Adam Wang wrote: > d) PTC, holds 50mA at 20?C[6], 1.5s time to trip when 150mA at 20?C, like > Tyco p/n: MICROSMD005F-2, Rmin = 3.6 Ohm, R1max = 50 Ohm, resettable way, > but recovery time is long, needs to carefully check few parameters to > ensure 0.305 usd/100pcs.
Yes, the R1max here worries me a bit. I haven't been able to find any information on the conditions under which R1max is reached (besides the fuse having tripped at least once). In our scenario, we could only afford a maximum resistance of 10 Ohm or a device drawing the maximum allowed 50 mA would drop more than 0.5 V (10%) of those 5 V. (Assuming here that a drop of 10% would still be acceptable, but I don't have any reference to back this up.) > Regards to that why selected 50mA, check [5] to meet two cases: > * If monitor is powered off, it may not draw more than 50mA. > * If monitor is powered on, it may not draw more than 10mA. In fact, we should deliver up to 55 mA, according to page 15 of [5]: "Note: The DDC required +5 volt power pin must be able to supply a minimum of 55 mA." Not sure where the 5 mA difference is supposed to go, though. Also not sure it we really care about distinguishing between 50 mA and 55 mA. The MIC2090's Ilimit is from 50-100 mA anyway, so it will almost certainly let a little bit more than 50 mA pass. Let's just hope monitors don't expect they can exceed their 50 mA by a lot. Well, the chip has a fault indicator. So at least M1r4 could tell us very quickly if it ever meets such device. > so indirectly shows in VGA system, draw current is > low even under 10 mA too. So we have one sample (out of a possible hundreds is not thousands) that would not need all that much effort :) Unless we can be sure monitors/TVs/projectors/etc. that draw more than just a few mA are extremely rare, it wouldn't be safe to design for less than at least 50 mA. I'm also not sure what would be the consequences of not providing a correct DCC5V supply. E.g., would the monitor refuse to power on ? Would is malfunction in some other way ? Or would this only affect the host's ability to read the monitor's characteristics, which should be a comparably minor issue. (As far as I know, we currently don't use any of this anyway.) > So Werner and me thought that candidate with Micrel part can work for it. > so will order few and test soon. Yes, I think this is the safest solution. Spending a few more cents on getting this right certainly beats having to worry about DCC5V as the potential cause each time a monitor problem pops up. - Werner _______________________________________________ http://lists.milkymist.org/listinfo.cgi/devel-milkymist.org IRC: #milkymist@Freenode
