Powering Teensy Audio Board Externally

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bcaldw1

Member
Is it acceptable to power the Teensy audio shield with an external 3.3 V regulator?

I would be keeping the ground wires connected but removing the 3.3 V pin from the audio shield to be powered separately, not by the Teensy itself.

Any advice or thoughts would be much appreciated. Thank you!
 
It is fine to power the audio from a separate 3.3V regulator as long as the grounds are common. That is how I do all my boards.
 
You probably should make sure the external 3.3V power comes up at the same time or only after Teensy's 3.3V power is on.

If you apply power before Teensy 3.3V power is up, current could feed through the 2K pullup resistors to the SDA & SCL pins, which then tries to feed though the ESD protection diodes and partially power up the chip on Teensy. The problem is if your external power causes Teensy's 3.3V power to come up to about 1 volt or more due to the feed through those ESD protection diodes, then Teensy's power up sequence could be incorrect. It can stall and not boot up if done incorrectly, especially if any part has power before the SNVS power.

To understand Teensy 4.x power up sequence, check out this page.

PJRC Store

www.pjrc.com www.pjrc.com

Scroll down to "Power Up Sequence" and try the little javascript buttons to see it visualized on the schematic.
 
If your plan involves using a LDO or switching voltage regulator to create your 3.3V power, the easiest way to be 100% sure you'll meet the power up sequence requirement is to choose a regulator chip with an active-high enable pin. Most modern regulator chips with enable are active-high, but some old ones might be active-low, so check the datasheet before you finalize the decision.

Then just wire Teensy's 3.3V power to your regulator enable pin. When Teensy's power comes up, it will enable your regulator to provide power to the audio shield.

You might also consider how to discharge the capacitors when power turns off. Many modern LDO regulators have an active discharge feature, where a resistor inside the regulator quickly and completely discharges the output capacitor when you disable the regulator. But if you don't have this, you might consider just placing an extra resistor across power and GND. Sure it wastes a little power while on, but when you turn off the power a real resistor will assure the capacitor fully discharges.
 
Thank you for all the excellent information!

I am wishing to use a high quality 3 pin linear regulator (made specially for audio use) to power the Teensy audio shield.

Would a MOSFET connected to the 3.3v power output to the of the Teensy (4.1) controlling the external 3.3v power into the audio shield be acceptable?

By extension, would this work for powering other I2C devices externally, keeping the 3.3v power draw to a minimum on the Teensy?
 
You could use a P-channel mosfet, but since it needs logic low to turn on you'll need to add more circuitry to control it properly. How well adding a mosfet preserves your ultra low noise goal is a good question. For example, if you use the mosfet at the LDO output, does high frequency noise at your drive of the mosfet gate get to couple through to the output? Mosfets are fundamentally a big capacitor between gate to source. While you only have small parasitic capacitance to the drain, the drain and source are virtually shorted together when the mosfet it on. If you try to make things quieter by adding more capacitors, remember to consider inrush current when you turn the mosfet on. Like almost all analog design, things usually aren't as easy as they first appear, so you should think carefully about how to do this....

You'd probably be better off just choosing a low noise LDO which has an enable input pin. Most of them do these days. For example, Analog Devices ADP150 and Texas Instruments TPS7A20 are both very good choices with low noise output and excellent rejection of noise at their input, and both come with enable inputs. Then you don't have to risk a DIY solution possibly ruining the ultra low noise specs you paid for in the LDO.
 
You might also consider whether an ultra low noise LDO is worthwhile. According to the SGTL5000 datasheet, PSRR is 85 dB. Since it's already pretty good at rejecting noise from the power supply, you could quickly get into diminishing returns with effort to make a special ultra low noise power supply.
 
This is excellent advice! Thank you!

Whether ultra low noise or not, at least your recommendations are way to add multiple external devices safely, without hitting the 250 mA limit on the 3.3 V power rail of the Teensy.

I’ll keep all this in mind designing my project.

Thanks again!
 
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