Electronics > Beginners

Put LC filter before or after LDO?

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antercreeper:
I have tested(filter in front of the LDO)
And result is I need to add more output capacitors maybe 10uF or more, because the load generate ripples on the output(about several millvolts). while the load is shut down and ripples are gone. (In fact I think the biggest reason is MLCC's capacitance degraded by DC bias  :-DD I forgot this and mess all things)
maybe later I can change the input filter capacitors from MLCC to tantalum one, but by now haven't observed any problems.

MathWizard:

--- Quote from: Nominal Animal on April 23, 2024, 09:08:04 pm ---

I usually power my own circuits from USB 5V, and that has a limit of maximum 10µF capacitance; so, I tend to use regulators that tolerate sufficient capacitance on the output to give me stable voltages.  For ADCs and similar having their own supplies, I like to use a ferrite bead to absorb/reflect the high-frequency noise (say, above 10 MHz).


--- End quote ---
So do mean for a typical +5V rail of a USB outlet, some spec says 10uF is the max capacitive load at some frequency? Surely I've seen much bigger caps on the input of USB devices.

Manul:

--- Quote from: MathWizard on May 04, 2024, 10:45:51 am ---
--- Quote from: Nominal Animal on April 23, 2024, 09:08:04 pm ---

I usually power my own circuits from USB 5V, and that has a limit of maximum 10µF capacitance; so, I tend to use regulators that tolerate sufficient capacitance on the output to give me stable voltages.  For ADCs and similar having their own supplies, I like to use a ferrite bead to absorb/reflect the high-frequency noise (say, above 10 MHz).


--- End quote ---
So do mean for a typical +5V rail of a USB outlet, some spec says 10uF is the max capacitive load at some frequency? Surely I've seen much bigger caps on the input of USB devices.

--- End quote ---

Yes, there is such spec for maximum capacitance. Frequency does not play here. Capacitance ceiling is meant to limit inrush current. There are a couple of reasons why such spec exists.

USB is designed to be hot plugged, so a high (and long duration) inrush current may cause momentary voltage dip on the entire bus, thus resetting or otherwise affecting other connected devices. Minimum bus voltage by spec is I believe 4.75V, so there should be no dips below that during hot plugging. Even one step further - it is possible that a high inrush may simply trip bus overcurrent protection.

Second reason is durability of connectors. Type A might seem like a fairly robust one, but mini B and micro B are real tiny. Those pins are certainly not designed with sparking in mind and will deteriorate quickly during hot plugging if inrush current is high.

Similar thing exists with boards meant for hot swapping, for instance, CompactPCI boards. They have hot swap controllers which control power rail inrush and usually also precharge I/O lines, so there is less glitch when you insert a board into a powered, working system.

Sure we live in a free world and rules can be broken, but it usually comes with consequencies.

Nominal Animal:

--- Quote from: MathWizard on May 04, 2024, 10:45:51 am ---for a typical +5V rail of a USB outlet, some spec says 10uF is the max capacitive load

--- End quote ---
Yes, as Manul wrote above.

If one wants their USB device to work with all sorts of USB hosts, then 10µF input capacitance should not be exceeded.
That does not mean that higher input capacitances won't usually work.


--- Quote from: MathWizard on May 04, 2024, 10:45:51 am ---Surely I've seen much bigger caps on the input of USB devices.
--- End quote ---
Me too.  In fact, for very power hungry devices on tiny SBCs (where the USB bus voltage is the internal 5V rail), extra capacitance can add stability.  For early 3G USB 2.0 modems having exactly such issues, Olimex created USP-CAP, with 2000µF bulk capacitance between USB +5V and GND.

However, using such a device will surely trip the overcurrent protection due to initial inrush current on most modern desktop and laptop devices.

MrAl:

--- Quote from: antercreeper on April 20, 2024, 05:27:40 pm ---Put LC filter before or after LDO?
(Attachment Link) (Attachment Link)

--- End quote ---

One other benefit to having an LC filter on the front end is that it will reduce inrush current when the circuit is hot plugged into a source of power, or the source of power is just turned on.  The inductor acts as a quasi-isolation mechanism for the regulator in that respect.

Every circuit with an added LC network must be tested carefully though because when inductors are involved they can act like temporary voltage boost elements which can cause high voltage spikes that can blow out other components either quickly or over time.

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