On Thu, Feb 9, 2012 at 8:53 PM, Werner Almesberger <wer...@almesberger.net>wrote:
> S?bastien Bourdeauducq wrote: > > This way the board can sustain continuous short circuits on the DDC > > power supply. > > That sounds like a wise precaution to me. Would a 500 mW heat > source on the PCB cause any issues for neighbouring components ? > > If yes, we'd have to find some other way to limit the current. > > > a) This assumes for short current at 100 mA, this two * 1206/100 Ohm would reach rated power at 70°C[1] is 1/4 W. Also DDC voltage to monitor will be dropped. Insensitive one could not smell short occurs up and resistors get aged, even PCB surface is still good. Of course shorting on DDC is extremely few chance in the end. Cheapest way(0.022 USD/10pcs), not resetable, no guarantee. Then considering to candidates: b) MOSFET circuit, I was wrong on thinking N-type that is low-side current limit[2] study. There still exists N channel high-side current limit chip. But I only checked: P-MOSFET NCP380LSNAJAAT1G[3] with adjustable at OCP(over current protection), 6-TSOP, 70 mOhm, reverse voltage protection, needs an external resistor used to fine set current-limit at 50mA threshold(waste time), 0.57 USD/100pcs, c) Werner found another MOSFET Micrel p/n: MIC2090-1YM5[4] with fixed 50mA SOT-23-5 exactly, 20ns fast reaction time to hard short, Rdson = 700 mOhm @ Iout = 50mA, auto-retry overcurrent and short protection, no needs to determine an extra resistor to fine tune, 0.729 usd/100pcs. 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. Be noted that although M1R3 have 2A PTC on 5V path already, keeping DDC 5V tripped prior to the protection of whole DC power in is must if using this way. From Figure S8 curve A, a fault current 1A[7] can let PTC tripped within 50ms. 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. I measured M1R3's J17:9 pin(5V) while vga monitor both OFF and ON, from scope to see voltage on monitoring DDC circuit stays the same as 5V. This means that drop voltage on R143/47 Ohm is very few though( assume under 10mA * 47 Ohm = 470mV). so indirectly shows in VGA system, draw current is low even under 10 mA too. A DVI-I monitor's DDC should work compatibly to VGA. So Werner and me thought that candidate with Micrel part can work for it. so will order few and test soon. Thanks, - adam [1] page 5 on http://datasheet.octopart.com/RC1206JR-071ML-Yageo-datasheet-118639.pdf [2] http://www.vishay.com/docs/70596/70596.pdf [3] http://search.digikey.com/us/en/products/NCP380LSNAJAAT1G/NCP380LSNAJAAT1GOSCT-ND/ [4] http://search.digikey.com/us/en/products/MIC2090-1YM5%20TR/576-3890-1-ND/ [5] page 15 on http://www.ddwg.org/lib/dvi_10.pdf [6] page 118 on http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Catalog+Section%7FSURFACE-MOUNT%7FA1%7Fpdf%7FEnglish%7FENG_CS_SURFACE-MOUNT_A1.pdf%7FRF1350-000 [7] Curve A in Figure S8 on page 127 on [6]
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