Protecting an IOIO VREG

[Andries Kruger]

Hi all,

So my VREG finally fried today. It might have been one of two things that fired the VREG:

1) Might have happened when I plugged in my phone

2) Was using a new power supply and might have happened when I powered on the supply.

For my applications, I will need to be running from a 12V power supply. I know the IOIO isn't great with anything more than 10V but I have no choice. I have ordered a new VREG and will hopefully be replacing this one in a week or so.

My questions are: 

1) Can I protect my IOIO with a Zener diode or TVS over Vin?

2) If this is possible, does anyone know of a nice TVS or Zener with the best possible spec to do this?

Thanks.

[Ytai Ben-Tsvi]

The issue is still under investigation. If you're curious, here's the thread I've been keeping with TI engineers to try to resolve it: http://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/p/390774/1379558#1379558. The issue is now believe to be more associated with load (output) switching rather than line (input) switching and has only even been observed with 12V or above.

A Zener or TVS would not help you here (you don't have enough margin). The next revision of the IOIO, as soon as this issue is resolved, will contain a simple protection circuit comprising a Zener and a FET that protects the voltage surges. However, this is known to be insufficient (because the failure had been observed with this circuit in place, while monitoring Vin and asserting that it has not exceeded the maximum ratings).

One thing that you can probably do in the near term if you have to use 12V input is step-down the 12V to, say, 9V using an external regulator that is more robust.

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[Andries Kruger]

Thanks for the reply, I'll make use of an external regulator then for the time being.

One more thing: I've been using a 12V DC power supply for the IOIO. A lot of the times when I turn the power supply on, the IOIO powers up but does not charge the phone or power a Bluetooth dongle, I then have to remove the crocodile clip that powers the device and put it back before it powers the phone or dongle. I'm thinking it is my power supply that is a bit shaky. Any idea what might be causing this and how I can prevent it.

[Ytai Ben-Tsvi]

I've never encountered this problem. When this happens, what happens if you don't power-cycle, but rather disconnect and reconnect USB?

[Andries Kruger]

Disconnecting and reconnecting the USB does not work with either the dongle or Android phone.

I am currently running the device from the 5V rail. Don't know if that might have an effect. Also my pot is set to give max current to the device.

[Paul McMahon]

I, too, fried a Vreg a while ago, and have a 12V application. I was using a bench supply at the time.
I decided to use a cigarette lighter USB charger circuit board in front of the ioio vin terminal, $1, and no issues since (at least 100 cycles).
The board is very small, too.
I made a slight tweak to the charger board, though not really necessary: I adjusted the voltage setpoint resistor to get an output voltage of 7-8 volts. To give some headroom over the phone's charging circuit.

[Andries Kruger]

That sounds like a very simple suggestion Paul. I am actually going to power my IOIO from a 24V supply, so I have an Orion 24/12-5A DC-DC converter to take me down from 24 to 12. The converter is supposed to handle transients very well, especially transients from vehicles. My VREG blew when I hooked it up to the DC-DC converter running from a 24V bench. So I was really surprised when the VREG didn't survive as I expected a very clean output.

[Paul McMahon]

Yes, I was surprised I blew it when using a bench supply, which should be very well behaved (it's a good supply).

I should add that the cigarette adapter I use is sadly inefficient, even when dropping from 12 to 7-8V.  The regulator on that board gets so hot you can't even really touch it for more than a half second.  I measured it with an IR thermometer at around 70C.   I guess that's what you get for $1.  Still, it's fine by me.

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[Ytai Ben-Tsvi]

Paul, you are probably using a linear regulator, which is inefficient by design (it reduces the voltage by burning the difference as heat.

Andries, sounds to me like your external 5V regulator is unstable.

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[Paul McMahon]

No, it's using the MC34063A switching regulator.  I'm an EE--so was thinking linear as well, but it's not.

[Ytai Ben-Tsvi]

Seems like efficiency is not its main strength, considering the BJT switch... But hey, if it works...