Author Topic: Can I damage a linear regulator by leaving Vin open and connecting Vout?  (Read 2906 times)

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Offline 0xdeadbeefTopic starter

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Let's say I want to put an existing CPU board with an onboard LM1117-33 linear regulator on top of a larger motherboard. Now the motherboard should provide the 3V3 voltages for the whole system including the CPU board and the 5V supply of the CPU board (Vin of the LM1117-33) is left disconnected. So the 3V3 supply from the mainboard is directly connected to the output (Vout) of the LM1117-33 on the CPU board.

My question is: could this damage the LM1117-33 or have other unfavorable side effects?
From what I can see in the LM1117's datasheet, a floating Vin should mean that all the bipolar transistors are inactive and so nothing bad should happen. I also did a small experiment with a LD1117V33 I had at hand: the (open) input could be measured at ~2.9V with 3.3V connected to the output, which I guess is the voltage drop over the NPN transistor between Vin and Vout. I measured ~4.8mA going into the output. On a scope, the output looked flat (no oscillation).

So does anybody have either the insight or experience to tell me if there are issues to be expected with this constellation?
« Last Edit: June 23, 2013, 04:12:11 pm by 0xdeadbeef »
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Offline SeanB

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Really depends if there is a load from the input to ground that will pull current and the actual regulator. Best is to place a low drop diode across the regulator so input is at near the same as the drive side, or disconnect it completely.
 

Offline 0xdeadbeefTopic starter

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Problem is that the CPU board has a fixed layout and before soldering diodes on it, I would cut the trace on the 3V3 regulator output or lift the according pin.
This is my emergency plan, but I'd prefer to keep the board unaltered.
Trying is the first step towards failure - Homer J. Simpson
 

Offline SeanB

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If it does not fry or stop working just leave it be, or connect the 3v3 rail to the input of the regulator as well to keep it happy. That way it will just sit there drawing quiescent current with the output drive shut off.
 


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