Boost converter for 5V on USB powered ATmega32U2

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Hi all,

 

I'm working on a circuit that uses a bus-powered ATmega32U2 but I need to run it at 16MHz.  I was thinking of adding a boost converter to get the bus voltage up to 5V to insure proper operation at 16MHz.  What I'm unsure of is how to connect this to the µC.  Below is the circuit as I have it now.  VBus provides power to the boost converter, and the output of the converter is powering VCC, UVCC, and AVCC (although I'm not using the ADC).  D- and D+ should be supplied from the internal regulator.  I threw in a solder jumper in case it turned out that I could run at 8MHz and did not need the boost converter (I can just leave off the converter components).

 

Does this look correct?  Is there anything else I need to do?

Cheers,

Tom

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Just checking here: your USB bus (connector on the left-hand side) is not supplying a good 5.0Volts to the ATmega32U4?  I feed a separately regulated 5.0V source to my ATmega32U4 as I found my bus voltage varying a bit and I wanted to use that voltage for the ADC reference.

 

If this is what you are trying to do I would be checking the temperature/voltage stability of the MAX756CSA also how well it regulates with any varying load.

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Dave,

 

Just going by the specs, The mega32 is only rated at 16MHz above 4.5V.  USB can be 4.35V so I figured for stability I should do something.  The MAX756 is good down to 0.7V with a minimum start-up voltage of 1.1 and output voltage regulation is spec'd at 0.8% typ to 2.0% max.  There is no data on temperature regulation.  I'm hoping that is accounted for in the max 2% value.  As long as it doesn't drop more than 10% I should be good.

 

I was mainly concerned if I had all of the µC supply connections correct.  The data sheet is a little ambiguous when it comes to connecting the mega32u2 up as a self powered device.  The datasheet shows USB Vbus being connected to a non-existent pin "Vbus" on the µC.

Cheers,

Tom

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Tom,

 

Sorry, I didn't realise it was the 32U2 part.

 

Dave

 

Last Edited: Sun. Mar 22, 2015 - 07:37 PM
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On the ATmega32U2 datasheet, figure 20-5 show connections for a self-powered device.  It shows two pins on the AVR device VBUS and UID that do not exist on the actual package.

 

Just wondering how these were really connected.

 

Cheers,

Tom

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Maybe, not too helpful but the ATmega32U4 datasheet shows a Vbus and an UID connection in Fig 21-6.  Vbus goes to pin 7 which is also present on my Adafruit ATmega32U4 dev broad.

The UID connection on the USB connector, which is called ID on my Adafruit board schematic is not connected to anything and there is no UID pin on the ATmega32U4.

 

Perhaps on the ATmega32U2 the Vbus is actually connected internally to Vcc?

 

Seems to be some inconsistent datasheet information here.

 

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The VBUS input pin 7 on the mega32U4 is a sense input, not a power input. The diagram above is one of the "typical connection" diagrams copied from another device, possibly the 90USB1287, which supports host mode. The UID pin, which should be connected to pin 4 of the mini or micro USB socket if present, is pulled to ground when a mini-A or micro-A plug is inserted, to notify the CPU that host mode and not device mode is called for. You need this for something like a camera that can either appear to be a disk drive (device mode) to a computer, or drive a printer directly (host mode). Note, a mini or micro A connector is exactly the same as a B except that pin 4 is connected to pin 5 internally.