LM1084: Output RC snubber if using film capacitor on output

Hi Mischa, 

1) For the LM1084-ADJ, what is the approximate output impedance across the audio band (20Hz-20Khz)

--The output impedance is not a standard datasheet item that we would have in the datasheet. You will need to make the measurements based on the load condition you have. 

2) The datasheet specifies an electrolytic or tantalum capacitor on the output of the LM1084 as a large enough ESR is necessary for regulator stability. Would an appropriately sized RC snubber in parallel with another low ESR capacitor provide the necessary zero pole at 500khz for regulator stability? If yes, how would I determine the required RC snubber component values?

--I would not recommend using RC snubber to be in parallel with a ceramic cap to meet the ESR requirement. When a low ESR ceramic capacitor is used, a series resistor needs to be added(no in parallel). 

3) If an RC snubber is not able to provide the necessary zero on the output, what ESR is sufficient for an electrolytic capacitor in order to assure stability? If low ESR electrolytic capacitors are used, is there a formula that allows calculation of the sufficient ESR of the capacitor relative to the capacitance value?

--The LM1084 design is done in the 1990s, and most of the LDOs designed 20-30 years ago are mainly working with capacitors with higher ESR than the ceramic ones. The 500KHz is given as an empirical number and I would suggest following the datasheet's recommendation: The recommended load/decoupling capacitance is 10-uF tantalum or a 50-uF aluminum. These values will assure the stability for the majority of applications. For newer LDOs, we have better recommendations/guide on stability and most of them work with ceramic capacitors and the low ESR will result in better AC performance. To verify the stability, I recommend using the load transients method described in this app note. 

Regards, 
Jason Song