Hi, the datasheet of 7182 (and I suspect this question would be relevant for all adjustable ADP71xx) says "RFB2 must be less than 120 kΩ...", but doesnt mention how low it is allowed to be.
I would like to have -9 V to balance 7102 (strange that ADI doesn't provide such fixed output option), so by going through the resistors availible on market I found a perfect pair at 10200/1600 Ohm. But I am not sure if it is appropriate. Should I perhaps scale it by 5 for 51/8 kOhm or by 10 for 102/16 kOhm?
HAW, it doesn't appear your values will work. Please try RFB2 = 115k and RFB1= 732K
for more help on this matter please download our linear regulator tool.
Why wouldn't they work?
It appears rather that your tool doesn't work: looks like it picked the biggest suitable value under 120 kOhm for RFB2, while Figure 2 and Figure 87 in the datasheet suggest that RFB2 can be (and probably should be) much smaller. And thats what I asked: how small it can be?
To increase the output voltage accuracy, choose the resistors with low tolerance, E96 (1%) or E192 for instance. RFB2 in several kohm would be fine. There is no specific min value for "how small it can be". You don't want to make it too low as the power dissipation due to the FB resistor divider is Vout^2/(RFB1+RFB2).
Also, to minimize the noise gain due to the external resistor divider, it’s good to choose the fixed voltage option that is closest to the target adjustable option.