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Original question:

LM338: LM338T Parallel Configuration

LM338: Using in parallel with shutdown

Alex C
Prodigy 10 points
Part Number: LM338
Other Parts Discussed in Thread: OPA990

Tool/software:

Hello,

I would like to use two LM338 in parallel and implement shutdown capability. For up to 28V, 8A output.

I see that figure 44 in the datasheet shows an example of the devices in parallel, but I see that its noted for +1.2V to 20V output. Is 28V output not possible with this configuration?



I also see that there is figure 33 with an example of the LM338 with shutdown using an NPN transistor. Could this be directly applied to the lower LM338 when used in parallel configuration?

  • 0
    Guru 316440 points

    28 V should be possible with a larger R2.

    The LM107 can be replaced with the OPA990.

    The minimum load current is necessary to get a minimum voltage drop over the two R resistors. To allow a lower load or increase the precision, use larger R. (What are the input voltage and your actual minimum load current? The maximum value of R is about (VIN − 3 V − 28 V) / (8 A / 2).)

    The circuit regulates the upper LM338 so that the current through the two R is the same. When the lower LM338 is shut down (actually, output lowered to 1.24 V), then the upper LM338 will follow.

    The power dissipation might become problematic. Do you really need to use linear regulators? Consider just a buck converter (which is likely to have a real shutdown), or a buck converter to 31 V followed by the LM338s.

  • 0
    TI__Expert 3805 points

    Hello Alex,

    Paralleling LDOs is possible but some design considerations are needed.

    In order to provide a more comprehensive answer, can you please provide the following:

    • What is the input voltage range? (minimum and maximum)
    • Will the input voltage be supplied by batteries or a power supply?
    • Do you intend to regulate to lower voltage(s), other than 28v?
    • What is the expected operating ambient temperature?
    • What package will you use: TO-3 or TO-220?
    • Is the application intended to operate at 8A continuously?
    • What is the nature of your load? (resistive, inductive, etc)
    • Is there any inrush current upon power up?
    • When in shutdown, what is the maximum output voltage your load can tolerate? 

    If your application requires an efficient circuit, there might be alternative options, like a buck converter as mentioned by Clemens.

    Best regards,

    Daniel Esparza