ARef input impedance? Other considerations?

[gatheround]

Hey all,
I'm just wondering if the ARef should be driven directly by an external reference source, or if a resistor should be used? Also what is the impedance of this input, and does it change based on what the reference is set to in the program (will I short out my reference if I want to use the internal reference, etc?)

 

[gatheround]

Hey all,
I'm just wondering if the ARef should be driven directly by an external reference source, or if a resistor should be used? Also what is the impedance of this input, and does it change based on what the reference is set to in the program (will I short out my reference if I want to use the internal reference, etc?)

I just wanted to add some more details to this problem. I'm using an LM431 shunt regulator to get my reference voltage (in this case 3.00V). When the teensy isn't powered up, my reference voltage drops to 2.5V or so. As soon as I power up the teensy, the reference voltage goes back to 3.00V. I'm not sure why, and for some reason, measuring the impedance of the ARef pin doesn't work so I can't see what is changing.

Any suggestions?

 

[JBeale]

If your Teensy power supply is 0 V, but you maintain the voltage reference, that means current is flowing from the CPU's voltage reference input through a protection diode (and/or other internal circuitry) into the +Vcc supply. I'm pretty sure the allowable Vref is in all cases less-than-or-equal-to the main power supply for the chip. So if you shut off the main supply, you must also shut off Vref, or suffer the excess current drain and voltage drop that you have observed. A series resistor (eg. 1k or more) would at least spare your LM431 from excessive load (and the Teensy chip some out-of-spec operating condition).

 

[gatheround]

If your Teensy power supply is 0 V, but you maintain the voltage reference, that means current is flowing from the CPU's voltage reference input through a protection diode (and/or other internal circuitry) into the +Vcc supply. I'm pretty sure the allowable Vref is in all cases less-than-or-equal-to the main power supply for the chip. So if you shut off the main supply, you must also shut off Vref, or suffer the excess current drain and voltage drop that you have observed. A series resistor (eg. 1k or more) would at least spare your LM431 from excessive load (and the Teensy chip some out-of-spec operating condition).

Thanks JBeale, I'm seeing about 12mA going through the Vref when the teensy is powered off.

 

[PaulStoffregen]

Teensy 3.0 has a 470 ohm resistor between AVCC and AREF. It's meant to provide the current for your LM431 chip.

Just connect the LM431 between AREF and AGND. You do not need to provide any power or current to the LM431. The resistor that's already on Teensy 3.0 does that.

 

[gatheround]

Teensy 3.0 has a 470 ohm resistor between AVCC and AREF. It's meant to provide the current for your LM431 chip.

Just connect the LM431 between AREF and AGND. You do not need to provide any power or current to the LM431. The resistor that's already on Teensy 3.0 does that.

Thanks for the reply Paul. That's a nice feature for sure!

Unfortunately when I do that though, I only get 2.9V through AREF and not the 3.00V I was getting previously. The resistors that I'm using around the LM431 are 960 ohm + 100 ohm trim between the Cathode and REF, and 5k between REF and Anode. As per the formula in the LM431 datasheet. It worked perfectly when connecting it to +15V through a 1K resistor. But now that I'm connected to Vin (5.05V) through the internal 470 ohm resistor, the max I can get out of the LM431 is 2.9V. Unless I increase the resistance between Cathode and REF to ~2.2K and then I can get 3V, but that's way off compared to the formula.

 

[PaulStoffregen]

The datasheet says the LM431 typically needs 0.4 mA for regulation, but only guarantees performance with 1mA or more.

You have 5100 (plus 0-960) ohms across the LM431, which will use 0.59 mA.

You need the resistor to provide a current of 1.59 mA for reliable operation.

The 470 ohm resistor is between 3.3 volts and the LM431 which tries to be 3 volts. That's 0.3 volts / 470 ohms, which is only 0.64 mA.

To make this work, you will need to add another resistor between the LM431 and either 3.3 or 5.0 volts. The 3.3 volt supply is probably better, since it's well regulated. This extra resistor must be 315 ohms or less.

 

[gatheround]

The datasheet says the LM431 typically needs 0.4 mA for regulation, but only guarantees performance with 1mA or more.

You have 5100 (plus 0-960) ohms across the LM431, which will use 0.59 mA.

You need the resistor to provide a current of 1.59 mA for reliable operation.

The 470 ohm resistor is between 3.3 volts and the LM431 which tries to be 3 volts. That's 0.3 volts / 470 ohms, which is only 0.64 mA.

To make this work, you will need to add another resistor between the LM431 and either 3.3 or 5.0 volts. The 3.3 volt supply is probably better, since it's well regulated. This extra resistor must be 315 ohms or less.

Thanks Paul, this worked great!