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Subject: Is USB bus power stable enough?
Date: Thu, 24 Oct 2002 02:19:05 +0200
Organization: XS4ALL Internet BV
NNTP-Posting-Date: 24 Oct 2002 00:14:20 GMT
X-Newsreader: Forte Agent 1.8/32.548
At the moment I am working on an USB smartcard programmer.
The smartcard reader is bus-powered and has a step-up regulator to
generate a Vpp programming voltage (Vout is approximately 13v).
I have some doubts about powering the smartcard from the Bus-power.
Is this voltage stable enough? What about glitches (I use a 4.7uF
buffer cap)? Is the voltage high enough if there are any hubs between
the programmer and the root-hub?
I can also derive the card voltage (with 78l05) from the step up
convertor (which works with an input voltage down to 3.5VDC)?
The disadvantage of using a regulator after the step up convertor is
the inefficient use of power. The advantage is the stable voltage on
the smartcard. The smartcard uses max. 25mA, but is normally less than
10mA. The output current of the step-up convertor would be a few mA
without the regulator for the card supply voltage.
Can someone give me some advice?
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