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Author Topic: Do I need cap's around 3V3 regulator when source is regulated 5V?  (Read 1848 times)
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Hi...  I have a circuit in which I am using a 7805 to regulate a 12V DC wall-wart into 5V.  Around the 7805, I have a 100uF cap across the input and a 10uF around the output.

Now, I need to have 3V3 also for a particular part.  So I'm introducing a LD1117V33 also, using the 5V as input.  The question is, do I ALSO need caps across the input and outputs on this regulator?  And if so, how do I determine the size of the caps?

Also, I am considering a LP2950-33LPRE3 regulator in place of the LD1117V33 to save space and because 800mA is overkill, 100mA is plenty.  Would that change anything wrt the caps?

Thanks!
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From the NCP1117 data sheet

Quote
Input bypass capacitor Cin may be required for regulator
stability if the device is located more than a few inches from
the power source.

As I assume they are close to each other I would say no for the input but yes for the output. For the MCP it's a minimum of 4u7.

Don't know about the 2950.

Another one to look at is the MCP1700.
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« Last Edit: March 22, 2011, 10:20:11 am by Graynomad » Logged

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Another one to look at is the MCP1700.

Ah yeah, MCP1700 is hotness.  Thanks!  And I can run it with 1uF's.  "A minimum output capacitance of 1.0 μF is required for
small signal stability in applications that have up to 250 mA output current capability. The capacitor type can be ceramic, tantalum or aluminum electrolytic."
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Output capacitor is more important than the input, especially since you already have one on the input due to the output capacitor of the 7805.

But for LDO regulators like the LD1117 and the LP2950 you have to be careful with the capacitor type. It cannot have too low an ESR (equivalent series resistance) nor too high an ESR else the regulator may oscillate. Generally tantalum or electrolytic capacitors are appropriate for LDO output caps, or a ceramic cap (very low ESR) with a 0.1 ohm resistor in series to ensure minimum ESR.

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But for LDO regulators like the LD1117 and the LP2950 you have to be careful with the capacitor type. It cannot have too low an ESR (equivalent series resistance) nor too high an ESR else the regulator may oscillate. Generally tantalum or electrolytic capacitors are appropriate for LDO output caps, or a ceramic cap (very low ESR) with a 0.1 ohm resistor in series to ensure minimum ESR.

Ok, thanks.  So how do you figure out the ESR?  I am looking at K105Z20Y5VF5TL2 Multilayer Ceramic Capacitors (MLCC) - Leaded 1uF 50volts +80/-20% Y5V 2.5mm LS. Datasheet.  But from the data sheet, I cannot find the ESR at all.  Or is it hiding in there somewhere?
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Multilayer Ceramic Capacitors are not normally used when you want a capacitor with low ESR, that's why they don't put it in the data sheet. You will find in in the sheets of polarised capacitors. By and large if it isn't in the data sheet they are not proud of it.
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Thanks, guys.  Man, electronics is bewildering...

Ok, so now I am looking at this part:  Lelon RXJ010M2ABK-0511P Aluminum Electrolytic Capacitors - Leaded 100V 1.0uF 105C 5x11 mm Datasheet.  It says this, "Low ESR, suitable for switching power supplies", and it has "Impedance" all over the place, which is I think what we're talking about, right?  7 Ohms for this guy.  Low enough?
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Quote
Multilayer Ceramic Capacitors are not normally used when you want a capacitor with low ESR, that's why they don't put it in the data sheet. You will find in in the sheets of polarised capacitors. By and large if it isn't in the data sheet they are not proud of it.

The opposite is true. MLCC's have extremely low ESR (see this presentation for example), lower than that of tantalum, and often too low for LDO's that expect a minimum ESR for loop stability.

The RXJ010M2ABK-0511P is definitely overkill for this application (100V??) but its impedance is fine -- 0.16 ohms at room temperature (I'm not sure where you got the 7 ohms number from).

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The RXJ010M2ABK-0511P is definitely overkill for this application (100V??) but its impedance is fine -- 0.16 ohms at room temperature (I'm not sure where you got the 7 ohms number from).

Ok, can you help me find where you got 0.16?  For my number, I looked at the tables on pages 2-3 until I found the section for this unit (100V (2A)), then looked at the row for 1uF, and followed over to the column for "Impedance / 20C".  Voila, 7!

Mouser has a category for "Low Electrolytic Capacitors", and this is the ONLY stocked product in that category at 1uF.
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Sorry, my bad, I thought you had picked out the 100uF capacitor. Indeed the 1uF capacitor has an impedance of 7 ohms, not really a good choice. I'd maybe get a 33uF 16V capacitor which only has an impedance of 1.3 ohms. 47uF 16V would be even better, with only 0.6 ohms of resistance.

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Brilliant.  Ok, I also have to factor for Mouser stock and BOM cost, so that leads me to ...

Panasonic EEU-FR1E470 Aluminum Electrolytic Capacitors - Leaded 25VDC 47uF 5x11mm LS2mm Datasheet 0.300 ohms
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