LDO Outputs

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I have two sources of 5v on a PCB. One is from an on-board LDO and the other is from an external DC supply. If the external supply is attached and there is no input to the LDO will it be damaged? I believe the LDO's transistors will all be back-biased so there will be no problem but I want some other opinions. See the attached schematic for details.

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The answer is "it depends".

We have had regulators that are sensitive to being "back-powered", and would fail/break/blow when subject to Vcc for a period of time.

Most will be happy being "back powered".

Look at the datasheet and see if there is a diode roughly from the output to the input. Then you are OK.

The datasheet for the LM1117 you showed doesn't have such a diode in the block diagram, so indeed be wary. The text, however, does mention it:

Quote:
output voltage of the regulator, and rate of decrease of VIN. In the LM1117-N regulators, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor (≥1000 µF), and with input instantaneously shorted to ground, the regulator could be damaged.
In this case, an external diode is recommended between the output and input pins to protect the regulator, as shown in Figure 22.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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You are already placing jumpers depending on external supply or not. Why not add JP5 which disconnects the LDO for 5V? Safety first!
"Either JP1, JP2 and JP5 installed or JP3 JP4."

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Quote:
Why not add JP5 which disconnects the LDO for 5V?

This is a current PCB design and there may not be enough space for an additional jumper. Since this is not a production design but rather a calibration machine (likely build less than 10 units) maybe the solution is to go for it since it is likely that everything will be OK. If not, we can just unpopulate U1.

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Check the data sheets, and then verify current flows, with a meter. (include a test of adding external power, when selected for a different choice )

How important is current drain ?
You can make jumpers SPCO (3 pin) so users cannot link both ways, or you can use schottky diodes, and devices like PUMD3, to do active-detect and eliminate the jumpers entirely...

There is a market for a smarter regulator, one that can accept 5 V from USB style power packs OR 9V.

To meet this, the regulator should be 4.85V or similar out, and have true low saturate (< 100mV), and the RST output should fire at ~ 4,5v.
Reverse connection protection should be std.

Some I've looked at come close, but no suppliers tick all the boxes. They seem stuck in the past.

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aero1 wrote:
I have two sources of 5v on a PCB. One is from an on-board LDO and the other is from an external DC supply. If the external supply is attached and there is no input to the LDO will it be damaged?
Have you asked the manufacturer? A couple years ago, I had a similar question about a Linear Technology part, and got a double-checked answer from them within a few hours.

- John

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Yes, I have contacted TI and they said they will respond within 48-72 hours. We cannot wait that long.

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if it is a callibration jig you want it to be solid.
then better add reverse diodes to be really sure that when a spike goes through the 5V you do not blow the regulator.