Using a 5v Digital Potentiometer With 8-12.6v input/output?

Discussion in 'Analog & Mixed-Signal Design' started by Bananaguns, Jun 26, 2016.

  1. Bananaguns

    Thread Starter New Member

    Jan 17, 2016
    2
    0
    Hey!
    I'm working on a PWM circuit which has a variable frequency controlled via an up/down control digital potentiometer.
    The problem is the digital pot has a max input of 5.5v, however the circuit is intended to be used with a 2 or 3S lithium battery setup. Right now I'm using an LDO regulator to drop the input from 8.4v to 5v then powering the digipot and astable 555 circuit with the 5v output. This does work, but I'd really prefer to be able to use the digital potentiometer and still have the full range of output that I would while using the same resistance trimpot.

    I feel like there is probably some easy solution that I'm missing, or perhaps the easier route is just using a micro controller and I2C digital potentiometer instead, which I've been trying to avoid.
    Anyway, my question is basically this: how can I regulate the voltage down to 5v to not burn out the digital potentiometer IC, but then some how step it back up or something so the output is adjustable up to close to 8v or so?

    I'm sure there has to be a way to control higher voltages with a digital potentiometer as I see digital potentiometers used in systems that surely aren't 5v, but the problem I've run into is that the leads from the 555 when powered by 8.4v will then send 8.4v through the wiper and outer leg pins of the pot and that would destroy it :[

    Any insight or ideas would be greatly appreciated!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Post you circuit schematic.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,395
    1,607
    Why don't you just choose a digi pot with a higher working voltage?

    I have 69 hits on Digikey when selecting supply voltages that range from 15, 18 and even 36 volts.
     
  4. Bananaguns

    Thread Starter New Member

    Jan 17, 2016
    2
    0
    Unfortunately if you change the filter to digital potentiometers with pushbutton control, i.e. not I2C, SPI, serial and so on... that leaves 109 results, none of which have a supply voltage higher than 5.5v, including the DS1809z - the one I am using. The higher voltage digital potentiometers generally need an MCU or some other circuitry to interface with, while the pushbutton pots are essentially stand alone, only requiring a few bypass caps and minimal optional components for wiper storage if necessary. In other words, it'd make things much more difficult, more expensive, and increase the size for essentially little gain when there is probably an easier way to just reduce the voltage from the 555's threshold and trigger pins to 5v before it gets to the pot IC, while still powering the 555 with 7.4-12.6v. At least that was my hope anyway.

    As far as a schematic goes, I don't have anything complete other than what I've put together in eagle for the PCB which would honestly probably be more confusing than anything so I'm working on a new one that will be easier to read. It's just a basic astable 555 with the potentiometer adjusting the frequency instead of a fixed resistor. Nothing special.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    What I'd do is; add a common base transistor or common gate MOSFET to the output in the manner of the upper stage of a casc-ode.

    The potentiometer needs to have either current sink or complementary output, and the common base/gate stage has no current gain - the potentiometer must be capable of supplying all the load current.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    Normally the frequency is fixed and the pulse width is varied. Why vary the frequency?
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784

    Not always - sometimes the on time is fixed, so increasing the frequency increases the duty cycle.

    Its the basis of the two 555 (556) PWM control - a variable frequency astable 555 triggers a fixed monostable 555 more or less frequently to vary the duty cycle.
     
  8. m zaid

    Member

    Jan 9, 2016
    46
    5
    The circuit steps down...then you want it to step up again?
    Operational amplifiers?
     
    #12 likes this.
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    If you use a pull-down resistor on pin 5 of the 555 you can adjust the pin 5 voltage to be something less than 5V, while still running the 555 from a 12V supply. The trigger and threshold voltages are thus reduced and so the 5V digipot should be usable. The pot would of course need a 5V supply, but the 555 output pulses would be at a higher voltage.
     
Loading...