Re: USB Power-Bus supply RIPPLE - How much??
From: Joel Kolstad (JKolstad71HatesSpam_at_Yahoo.Com)
Date: 10/28/04
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Date: Thu, 28 Oct 2004 09:20:32 -0700
"Alexandre Heil Franca" <alexheil@hotmail.com> wrote in message
news:pan.2004.10.28.14.06.14.402446@hotmail.com...
> I am building a device that should include a USB-Host. As a USB-Host, it
> should supply 5V x 500mA.
That's slightly wishful thinking. Download the USB spec from www.usb.org
... you'll see that it specifies 4.75V for most nominal connections but can
get as low as 4.4V if you're at the end of a bunch of lower power hubs.
Transiently, it can drop to 4.07V. The upshot of all this is that powering
5V logic off of the USB bus directly is a little iffy... in actuality, it
wil usually work, but it is poor engineering practice and shouldn't be used
on commercial products.
Also, keep in mind that the 500mA number is what you'd typically 'expect'
from a full port USB port (either a motherboard port or a powered hub's
downstream port). Unpowered hub typically only provide 100mA to their
downstream ports. Your device isn't 'allowed' (by the spec) to pull more
than 100mA until it's been 'authorized' to do so by the host. Many cheap
high power devices violate this part of the spec, however, but again -- it's
a lot easier to get away with poor engineering design if you're somewhere in
Taiwan or China and your customer service department is non-existant or
e-mail only (and most of the time everything still works anyway).
> I hasn't been able to find any reference about
> the max. ripple for the power-bus.
I don't believe they specify 'ripple' -- just the maximum and minimum
nominal voltages and the minimum transient voltage. Hence, depending on how
you define 'ripple,' it could be well over a volt.
> In reality I am not sure, if all clients must include a power supply it
> self, or at least any kind a filter at the input of the power-bus. If any
> of these options is true, I don't need to care about ripple. Does anyone
> could point me any reference? Does anyone know how much it should be?
The most common power supply you see on USB peripherals is that they use a
3.3V logic (with a low dropout regulator) and don't have to worry about any
of this.
---Joel Kolstad
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