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Power up problem

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  • Power up problem

    I have a Picaso GEN4-ULCD-32PT that I am building into a portable handheld device run from a 3.7 volt lipo battery. In order to get 5 volts for the display, I'm using a purchased booster circuit that generates 5 volts from the 3.7 volt input. There is an embedded microprocessor that will control the display, but it is not being test yet. Problem is that the module sometimes won't power up when the power is turned on. It works every time when I plug it into the USB programming interface, but when attached to the device it only powers up about one time in four. Is there something that I should be looking at in the power supply that may be preventing a good start-up? The converter is rated for 500 milliamps, which should be plenty of power for the display..

  • #2

    Based on the gen4-uLCD-32PT datasheet[page 19], the display will take 4.0V minimum and 5.5V maximum supply voltage. You may directly connect it to the LiPo battery if you wish. However, operating under the required minimum voltage would mean you will have a dimmer screen as the backlight needs 5V.

    A word of caution, when fully charged the LiPo battery has a potential of 4.2-4.3V sort of range. However, as they drain they go below 4V and once that starts happening you lose the head room for the 3.3V regulator, and things like the uSD card will likely fail first. You can certainly do it, but won't likely get the full run-time given the voltage drop that happens as it drains.

    The converter most likely cannot supply the inrush current required to start the module. You might want to try adding a big capacitor to the output of the converter and a switch after that, to be able to power the converter up, which will charge the cap, and then when you turn the switch on, the converter + cap will power the module inrush.

    Kind regards,


    • #3
      I understand what you are saying about the inrush. I assumed this was the problem since it works fine when plugged into a powered USB port. Would it help any if I reset the display a few seconds after power up, or is it hopelessly hung at that point? I know the boost converter I'm using has enough capacity once it starts up and the output is 5.1 volts, but I'm not sure about its startup cycle. I know these units take a few milliseconds to get the voltage up to the rated level. The startup is probably dependent on the charge level of the lipo battery, so it may take longer to start up if the battery is low. If I have to hold off the display until the power supply ramps up, that will be a major redesign of the circuitry which I'd like to avoid if possible!