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In most of my designs recently I've been quite liberal in using decoupling caps (especially 0.1uf X5Rs) on most if not all of the supply pins for anything that performs some remotely significant switching (especially when the datasheet/application schematic specifies fewer caps than it has supply pins).

Is this an ok/good practice or is it important to stick to the manufacturer guide and just use what they specify?

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  • \$\begingroup\$ Nowadays I usually use 1 uF. A 1 uF cap in 0805 package has lower impedance across frequency than the 100 nF and 10 nF thru hole decoupling caps used back in the pleistoscene. Even though the resonant frequency of a 1 uF is a little lower than 100 nF, most still have lower impedance across a wide spectrum. Unless you have a unusual RF application, just use 1 uF SMD. In one RF application I actually used 100 pF decoupling caps in parallel with 100 nF just to cover a larger frequency range. \$\endgroup\$ – Olin Lathrop Apr 16 '15 at 15:04
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Not really, no. The only issue I can think of quickly would be inrush current on turn on (which can be a problem for e.g. USB peripherals) but this isn't too hard to deal with with a slow ramp up circuit.

Of course there's cost and board space, and if the circuit works fine and meets emissions regulations then adding extra capacitors isn't necessary.

Although they are a starting point, don't always assume the app notes are the final word on how to design your circuit - if it doesn't function as desired following their advice, then do it your own way. App note vary considerably in quality - some are generally reliable (e.g. LT, National Semiconductor, TI usually), and some app notes are pretty bad and contain mistakes, so always double check things.

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  • \$\begingroup\$ Actually it can be a problem with a Switch Mode PSU driving the circuit. If the ESR of the combined decoupling is too low the SMPS goes nuts as it can't create a large enough error feedback signal. Even with a slow start it just oscillates (Though there are tricks to fix this like injecting a fake error signal) \$\endgroup\$ – Jason Morgan Apr 3 '14 at 8:18
  • \$\begingroup\$ @JasonMorgan using decoupling caps only on the output of a SMPS is a terrible idea anyway, their datasheet should specify minimum output capacitance (as close as possible to the switching output inductor and IC) and usually specify a max ESR (and usually omit minimum ESR, which is your point) but it's expected that you have a "medium"-ish ESR worth of filtering capacitors there - some ceramics, and some tantalum/niobium/aluminium electrolytics. \$\endgroup\$ – KyranF Apr 16 '15 at 15:25
  • \$\begingroup\$ @KyranF I did not suggest anything about adding caps only at the output of an SMPS. I stated there CAN be issues with too much decoupling. \$\endgroup\$ – Jason Morgan Apr 18 '15 at 10:00
  • \$\begingroup\$ Some Voltage regulator chips from some manufacturers have gone unstable with overly good decoupling .these chips are still used today and are shown on www.badbeetles.com \$\endgroup\$ – Autistic Sep 10 '15 at 4:33
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In the case of a circuit containing a device with a power on reset mechanism, too much capacitance on the VCC (or power input pin) can cause the voltage at the power pin to rise too slow to meet the devices dV/dT requirement. This is sometimes remedied by adding an RC network at the devices 'RESET' pin to allow the Reset pin to stay active long enough to allow the power pin to stabilize to a valid operating voltage level.

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The trouble with too much decoupling capacitors is that if the layout is bad, i.e. high trace inductance all the decoupling capacitors in the world won't help. So you might actually never realize you have a decoupling issue, because you think it can not be the decoupling...

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