OLED charge pump schematic help

Discussion in 'Analog & Mixed-Signal Design' started by paulsoulsby, Nov 6, 2017.

  1. paulsoulsby

    Thread Starter Active Member

    Jan 9, 2013
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    Hi,
    I've got a datasheet for an OLED and it shows recommended input circuitry for the charge pump and various other pins. I've got 2 questions.
    1) It all seems fairly simple apart from the input labelled GPIO. Why would you need a GPIO input at that point? What should it be connected to?
    2) It's a bit annoying that the VBAT_in voltage is 3.5-4.2V. What would I need to change to use 3.3V or 5V for VBAT_in?
    thanks!
    Capture.PNG
     
  2. RichardO

    Late Member

    May 4, 2013
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    890
    A part number, or better yet a data sheet, for your OLED would help a lot in getting good answers.

    If they say VBAT must be 3.5 to 4.2 volts then that is what it must be.

    You will need a voltage regulator to supply that voltage if you don't have it already available. Either a step-up switcher from 3.3 volts or a step down from 5 volts.
    A low dropout linear regulator would be the easiest way to get to the required voltage from 5 volts in.

    The data sheet will say how much current is needed for the power supply.
     
  3. paulsoulsby

    Thread Starter Active Member

    Jan 9, 2013
    29
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    Part number is: QG-9632TSWEG01
    Manufacturer: All Vision
    Website is http://www.szallvision.com but this doesn't give much info.

    Any idea what the GPIO in/out would be for? There's absolutely no mention of it in the datasheet (other than that pic) and I can't find anything similar in other products by other manufacturers.
     
  4. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Where did you get the datasheet? Can you link to it? Their website doesn't appear to have datasheets, or else I'm just failing to search it properly.

    The GPIO circuit appears to make a connection between your battery voltage and the OLED battery input. Since I can't get to a datasheet, I'm not sure how the OLED is using battery and vdd differently. This could be effectively enabling and disabling the whole device, or perhaps vdd keeps the memory portion going and battery power lights up the display. My guess it's effectively a display enable function, and that if you never wanted the display to be off you could just bypass that circuit... but keep in mind this is only a guess, and a fairly wild one at that!

    If you can get us access to a datasheet, there might be more answers. Otherwise, just guesses, at least from me.
     
  5. paulsoulsby

    Thread Starter Active Member

    Jan 9, 2013
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  6. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    I thought each OLED pixel was its own light source (essentially the screen is a matrix of tiny LEDs) and that there was no backlight in OLED screens.
     
  7. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Ok, the datasheet isn't too bad.

    VDD is supply voltage for logic (~2.8V)
    VCC is supply voltage for OLED display - this can be supplied directly (7-9V) or created with internal charge pump, which relies on 3.5-4.2V at VBAT

    If you provide your own 7-9V at VCC, you can connect VBAT to VDD and leave C1P, C1N, C2P, & C2N all floating.

    If you use charge pump, you must provide the spec'd VBAT voltage and add the various charge pump and stabilization capacitors as drawn. I don't see any way around coming up with some voltages besides 3.3 and 5.

    As for the GPIO, I get it now. In order to protect OLED elements, they have a specific power up and power down sequence they want you to follow. This involves controlling power to the OLED (either VBAT or VCC depending on whether you utilize built in charge pump) separately from VDD logic supply voltage. Power sequences are detailed in section 4.

    I must confess, all the tedious details of stuff like this are why I usually use cheater products from places like Adafruit and Sparkfun, where all of the extra components and all the drudgery of programming startup sequences is already done. Obviously when you're designing a thing to mass produce, it's worth taking the time to do everything right at a low level, but when I'm doing hobby projects and one-offs, I outsource the dirty work!
     
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  8. paulsoulsby

    Thread Starter Active Member

    Jan 9, 2013
    29
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    Thanks so much for your help - that answers all my questions. GPIO input all starting to make sense now, thanks for the pointer to section 4.

    Yes I totally agree with you, pre-built units are a lot easy for prototyping and it's normally the route I take. However this is for mass production - there's gonna be 8 OLEDS per product, so had to source something that is a fraction of the price of the pre-built units! I am slightly dreading getting them to work though ;)
     
    ebeowulf17 likes this.
  9. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Glad I could help. Good luck and have fun! Let us know how it turns out.
     
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