Teensy 4.1 not running on external power (LiPo-Battery/3 AA-Batteries)


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As written in the title my Teensy 4.1 is not running on external power. I have a 3.7 Volts LiPo-Battery. I connected VBatt+ from the battery to the VIN pin on the top right corner og my teensy and afterwards i connected GND to GND. But the LED which is supposed to blink (which it does when i connect the teensy to my computer with MicroUSB) does not blink. I also took 3 AA-Batteries and connected them in series. Then again i connected + of the batteries to the VIN pin and afterwards GND to GND but with the same result that the LED is not blinking. I measured the voltage on the pins of my teensy and had 3.72 Volts with the LiPo-Battery and 4.52 Volts with the 3 AA-Batteries. I then did a little research and but the CPU-Speed from 600 Mhz down to 24 MHz, because in a thread referring to the Teensy 2.0 i red that the Microcontroller only runs at a Speed below 16 MHz (highest for Teensy 2.0) when running on 3.3 Volts. But that also didn't do the trick. Any thing i am missing here? Anyone has any advice? Thanks in advance!
If you don't mind adding another module to your project, I've used the <PowerBoost 1000C Charger> from Adafruit successfully in several of my portable projects. This module will take care of boosting the output voltage from the LIPO battery to a clean 5.2VDC that the Teensy will just love. It'll also take care of managing the charge on the LIPO< & can even act as a 5VDC pass-thru when the USB input is connected & the LIPO is charging and/or fully charged.

Hope that helps . . .

Mark J Culross
Seems a new sketch for blinking was loaded to drop the speed to 24 MHz?

Any chance that sketch has something like:
while( !Serial ) {

That will cause this behavior where the program waits for USB to connect to Host computer.
When I check the schematics for the Teensy 4.1 board:

What I see:
a) VIN is the same as VBUS (from USB). There is not any decoupling diode! So, you have to make sure,
not to power via USB in parallel with an external battery!!!! (exclusive!)
(VIN becomes 5V with USB plugged in!)
b) There is Q1 and VIN goes via it to the LDO, the U4 (TLV75733P).
This generates the 3V3 voltages for the MCU (coming out as 3V3 on pin 3V3).
c) BUT!: this TLV75733P has a max. dropout voltage of 475mV (when 3V3..5V is applied):

So, it means to me:
you want to provide 3V3 out of LDO to MCU, plus 475mV dropout voltage, plus dropout on Q1 (? maybe 500mV ?) - you need at least: 4.0V ... 4.3V as VIN.
So, a fully charged LiPo with 3.7V might not be enough (for sure not: just 3V7 - 3V3 = 400mV margin but also Q1 is there!)

Measure, what you get on the the 3V3 pin, with your 3V7 LiPo connected (potentially "nothing" or way below 3V3 (LDO not regulating anymore)).
The 3V3 pin (the third from top) tells you what the MCU would see and use (it should be 3V3).

And: never connect a LiPo and USB at the same time!
Or: use a protection diode outside from your LiPo battery. But this would reduce also the internal voltage (depending on which type of diode: 0V3...0V7 voltage drop by diode).

I think, the U4 needs a bit more as 3V7 (and consider voltage drop due to Q1).
The marking as: "Vin (3.6 to 5.5 volts)" is potentially not correct:
4V needed, at least, and 5.5 volts is the absolute maximum for the chip U4!
b) There is Q1 and VIN goes via it to the LDO, the U4 (TLV75733P).

I need to update the web page, yet again. In early 2021 all TLV75733P disappeared from the market. Robin bought a large amount just before they vanished, but the shortages lasted longer. In late 2021 we were forced to redesign. We used an OnSemi part during the 2022 shortages. More recently, we've switched back to TI, but using TLV75533 now.

So, it means to me: you want to provide 3V3 out of LDO to MCU, plus 475mV dropout voltage, plus dropout on Q1 (? maybe 500mV ?) - you need at least: 4.0V ... 4.3V as VIN.

With all of these chips, the dropout voltage is under 0.2 volts at the current Teensy 4.1 actually uses. TLV75733P is rated for 1 amp maximum, and its maximum dropout voltage is at that 1 amp rating. If you look at the charts in datasheets, you'll see the dropout voltage scales roughly linear with the actual current.


Teensy 4.1 doesn't use anywhere near 1 amp, so the 475mV dropout spec at 1A doesn't apply.

Q1 drop is very low. It is rated for 52mΩ. At 250mA (CPU running with SD and Ethernet active) the loss is about 13mV. It's voltage drop is nowhere near 500mV.
I was able to get the teensy running on external power with the setup describen. I guess it was my bad soldering, which may have led to a high resistance. I soldered the pins again and made sure there was plenty of solder to connect the teensy and the pin. Now it is working with both, the 3.72V LiPo and the 4.7V batteries. Thanks for your support!