Better Late Than Never?

A project log for VFDuino

An arduino VFD clock

Brad ArnettBrad Arnett 09/01/2016 at 02:122 Comments

So, I kinda let this fall by the wayside (again), but I'm going to try not to do that YET again.

Here we go:

The NCP1403. It's a step-up DC-DC converter in a tiny surface mount package produced by ON Semi. The datasheet is available at

This is where things start to get complicated, but lets start with the pinout.

    1. CE - Chip enable. >0.9V = Enable, <0.3V = Disable, Floating = Enable
    2. FB - Compensator inverting input. Connected to resistor which sets output voltage
    3. VDD - Power supply
    4. GND - Ground (obviously)
    5. LX - Inductor connection pin

    Tolerance on the VDD is -0.3V to 6.0V, so I should be good for mobile applications (probably going to be 3.4V) as well as something powered by mains (probably 5.5V). Thats good. I set CE to a pin on the Arduino. That means a little more power draw, but that will allow me to do things down the road like put the screen to sleep. Overall it should save more power than it uses, and if it doesn't, well, nothing solves that problem like an xacto knife across the trace on the board.

    We need to find the voltage for our feedback resistors so that we can set our voltage. The equation is:

    Err... well then. I know what I have laying around my apartment, and I know what I need for an output voltage, so I guess let's plug some numbers in and see what we get.

    I know I need at least 10V. So we'll start with Vout = 10, and then figure the closest values to real capacitors I have otherwise. Expressing the formula another way:

    So increasing the overall voltage is dependent on increasing the value of Rfb1. Tossing values in there for resistors (I'm writing the book so I can read ahead anytime I want :) ) with Rfb1 = 1M and Rfb2 = 82.8k, V = 10.46. That actually comes out being closer to 10.35 in real life, because life isn't ideal. But so it goes, and so do we, onto talking about the circuit design.

    There's a lot of ways you can put this SOIC to use. The one I'm going to use is straight out of the datasheets. Page 2, Figure 1. Other than the resistors we hammered out earlier, use values as close to what is recommended as possible. You want the 47uH inductor and the MBR0520LT1G diode, as it's the current revision. From what I can tell, it's somewhat of a specialty diode, and as far as I know it only comes in the SOIC package, but it's pretty cheap. Mouser currently lists them at 0.32USD per unit.

    One last note, they don't outright list a recommended capacitor for Cc in that diagram anywhere that I noticed, though they recommend a disc capacitor. I used a 10 nF one. I get some voltage flux within about a volt or so, but I haven't noticed any real flicker or anything, so I'm calling it close enough for my hack. I imagine that if I were someone who actually knows what they're doing, I'd probably start tweaking things there to get more consistent voltage.

    I'm going to start talking about the MAX6920 shift register in the next update. It's wired up, so I'll start writing that up tonight but we'll see if I finish it in time to post. I imagine that I will need to start talking code at that point as well.


    Brad Arnett wrote 09/01/2016 at 04:19 point

    Whoops!  Good catch.  Editing now.

      Are you sure? yes | no

    K.C. Lee wrote 09/01/2016 at 03:19 point

    >CE - Chip enable. <0.9V = Enable, >0.3V = Disable, Floating = Enable

    Actually the datasheet reads this way.
    < 0.3V (max) = disable.  >0.9V (min)=  Enable. 

      Are you sure? yes | no