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I've two questions:

  1. If I've a voltage regulator that says in the datasheet to use electrolytic capacitors is it possible to use 'normal' capacitors with the same capacitance?
  2. I've the circuit drawn in the figure below. Can I use 2,2 microfarad capacitor as input capacitor (instead of the 0,33 microfarad one) and a 1 microfarad output capacitor (instead of the 0,1 microfarad one)?

enter image description here

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  • \$\begingroup\$ The caps shown are the minimum for the IC stability under all conditions. The total input cap can be much higher and usually is, but you should have that minimum cap close to the regulator. The output cap can be larger, but be aware it will alter the risetime of the output when power is applied. This normally isn't an issue for rectified mains low voltages but may be critical where you have multiple supplies that are expected to rise in a given order. \$\endgroup\$ – Jack Creasey Jun 25 '18 at 22:36
  • \$\begingroup\$ So for voltages up to 20 volts with max 100 mA, that will be no problem?! \$\endgroup\$ – Hhsja Jun 25 '18 at 22:42
  • \$\begingroup\$ Depends where you input DC comes from. If it's from a battery or another DC supply you may not need to add any capacitance. If you are rectifying the output of a low voltage transformer, then you'd need more on the input to filter the rectified DC. \$\endgroup\$ – Jack Creasey Jun 25 '18 at 22:52
  • \$\begingroup\$ In this case bigger is better and you are good to go. From TI "A dangerous precedent was established by the first linear regulator semiconductors sold commercially like the LM7805 type devices: they require no input or output capacitor and are completely stable under virtually any operating conditions. Some of the newer LDO regulators require careful attention to external capacitors to operate in a stable mode. The main reason the 7805 is unconditionally stable is because the power pass transistor was made up of an NPN Darlington" \$\endgroup\$ – lucky bot Jun 26 '18 at 3:24
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You have to check the datasheet for the specific regulator. Some require a certain level of resistance in series with the capacitor, and recommend electrolytic (or tantalum) capacitors due to the high ESR. However, ESR is specified as a maximum, and future capacitors with far lower ESR may still be within their own specs but have too low ESR for the loop stability.

These types can be used with ceramic caps, but to ensure stability, it is necessary to put a small series resistance (typically <1ohm) in to ensure stability. Other LDOs are specifically designed to be stable with ceramic caps.

Note when using ceramic caps that you won't always get the capacitance you're paying for. Using class 2 ceramics for supply bypass is worst case in that your AC voltage across the cap is minimal (lowering the capacitance) while the DC voltage is probably close to the rating (further lowering the capacitance). This is a good tool to see those effects...ceramic chip caps will behave about the same for the same size and value from different vendors.

Although as a general rule more capacitance means better performance, each regulator has its own idiosyncracies. Also remember that the output capacitance doesn't just include the capacitor closest on the schematic...it includes all the bypass caps connected to that supply.

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