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Connecting propeller directly to Li-Po — Parallax Forums

Connecting propeller directly to Li-Po

average joeaverage joe Posts: 795
edited 2012-09-10 18:47 in Propeller 1
I've read several threads that suggest the propeller chip is tolerant of excess supply voltage. Tried searching for these threads but couldn't seem to find them.

I'm trying to eliminate the 3v3 regulator from a design powered by a Li-Po, with nominal voltage @3.6v. Reading the datasheet suggests this won't be a problem, until the battery is charged and providing 4.2v. That's only two tenths of a volt over absolute maximum listed on the propeller datasheet.

My questions for others who have done this:
#1. How well does the propeller handle 4.2v supply?

Any thoughts, recommendations, greatly appreciated!

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-09-10 15:30
    Please bear in mind that any chip's Absolute Maximum Ratings are not intended to be operational limits, but the limits above which actual damage may occur. It's always better to operate a chip within its operational specifications. In your case, that would mean using a voltage regulator.

    -Phil
  • lanternfishlanternfish Posts: 366
    edited 2012-09-10 15:48
    Google low drop out regulators/regulation. Or you could try a simple Zener Regulator. Some design info here.

    EDIT: And then I searched the forums and found this thread that may help.
  • jmgjmg Posts: 15,183
    edited 2012-09-10 16:20
    average joe wrote: »
    Any thoughts, recommendations, greatly appreciated!

    You have got a reverse polarity protection diode, right, and a small series decoupling resistor ?
    That will drop ~ 600mV

    If you want to run down very low, and have any battery monitoring, you can flip the Diode for a P-FET-Diode, and enable the PFET when the battery sags and you need the 'last few drops'.
  • jmgjmg Posts: 15,183
    edited 2012-09-10 16:22
    lanternfish wrote: »
    Or you could try a simple Zener Regulator.

    Low voltage Zeners are quite poor regulators, with a very soft knee. Best avoided as regulators.
  • average joeaverage joe Posts: 795
    edited 2012-09-10 16:26
    Thanks for bursting my design bubble, Phil ;)

    I usually run everything 'in spec' but this specific design mandates NO REGULATOR! Perhaps a voltage-dropping diode?

    The thread lanternfish pointed out suggests ADP2503 - ADP2504 and I will read up on this for possible future use.

    So thoughts about using a voltage-dropping diode?

    *edit*
    While I was typing jmg came back with the lines I was thinking of. Current design DOES NOT have reverse polarity protect diode but was on the drawing board. I will need to think about this a bit as I'd like about 400mv drop. 600 complicates things slightly because there IS a regulator in the design, it just does not power the propeller. The regulator I'm using has a 400mv output switch drop and I'd like to match this fairly close.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-09-10 16:30
    average joe wrote:
    ... but this specific design mandates NO REGULATOR!
    Who wrote that mandate, and why?

    -Phil
  • average joeaverage joe Posts: 795
    edited 2012-09-10 16:38
    I wrote the mandate, since I plan on controlling a 3v3 buck regulator from a propeller pin. There will be no "physical" power switch, the propeller will just "sleep" most of the time with regulator to rest of circuit turned off. The propeller needs to detect a power on pulse on pin and turn everything else on.

    I think the diode/decoupling resistor solution will work quite well.

    @jmg, The P-FET is brilliant and may go into a later design. Best to KISS at first!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-09-10 16:58
    Have you seen this? It's a combination LDO and buck regulator with separate enable inputs for each:

    'Seems ideal for your app.

    -Phil
  • jmgjmg Posts: 15,183
    edited 2012-09-10 17:00
    average joe wrote: »
    *edit*
    While I was typing jmg came back with the lines I was thinking of. Current design DOES NOT have reverse polarity protect diode but was on the drawing board. I will need to think about this a bit as I'd like about 400mv drop. 600 complicates things slightly because there IS a regulator in the design, it just does not power the propeller. The regulator I'm using has a 400mv output switch drop and I'd like to match this fairly close.

    Reverse connected FETs make good ideal diodes.

    If you have a larger regulator powering other items, then you could use a dual diode approach.

    Use Series R + Silicon diode(s) for the Deep sleep mode, and Diode OR a schottky diode to the Buck output, so when that comes up, the Prop is never more than a Schottky diode drop away.
  • average joeaverage joe Posts: 795
    edited 2012-09-10 17:28
    jmg wrote: »
    Reverse connected FETs make good ideal diodes.

    If you have a larger regulator powering other items, then you could use a dual diode approach.

    Use Series R + Silicon diode(s) for the Deep sleep mode, and Diode OR a schottky diode to the Buck output, so when that comes up, the Prop is never more than a Schottky diode drop away.

    This suggestion is PERFECT, I think. I really like the propeller never being more than a schottky diode drop from buck regulator. This might help push voltages even lower and that would be GREAT for battery life. 2v8 to 3v0 is where I'll get the best consumption trends, but I had stuck with 3v3 due to difference in supplies.

    Phil: I took a quick look at the regulator and "micropower shutdown" of 1ma is a bit higher than I'd like. Trying to keep sleep mode under a couple micro-amps *not including propeller*.

    Thanks again for the input. I have plenty to think about now!
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-09-10 18:47
    A P-FET driven by an ADCMP356 and a pair of resistors will get you a simple regulator with almost zero drop-out voltage. This circuit regulate at 3.42 volts. It should draw ~11uA, though it will oscillate a bit (~0.08v). The lower the RdsOn of your MOSFET, the lower the drop-out voltage. The higher the RdsOn, the softer (lower frequency) the oscillation will be.
    {{
Vin ───┳────────┐
        │        │
     ┌──┴───┐    │
     │ Vcc  │    │
     │      │  G │S
     │  OUT ├───
     │      │    │D
     │ADCMP │    └────┳──── Vout
     │  356 │          4.7M
     │      │         │
     │   Vin├─────────┫
     │      │          1M
     │  Gnd │         
     └───┬──┘
         
}}
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