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Powering Circuit Q's


I am have some questions regarding the Schematics of the BF548 ezkit (attached). Specifically

the powering sections.

Firstly, a basic questions, what is the difference between the filled square and hollow

circle schematic symbol for the power buses? The page Power Page 1, for example,

on the bottom right shows the circuitry for the VDDEXT and 3.3V, shown as filled square

and hollow circle respectively. They are both 3.3V so what is the difference?

Looking now at Power Page 2, I don't really understand the power supply for VDDDR and

5V. What does the adp1823 chip setup do? I looked briefly at its datasheet but didn't

understand how it relates to the regulation of VDDDR and 5V (again the symbols are different?).

On the bottom left of this page there are two outputs for 5V_USB? Why?

Also, on the bottom right there are these symbols (MH4, MH5, etc) but they aren't connected

to anything? What are they for?

It's pretty evident there are lots of things that I'm confused with - the power circuitry has been

a bit of a hurdle. Is there an easier design for the power supply? Would really appreciate

any reference designs that have a simpler powering system.

Thanks to anyone who can be bothered to try and help. I know it is not exactly 'fun', but it

will be much appreciated.


  • Hello,

    Hope this answers your questions.

    1.) There is no difference between the hollow circle and the filled in sqaure. They are both global nets that are used throught out the schematics.

    3V goes throughout the board to mulitple circuit on the board and the VDDEXT goes only to certain power pins of the BF548. JP5 and R194 allow a user to calculate the amount of current that the VDD external pins of the BF548 are consuming. The manual for the BF548 EZ-KIT elaborates more on this.

    2.) The ADP1823 is a voltage controller that helps sequence your power rails based on the enable pins. In this case 2.5V comes up and then 5V.  This circuit takes in VIN of 7.5 and is used as a dual supply. Since it is a switching circuit it brings down the 7.5V to 2.5V with little to no heat dissipation. If you need further explaination or need help is a custom power circuit design that works for your needs, we can put you in touch with a power specialist from ADI.

    In terms of the symbols the 5V, 2.5V and 3.3v are standard symbols in our schematic library. The others are just for this design. A user should treat them the same; as global power nets.

    3.) The 5V_USB connected via R280 is actually not used. There is a DNP (Do Not Populate) resistor there. It is there in case we needed to debug the circuit and needed contstant 5V applied to the USB VBUS pin.

    Currently the 5V_USB is controlled by U39 and is off when the BF548 is a device and is connecting to a USB Host (i.e. PC). Simarly, U39 is on when the BF548 is the host and is connected to a device (i.e. memory stick).

    The MH's in the schematics correspond to Mounting Holes on the board. Some are just hole, while other are connected to the ground plane. The Hole corresponding to the STAMP connectors have the Mounting holes connected to ground as this was a requirement.

  • Thanks a lot for the detailed response..

    Regarding the power specialists, do they provide free support / power design advice? This could be very helpful.

    I would like to know whether the powering circuit could be simplified if the complexity/functionality of the system

    is much simpler than the bf548 ezkit. For example, if we didn't have a hdd in the system could it save some

    power circuitry etc..

    Thanks again..

  • Hello,

    Yes, they can help you out to simplify the power circuit.

    I would recommend  you formulate an email with your power requirements and send an email to

    Important information they would need in the email are:

    1.) your Vin (power supply) voltage and current,

    2.) what voltage rails you need and the current requirements

    3.) any special power sequencing needed.

    Not using the hard drive should help you to not have to use such a large power supply.

    The hard drive requires between 0.5 to 1A instantaneous current at power up of the hard drive.

    Good luck.

  • Hey,

    I have a further questions regarding the bf548 ezkit schematic. The attached image

    is a part of the schematic for powering the vddusb. The data sheet says that it should be

    about 3V. The 3.6V input makes sense - because after the diode Vf loss its about 3V. However,

    the 5V_USB is much less than 3V? The Zener is around the 2.3V mark, then after the Vf loss

    it will be down around 1.7V?? Have they put the wrong schematic reference for the Zener

    or is there something that I am missing?


  • Hello,

    You can disregard all the diode circuitry( D2, D3, D7, and JP11). You will not need it.

    Simply connect Pins G4 and  F5 of the BF548 to 3.3V in your design.

    This circuit was there for intial silicon testing.


  • Sorry for the confusion,

    I think I may have mixed up this circuit with another product.

    The diode circuitry is actually necessay.

    A colleague will post shortly and help clarify any questions you may have about this circuit.

    Good Luck.

  • Hi D H,

    A close examination of the tolerance specification for the BF54x USB pins shows that they are only 5volt tolerant if VDDUSB is at 3V.  The specification may be violated if your device is powered off but you are connected to a live USB host or hub.  If this condition occurs for a long time, the processor may fail.  I can’t be explicit here but my belief is that it would take many months of elevated temperature to cause a failure.

    Therefore we suggest that for applications like PC peripherals that can expect to be powered off for very long periods, that you find another way to power VDDUSB.  That is the purpose of these diodes, when local power is on; the 3.6V supply through D2 puts 3.3V on VDDUSB.  When there is no local power, the VBUS pin caries 5V from the host and will provide about 3V to VDDUSB through D3 and D7.

    This extra circuitry is only required for the BF54x family.  There are also many BF54x applications that do not require this circuit.  USB powered devices are an example of a designs that do not require the additional diodes.

    This and other hardware topics are discussed in EE-281.


  • Hi,

    I am still confused how this circuit gets the 5V_USB to 3V. The zener voltage for D3 is 2.2V and

    the Vf of D7 is about .5V? Not sure how the 3V is derived from this?

    So basically if, if the PC provided 5V to the VBUS of the BF548 while it was is not powered it

    may cause damage. But if USBVDD is powered with 3.3V then it will not damage it?

    Also, this is a bit off on a tangent. If I want to use the VBUS to power the board, what sort of

    cap values or filtering would be required on it (other than regulation). From what I know, the USB power

    is pretty noisy?

    Thanks a lot for your help so far. It is really appreciated and has helped a lot..


  • Hi dlh,

    Sorry if I misspelled you name before.

    Yes you are correct. The Zener is about 2.2V.  The forward drop of D7 is only .3V.  Likewise D2 drops the 3.6V supply to 3.3V when the device is operational.  The unpowered voltage may be closer to 2.5V than to 3V.  That’s ok because you are not operating the device.  The processor is powered off and you do not need the minimum 3.0V.  Even less than 2V should prevent any damage caused by ageing.  Before I said months of ageing at worst-case temperature.  That was conservative.  We know of no failures.

    I have not studied the noise on VBUS.  Don’t quote me but I think there may be a maximum inrush current spec for USB VBUS.  If I remember correctly, that limits you to about 10 or 15uF.

    Thanks for you interest.


  • Hello,

    To elaborate further in what BobK mentions:

    The BF548 schematics currently does not show a 150uF cap that is needed when the BF548 is a USB Host. (Meaning when EZ-KIT supplies the 5V_USB).

    This needs to be added on the side of U39.7 (next to C279).

    When the BF548 is a device this FET (U39) is off and the cap is not seen on the 5V_USB line.

    It only sees the 4.7uF cap (C198).

    We will be adding this 150uF when we update the BF548 schematics.