AT90USB647 USB host conectivity. Discass.???

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Hi there,

I am designing a battery based USB host circuit uses the AT90USB647-AU. Only USB flash memory sticks can be applied at the USB A connector. The supply of the citcuit is a battery (6.4Vmax, 5Vmin).

Looking at the datasheet of the uC:
http://www.atmel.com/dyn/resourc...
Chapter 21/Host Mode Figures 21.7 and 21.8 (pages 249-250), there are 2 suggested circuits to use. The first one is used for Vcc=3V, with an On/Off controled DCDC converter, to apply 5V at VBus and the second one is used for Vcc=5V, with just a switch to upply 5V at VBus.

In my case the things are a little bit different. According to the USB2 specs. the VBus cannot be less than 4.85V. Because of the supply max. voltage which is 6.4V a diode is used to make a voltage drop of ~0.6V, in order to feed the uC Vcc. An very low dropout LDO is also used to apply 4.85V at VBus, which is input is connected directly to Vbatt.

There is no need of OTG so UID is not connected.

One important question is that if I have to connect the UVCC pin or not???

Furthermore, do you agree with the VBUS pin connectivity?

Also if you are able, could you please critisize the whole circuit?

Thanks in advance.

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Michael.

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UVCC must be connected, it's the input of the AVRs onboard 3V3 regulator that feeds the USB controller.

If you don't want to use th internal regulator, you must feed the USB controller via UCAP (Figure 21.6 in the datasheet).

Are you sure you want only use a diode to feed the AVR from the battery; a diode will only drop 0.7V and with a full battery that gives 6.4V (what kind of battery is that btw?) the VCC will be 5.7V. A bit high for my liking though the datasheet shows 6V absolute max.

I would use the LDO to feed the whole system, including the slave USB device. It will give a nice 5V then. Like figure 21.8 in the datasheet.

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Yes, I am sure about the way of supplying the whole circuit. This design is an update of a circuitry that works fine some years now. I am doiing this because the whole product functions with an average current consumption < 100uA. So there is no way of using an LDO or a DCDC. The battery consists of 4xAA Alkaline in series.

Now, going back to my original question, your repply and what the datasheet says, I guess that UVCC must be connected at Vcc.

Can you please confirm the above?

Michael.

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Quote:
So there is no way of using an LDO or a DCDC

Why not? I've seen LDOs with an Iq of 1.1uA. Switchers with an Iq of 12uA.

You must connect Uvcc because otherwise the USB controller would not work and the rest of the circuit would be completely superfluous then :)

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1. Do you have an LDO part number with Iq < 5uA in your mind?

2. Ok, I understand the reason of connecting the UVcc, but I asked if it's a good idea to connect it to Vcc, of the circuit I gave in my first post?

Thank you.

Michael.

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Yes, you can connect UVCC to VCC, you can turn off the regulator in software to save power.

Low Iq LDOs I've come across while looking for them:
* Seiko S-1167 series
* Onsemi MC78LC30.

Switching micropower stuff:
* LTC3406
* LTC3560

There are more.

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Thanks for all your time.

Best regards.

Michael.

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Something else:

There is nowhere defined in the AT90USB647-AU datasheet the maximum voltage that can be applied at UVcc.

http://www.atmel.com/dyn/resourc...

Michael.

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Likely the absolute maximum of 6V. At least 5.5V. It would be pretty useless if it didn't have the same range as the AVR itself.

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This reminds me the non defined value of the I/O pins internal clambing diodes, max. current.

Thanks for your very fast repply.

Michael.

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Hi,

Sorry for hijacking this thread, but I think my question is not completely unrelated. I am dealing with an existing AT90USB646 circuit that is built for 5V bus power operation (cf. Fig. 21-3 in datasheet #7593). For some new periphery, however, I need to take the I/O lines down to 3.3V. The problem, though, is that all VCC / UVCC are wired together and there is not much I can do about it since the wires are underneath the QFN64 chip. Hence, I cannot, e.g., separate UVCC from the other VCCs.

Because I want to avoid the use of logic level translators, I'd like to disable the internal USB pad regulator and instead supply the entire chip with 3.3V via an external regulator. The question is, what happens electrically when I "disable" the UREG? Could I just go for something like the sketch I attached, or are there any problems that I fail to foresee?

Thanks,
Matthias

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