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Part Number: LM317
My application is to post regulate an SMPS output with +5V and -5V.
Is it ok to use LM317 where the input is floating, and the output is nailed to ground. A picture will make this clear.
This is the normal way to connect up the LDO. Vbat- connects to ground and Vbat+ connects to LM317 input. Vout+ is a positive voltage relative to ground. Okay.
Now consider this alternative. The input battery is floating, so just connect the LM317.OUT to GND and use the Vbat- as the output. I think this works, just not sure how good.
In my case, Vbat is the floating output of an SMPS supply.
I am considering this because the negative LDO LM337, has crummy frequency response as compared with the LM317.
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In reply to gene glick:
Here is the normally configured LM317
Are you attempting to convert a positive input voltage to a negative output voltage? Linear regulators are designed to convert a positive input voltage to a lower positive input voltage, or to convert a negative input voltage to higher (less negative) negative output voltage. Although the LM317 could be used in the circuit shown by the first picture to achieve a negative output voltage, it was not characterized or tested for operation this type of application. As such, the performance specifications will likely differ from those listed in the datasheet.
If you are attempting to convert a negative input voltage, LM317 can be used with the following circuit to regulate a negative output voltage. The output voltage is calculated with the following equation:
Vout = -Vref * (1 + R2/R1) - Iadj * R2
I would also like to point out that the TPS7A33 is a good alternative to the LM337 if you do not require more than 1 A output current. It is a negative output ultra low noise LDO with high power supply rejection (PSRR) making it very effective as a post regulator.
In reply to Gerard Copeland:
In reply to gene glick72:
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