Propeller Project Board Rev. B, A Linear Power Supply, And The LDO Regulator Pad

idbruceidbruce Posts: 5,375
edited May 11 in Propeller 1 Vote Up0Vote Down
Hello Everyone

This current thread is related to this previous thread (, but this thread specifically relates to the Propeller Project Board Rev. B And LDO Regulator Pad. I am creating this specific thread, because I would like to use this board feature if I can, I need some assistance, and I think this discussion will be useful to someone else in the future.

I would like to feed the Power Input - VIN (5~16VDC, 3A) of the Propeller Project Board, with the output of a linear power supply, which is rated at 13.8VDC, 2.5A, but when tested with a DMM, the output is 15.8VDC. Of course, VIN feeds the switching regulator of the Propeller Project Board, to provide power to the board itself and to the Propeller chip, but I also want to add a 12VDC LDO regulator, to the LDO Regulator Pad, of the Propeller Project Board, for specific power 12V requirements. The LDO regulator that I would like to add is the LM2940CS-12/NOPB (Datasheet:, which is manufactured by Texas Instruments.

Within the datasheet, it states that C1 is required if the regulator is placed far from the power supply filter, and it further states the following:
The LM2940-N and LM2940C positive voltage regulators feature the ability to source 1 A of output current with a dropout voltage of typically 0.5 V and a maximum of 1 V over the entire temperature range. The output capacitor, COUT, must have a capacitance value of at least 22 μF with an ESR of at least 100 mΩ, but no more than 1 Ω. The minimum capacitance value and the ESR requirements apply across the entire expected operating ambient temperature range.

Now that I have said all that, let me backtrack a little, just to give you all a few details about the linear power supply. It is a "Micronta Regulated 12-Volt Power Supply", Cat. No. 22-120, which was once distributed by Radio Shack, many moons ago.

Although it is small and a little hard to read, here is a link to an image of the schematic, within an original owners manual:

At this point, I have already soldered a two position terminal block to VIN of the Propeller Project Board, so now I am moving onto the important questions, which are:

1. As it pertains to the schematic of this linear power supply, if I place the power supply in very close proximity of the Propeller Project Board and only utilize very short jumpers, will C1 referred to within the LM2940 datasheet be required?

2. As referred to in a previous quote from the LM2940 datasheet, as well as other stern notifications within the datasheet, the ESR of COUT is critical to proper operation of the regulator, but I am having serious difficulties finding a capacitor to satisfy the stated requirements, so could someone please provide me with a good part number for COUT?

3. The output of the LM2940 is intended to feed the input of a laser diode driver, which basically requires a smooth and steady 12V supply, so is there any other components that I should add to the typical application of the LM2940 to eliminate noise and make it as smooth as possible?

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