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I order the REG1117-3.3V IC and after reading the REG1117 datasheet I found the following on page 8 :

Figure 1 shows the basic hookup diagram for fixed-voltage models. All models require an output capacitor for proper operation, and for improving high-frequency load regulation; a 10µF tantalum capacitor is recommended. Aluminum electrolytic types of 50µF or greater can also be used. A high-quality capacitor should be used to assure that the ESR (Effective Series Resistance) is less than 0.5Ω

I will use this regulator to power from microUSB (5V to 3.3V) an ESP8266 and several MPR121 (IC sensor). I was wondering which value for the ESR capacitor should I order ? I don't know which value for the capacitor because there is no other information on the minimal value to avoid instability.

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  • \$\begingroup\$ It's not a bulk capacitor and a low ESR capacitor. It's a bulk capacitor with a low ESR. The value is given in the text above. That's an old part and datasheet, these days a ceramic may be easier to find that meets the requirements. \$\endgroup\$ – Andrew May 9 '17 at 11:08
  • \$\begingroup\$ @Andrew is spot on, go for a ceramic 10 uF or two in parallel if you can. For the record, using smaller devices in parallel to get the same total capacitance costs you more parts and assembly but brings the effective ESR down because each capacitor's ESR is in parallel with the other capacitor ESRs. \$\endgroup\$ – TonyM May 9 '17 at 11:13
  • \$\begingroup\$ @Andrew I will go for ceramic 10 uF, but when looking for an ESR value below 0.5 Ohm, should I go for 0.4 Ohm or really more slower like 0.05 Ohm ? \$\endgroup\$ – Henri Koch May 9 '17 at 11:19
  • \$\begingroup\$ @HenriKoch 0.4 should be fine for normal applications. You could even get away with over 0.5 in some situations depending on the nature of the connected loads and the board layout. \$\endgroup\$ – Andrew May 9 '17 at 11:33
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    \$\begingroup\$ You need to be careful when looking at output ESR on older LDO regulators as many of them required a minimum ESR as well as a maximum. In this particular case, it looks ok for really low output ESR as the output is a NPN emitter. \$\endgroup\$ – Peter Smith May 9 '17 at 12:48
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Comment reposted as an answer so that the question can be closed:

The datasheet isn't describing a bulk capacitor and a separate low ESR capacitor. It indicates you need a bulk capacitor with a low ESR. The values is given in the text above.

Keep in mind that this is an old part and old datasheet, these days a ceramic capacitor that meets the requirements is fairly easy to find and is probably a better choice.

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  • \$\begingroup\$ I can't find on Farnell ESR for ceramic, so any ceramic capacitor should be ok for the stability ? \$\endgroup\$ – Henri Koch May 9 '17 at 13:00
  • \$\begingroup\$ The ESR curve showing ESR Vs Frequency should be in the parts datasheet. But generally yes, the ESR on ceramics is so much lower than for Aluminium or Tantalum parts that virtually any of them should be fine. \$\endgroup\$ – Andrew May 9 '17 at 13:14

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