LM317-N-MIL: Multiple LDO devices, Max/Min input voltage request

Hi Archie, 

The LM317K has an adjustable pin rather than a ground pin - also known as a floating ground pin. 

Because the regulator is floating and detects only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input-to-output differential is not exceeded. That is, avoid short-circuiting the output. 

The max differential being 40V means that the part could potentially support a 77V VIN to 37V VOUT with a current of 1.5A. 

That being said, the thermals of the part must be considered.

PD = (VIN-VOUT)(ILOAD) + (VIN*IGROUND), which influences the temperature at the junction of the package. The junction temperature cannot exceed 125C. 

Depending on your application and PCB layout, you should review the "10.3 Thermal Considerations" of the DS and add the appropriate heatsinks if possible. 

The heatsinks necessary to support a large power dissipation may require a large amount of space. 

Best,

Juliette

Sorry for the misunderstanding Archie, 

Figure four in the DS goes over the dropout voltage based on the output current of your application. 

3V is the headroom that would apply to any application regardless of output current. 

Best,

Juliette

Hi Archie, 

I have looked into this for you and these parts are not spec'd with a min and max value.

They are only spec'd with a differential between IN and OUT, not with respect to ground.

As long as the thermals of the part are not violated and the differential between IN and OUT are not exceeded, the part is expected to work. 

Best,

Juliette