LDO reverse current protection diode

Thread Starter

upand_at_them

Joined May 15, 2010
766
I'm familiar with the two typical ways of using a diode for reverse current protection of a voltage regulator: 1) diode from reg output to reg input and b) diode from power supply to reg input.

My question is: If I put a voltage at the reg output, and leave the reg input disconnected, is it still safe? Because it appears to be the same thing as having the diode between supply and reg input, i.e. no current can flow. But I'm wondering if I could still damage the regulator.
 

jpanhalt

Joined Jan 18, 2008
11,088
Source: LM117/LM317 datasheet
PROTECTION DIODES
When external capacitors are used with any IC regulator it is
sometimes necessary to add protection diodes to prevent the
capacitors from discharging through low current points into
the regulator. Most 10 μF capacitors have low enough internal
series resistance to deliver 20A spikes when shorted. Although
the surge is short, there is enough energy to damage
parts of the IC.
When an output capacitor is connected to a regulator and the
input is shorted, the output capacitor will discharge into the
output of the regulator. The discharge current depends on the
value of the capacitor, the output voltage of the regulator, and
the rate of decrease of VIN. In the LM117, this discharge path
is through a large junction that is able to sustain 15A surge
with no problem. This is not true of other types of positive
regulators. For output capacitors of 25 μF or less, there is no
need to use diodes.
Yes. The diode conducts to the input rather than the internal circuitry, so it is no problem. Now, if you plan to use it for perpetual motion, then it won't work work in that application. ;)
 

Thread Starter

upand_at_them

Joined May 15, 2010
766
My application is this. I have an 5V LDO for a microcontroller circuit. The few times when I would update the firmware I'll be applying Vcc directly to the microcontroller, which is also the LDO output. And in this case I won't have anything at the LDO input.
 

MisterBill2

Joined Jan 23, 2018
8,760
A diode from regulator out back to regulator in is the only one that makes any sense, since the goal is to protect the circuit inside the regulator IC. The problem is that without the internal circuits powered current will flow into the output and cause damage.
 

jpanhalt

Joined Jan 18, 2008
11,088
I think the TS is missing the point. Those VR's are designed to accommodate an input voltage well above the output + a diode drop. However, they are not designed to have the output a lot more than the input, say 5V more. A diode from output to impt will will avoid that condition in ALL situations, regardless of whether the input is connected or disconnected.
 
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