Author Topic: Did I order the right caps ?  (Read 1806 times)

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Offline cosmicrayTopic starter

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Did I order the right caps ?
« on: January 30, 2015, 03:42:50 am »
I ordered some caps, to build the Simple Negative Voltage Converter as shown in the ICL7660A data sheet (same basic circuit in most any 7660 data sheet). It calls for two 10uf caps (polarized electrolytic) for the charge pump.

In ordering the caps, I was more focused on the ESR, voltage, and uF, and totally blitzed out to other specs (that now I am waking up to). What I ordered was a 1210 MLCC 10uF 16V X5R ceramic cap (Samsung). The datasheet shows an ESR of about 0.03 ohms at 10 kHz.

But now it dawns on me that these are not polarized. In researching this, I noted several threads talking about MLCC caps, and how they present quite different capacitance values depending on the DC bias voltage. The Vcc for the 7660 will be +5V, so that I can generate a -5V rail. Current draw will be negligible, likely less than 5 mA.

Will these caps work, or do I need to go back and get real polarized electrolytic type ?
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Offline helius

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Re: Did I order the right caps ?
« Reply #1 on: January 30, 2015, 04:03:07 am »
I don't know if it will work (X5R can present a capacitance of 40% less than nominal with 5V bias), but you can either overrate the capacitor, or use a type that doesn't suffer that effect. SMD tantalums are very good for that application.
 

Online wraper

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Re: Did I order the right caps ?
« Reply #2 on: January 30, 2015, 04:11:53 am »
There is no obvious reason why they would not work. About capacitance drop under voltage, As they are 1206, likely it is not so horrible like if they would be 0805. Anyway, there is no reason why IC would not work with half of the capacitance.
 

Offline c4757p

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Re: Did I order the right caps ?
« Reply #3 on: January 30, 2015, 04:18:41 am »
Nothing requires polarized types, they were just emphasizing that they don't have to be non-polarized (a fact that might not be obvious to someone unfamiliar with charge pumps).

They're pretty big (physically) caps and X5R, so they'll probably be fine. You might be able to find a capacitance-vs-voltage plot for them (Samsung had them, but they seem to have fucked up their website and I can't get it to produce the characteristics sheets anymore; DigiKey appears to be caching them though) if you're concerned.
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Offline cosmicrayTopic starter

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Re: Did I order the right caps ?
« Reply #4 on: January 30, 2015, 04:36:33 am »
They're pretty big (physically) caps and X5R, so they'll probably be fine. You might be able to find a capacitance-vs-voltage plot for them (Samsung had them, but they seem to have fucked up their website and I can't get it to produce the characteristics sheets anymore; DigiKey appears to be caching them though) if you're concerned.
I have the characteristic sheet, which I found via DigiKey. The part # is CL32A106KOJNNNE. I suspect the chart is the marked "DC Bias characteristics". My impression was around 5-10% down at 5v.
it's only funny until someone gets hurt, then it's hilarious - R. Rabbit
 


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