Regulator long start up

Hi everyone, I’m having troubles with the D24V5F5 and the D24V5F3 regulators and I would be happy if somebody could help me out.

The setup is a custom PCB where a 12V lipo battery powers the 5V regulator to power an avr, and then the 5V is fed to the 3.3V one to power an ESP chip. (In retrospect, I should have fed the battery power independantly to the 5V and 3.3V regulators)

When only the 5V regulator is plugged in, and I switch on the power, the indicator LED will take between 12 and 15 seconds to light up, and if turned off and then on again, startup will be almost immediate. After waiting about 2 minutes, the same issue arises.

Now the problem is when both the 5V regulator and 3.3V regulators are plugged in, they don’t seem to be able to start up, the 5V regulator being stuck at a 1.08V output and the 3.3V one at about 0.4 volts.

I need to add that the board is supposed to control stepper motors, so there is 400µF of capacity between the positive and negative side of the battery because of the 4 drivers the battery is hooked up too.
When the drivers are plugged into the board, the 5V regulator alone won’t even start.

Last strange thing, is that if I unplug and plug again either one of the regulators, then they start up immediately.

I checked the solder joints on both the regulators and they are fine, covering all the pad, I can provide board schematics if necessary.

Does somebody have an explanation or a fix to this problem? Thanks in advance :slight_smile:


Those regulators have a built-in soft start features that slowly ramps up the output voltage, but that should happen much faster than a second. Do you have a lot of capacitance on the 5V or 3.3V lines?

I suspect that when you try to power the 5V and 3.3V regulators at the same time it is too much load for the 5V regulator so it is not ever able to fully turn on. Those regulators can only output around 500-600mA. What is the current draw of your AVR and ESP? You are correct that it would make more sense to power the 3.3V regulator directly off the LiPo.

As for the 5V regulator not turning on when you power your stepper motors, it could be that the motor current draw is causing the LiPo voltage to drop. Could you measure that voltage?

Could you post pictures of your setup that show all connections and a wiring diagram of your setup? What is the capacity of your LiPo? What are the voltage and current ratings of your motor?


Thanks for your answer, I checked the Lipo voltage, but being a 50C drone battery, it was no surprise that there wasn’t any drop. I also used the 5V regulator off an arduino to check if it was the capacitance for the drivers causing a problem.

After checking the connections and carefully reading the datasheet, I noticed I connected the Shutdown pin to the output of the regulator, which seems to be the problem, I’ll cut the track on the PCB and everything should go back to normal since the output is held high by default on the board.

Edit: Indeed the problem was solved by cutting the shutdown pin, now both regulators start up instantly, appart from the obvious mistake of power the 3.3V line off the 5V line from an efficiency perspective, everything works fine.

Thanks again for your reply.