[time-nuts] FE-5680A Voltage?

[Bob Camp]

Hi

The issue is the efficiency drop off as the voltage differential goes low. The ebay boards normally  don't have any heat sinking on them. The LPRO pulls full current for quite a while when it warms up.

Bob

On Sep 12, 2013, at 10:40 PM, Orin Eman <orin.eman at gmail.com> wrote:

> Yes, my reading of the datasheet was the same.  18V in would be fine for
> 15V output.  My concern would be how noisy the output would be.  I have 9
> of the from China ebay boards here (gave one to my boss to play with).  I
> should wire one up and take a look.  At any rate, I think a turn or two of
> the output wires through a ferrite bead would be in order.  The data sheet
> shows a secondary filter of 3uH/180uF as well which I'd certainly want to
> use to feed a 5680A.  Shouldn't do any harm, might do some good.
> 
> Orin.
> 
> 
> On Thu, Sep 12, 2013 at 7:19 PM, Robert LaJeunesse <
> rlajeunesse at sbcglobal.net> wrote:
> 
>> Looking at the LM2596 datasheet the switch transistor saturation voltage
>> is 1.5V max at 3A (1.16v typ). That and an educated guess of about 0.25V
>> for inductor loss combine to set the minimum difference between input and
>> output. The 1.23V reference is about +/-3% accurate, so after setting the
>> "buckboard" to 15.00V out (at 25C) expect it to shift at most 0.5V over
>> time and temp. Add that to the minimum in-out difference and now you are at
>> a minimum safe input of 2.25V above the nominal 15V output, or 17.25V in.
>> An old 19V 3A laptop supply might be a good supply to use for this, if you
>> were local I'd hand you one.
>> 
>> As for efficiency, that just tells you the amount of heat the regulator,
>> plus inductor, plus diode, etc. will throw off. FWIW the datasheet Figure 5
>> indicates you should expect about 88% or better efficiency for 15V out and
>> at least 2V higher input voltage. (Doing some switching power supply design
>> for a living does help with my understanding of them, but not ever enough!)
>> 
>> Bob LaJeunesse
>> 
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