[sdiy] Reducing maximum current through analog switches in ASM-2 ADSR

[Simon Brouwer]

Tom Wiltshire schreef:
> Simon,
>
> On 23 Nov 2008, at 20:16, Simon Brouwer wrote:
>> What do you guys think? The minimum attack time would become about 3 
>> to 4 ms. Is that something to be content with that or should I aim 
>> for a shorter attack time?
>
> 3 to 4 mS is still a reasonably fast attack. For really snappy 
> envelopes, you need to be closer to 1 mS - think Minimoog, Sequential 
> ProOne, SH101. But then again, lots of famous polysynths from the 
> early era of microprocessor-based envelopes couldn't manage times 
> anywhere near that short (Oberheim Xpander only updated the envelopes 
> every 10mS, and the Waldorf Wave is notoriously sluggish too).
>
> Do you really need to reduce the current so much? Is the chip actually 
> in any realistic danger?
I consider it bad practice if a design allows exceeding absolute maximum 
ratings, because you simply have no way of knowing if you can get away 
with it. Of course if your DIY synthesizer breaks down it's not a 
disaster like when you have to recall thousands of products for repair 
or modification if your decision to cut corners turned out not so well. 
But still...
> I'd have thought that although the peak current might exceed the 
> absolute maximum rating, if this is done briefly (say for 1mS!) it 
> might not cause any permanent damage. As the capacitor charges, the 
> voltage drop across reduces, and the current drops correspondingly. 
> Hence the period when this excessive current flows is actually very 
> short.
All I know is that the manufacturer specifies absolute maximum current 
of 25 mA, and they don't specify a peak pulse current (which you can see 
in datasheets of diodes, MOSFETs etc.). Who knows what the thermal time 
constant is of a switch in the 4053. It might heat up in much less than 
1 ms.
> Have you worked out roughly what the *average* current through the 
> switch would be during the attack curve?
It would be somewhat less than half the maximum (and that would still be 
too much).
> Alternatively, is there another version of the 4053 that you could use 
> that has a higher current rating without increasing the on resistance?
Maybe the Fairchild one, its datasheet does not specify an absolute 
maximum for the switch current at all...
However, 15V times 0.167A amounts to 2.5W which exceeds the maximum 
power dissipation of 700mW.

http://pdf1.alldatasheet.com/datasheet-pdf/view/50867/FAIRCHILD/CD4053.html

-- 
Vriendelijke groet, 
Simon Brouwer. 

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