Power Supply Design - Input Protection Scheme

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Fred
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Re: Power Supply Design - Input Protection Scheme

Post by Fred »

If it's as high as 5.3 it's a reject. Spec says 4.9 to 5.1 IIRC. adjustable can be more finely trimmed.
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Re: Power Supply Design - Input Protection Scheme

Post by Dan »

ah yes, I am referring to the adjustable one in the RavAGE schematics (LM2931S I think it is) and not the fixed one (LM2937-5.0).
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Re: Power Supply Design - Input Protection Scheme

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Fred wrote:If it's as high as 5.3 it's a reject. Spec says 4.9 to 5.1 IIRC. adjustable can be more finely trimmed.
What spec? What's an IIRC?
Dan wrote:regulator output voltage is approx 5.02V when using correctly spec'd resistors (attention paid to tolerance)should never ever get to 5.30V.
The reference voltage for the LM2941 has a +/- 5% tolerance over temperature (see data sheet), so you are starting off with that.
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Re: Power Supply Design - Input Protection Scheme

Post by Dan »

TonyS wrote:The reference voltage for the LM2941 has a +/- 5% tolerance over temperature (see data sheet), so you are starting off with that.
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Table from datasheet below!

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Where does this +/- 5% you speak of exist?
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Re: Power Supply Design - Input Protection Scheme

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Re: Power Supply Design - Input Protection Scheme

Post by TonyS »

Look at the first Parameter ("Reference Voltage") on page 3 (sorry, but I do not know how to copy / paste from the data sheet).
So even if you use 0.01% resistors to obtain 1.275V at 5.00V, the regulator may try to regulate the output to obtain 1.211 to 1.339V at it's adjust pin.

Note that the "Output Voltage" graph is "Typical" and assumes that you were at 5.00V @ ~ 20C to begin with.
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Re: Power Supply Design - Input Protection Scheme

Post by Dan »

here you go

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Re: Power Supply Design - Input Protection Scheme

Post by Fred »

https://duckduckgo.com/?q=(1.313+%2F+1.275)+*+5 == 5.14901960784314 absolute worst case at 25C
https://duckduckgo.com/?q=%281.339+%2F+1.275%29+*+5 == 5.25098039215686 for full temperature range extreme

Typical values are MUCH better than specced, though. This can/could/should/will be part of test procedure. If your reg is pumping out too much, trim it down a bit before installing the clamp. And/or acceptable steady state current draw fully assembled post production before shipping.
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Re: Power Supply Design - Input Protection Scheme

Post by TonyS »

HUH?
So "Quality" and "DIY" and "NOOB" go out the window cause we don't like reality? A 5% part was picked, if that is not acceptable, pick another part or review the topology.
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Re: Power Supply Design - Input Protection Scheme

Post by Fred »

Ahh, what? I don't even understand what it is that you're *trying* to say. How many of these parts have you handled and measured? I've never seen one over 5.1V, and I've measured plenty. Nor under 4.9, for that matter. Those values (both hot and cold) represent the absolute corners of a bell curve, whereby the vast majority of parts come no where near those values. As with any design decision, you play into statistics with it. If you're happy for 5% to require rework to work properly, then you can pick a voltage based on that and a known statistical distribution.
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