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Reply-To: "Ultimate Buu"
From: "Ultimate Buu"
Subject: Re: Is USB bus power stable enough?
Date: Sat, 7 Dec 2002 21:51:06 +0100
Organization: DragonBall Z Universe
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
NNTP-Posting-Date: 07 Dec 2002 20:52:27 GMT
"Erik" wrote in message
> 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.
Your design is OK, always use seperate regulation preferably using DC-DC
converters. A large buffer cap isn't really needed.
Use multiple DC-DC converters for each voltage you need. Pro: efficient Con:
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