Hi, On Tue, Apr 19, 2011 at 8:00 PM, Sebastien Bourdeauducq < [email protected]> wrote:
> Hi, > > On Tue, 2011-04-19 at 19:32 +0800, Adam Wang wrote: > > c) microSMD110F-2, hold 1.1A, trip 2.2A, 6V, 0.21 Ohms, PD MAX 0.8W > > > > **MM1 needs 0.42A under reconfiguration** > > **MM1 needs 0.79A after booted** > > I'd rather use a 2A total hold current, since the USB ports can draw up > to 1A plus those ~0.8A of MM1 circuits. > > Also that 0.21 ohm series resistor would drop 0.42V @2A, which is quite > a bit. > > I'd go with a stronger fuse with less resistance. This might require a > more robust zener diode as well. > > When used miniSMDC125F-2, hold 1.25A, trip 2.5A, 6V, 0.14 Ohms, without MM1 circuit, it needs at least 2 seconds to trip. Meanwhile a 5W Zener can't afford normally. Mouser or Digikey is hard to get over 5W 5.1V though. If 2A like miniSMDC200F-2, trip @ 4A, 0.07 Ohms, although drop 0.14V @ 2A holding. Thus when input stays 5.1V + 0.14V = 5.24V. Under this condition. fuse starts to hold even all works well with two USB host ports @ 1A driven. See page 12 of http://www.circuitprotection.com/catalog/smdDS.pdf If I needs 1 second Time-to-trip Curves at 20°C only when fuse meets 8A occurred. And 10s @ 4A. The fuse actually won't trip less than 1s whenever either fault current on MM1 circuit or more higher input injection. Also I quite doubt that I can find a suitable Zerner to afford this time that diode doesn't fail within 10s. If with regards to all considerations above, I'd rather to use regulator though. How do you think? -adam
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