) for 5mA ≤ IOUT ≤ 1A, which we can interpolate to 0.5A by:

    50mV * (500mA - 5mA) / (1A - 5mA) = 24.87mV

    Line regulation is 50mV (max) for 8V ≤ VIN ≤ 20V at 500mA, which we can interpolate by:

    50mV * (12V - 8V) / (20V - 8V) = 16.67mV

    With these we can reduce the overall accuracy spec to 200mV - (24.87mV + 16.67mV) = 158.5mV. This corresponds to an accuracy spec of 158.5mV / 5V = 3.17%. As you can see, this is still far from 2%. Additionally, a 50mV ripple requirement (again, assuming that you mean to include line/load transients in this ripple requirement) is likely not met by this old device. 

    Karthik A said:
    if you could provide any reference documents or calculations with respect to linear regulator selection and design parameter, it is helpful.

    Do you mean linear regulator selection in general? If so, defining the load requirements is the first step, which it seems that you have defined the basic requirements at least. Other than that, other considerations should be addressed such as quiescent current requirements and PSRR and noise requirements. 

    Regards,

    Nick