[gta02-core] Caps for bass fix (ECN0022)
werner at openmoko.org
Sat Aug 15 20:34:49 CEST 2009
Joerg and I discussed the choice of capacitors for the bass fix on
#gta02-core. Some conclusions:
- as explained in ECN0022, the capacitor should have about 100 uF
- tolerance is only a minor concern, given that parts from the same
reel are likely to be similar
- the capacitor should be rated at 4 V or higher
- ceramic is unsuitable because of harmonic distortions
- Tantalum seems to be the best choice
- due to the frequency-dependent ESR of tantalum caps, the ESR should
be low (<< 1 Ohm)
- component size and height are critical. The tallest component we
have inside the GTA02 main can is C1707 with a height of 1.6 mm.
We can place the capacitors outside the can, but given that we now
have plenty of room inside, that may not be optimal.
Here's a document that compares various capacitor technologies:
Particularly the ESR chart on page 5 is interesting.
With this data, I went to Digi-Key and did a search with the
- SMT, in stock, lead free, RoHS compliant
- available as cut tape
- 47 uF ... 120 uF
- 4 V ... 6.3 V
- ESR <= 200 mOhm
- case EIA 3528-21 or smaller
The leaves a choice of products from AVX, Kemet, and Nichion.
Possibilities for inside the can, maximum height 1.6 mm:
- AVX TCJT686M004R0080, 68 uF, 4 V, 20%, 80 mOhm, EIA 3528-12
Digi-Key 478-3480-1-ND, USD 2.58/1
- Kemet T520T107M004ATE150, 100 uF, 4 V, 20%, 150 mOhm, EIA 3528-12
Digi-Key 399-5288-1-ND, USD 1.55/1
- Nichicon F320J476MAA, 47 uF, 6.3 V, 20%, 180 mOhm, EIA 3216-16
Digi-Key 493-3149-1-ND, USD 0.84/1
Note the smaller footprint of the Nichicon part. It could probably
still be considered a viable 3rd source, though.
Outside the can, height unlimited but trying to minimize footprint:
- AVX TCJA107M004R0200, 100 uF, 4 V, 20%, 200 mOhm, EIA 3216-18
Digi-Key 478-3453-1-ND, USD 2.48/1
- Kemet T528I686M004ATE200, 68 uF, 4 V, 20%, 200 mOhm, EIA 3216-10
Digi-Key 399-5243-1-ND, USD 4.85/1
- same as above
So it seems that we should plan on using a 3528 footprint, if possible.
If we can place it outside the can, that would be even better.
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