I'm trying to power a microcontroller off a DALI communication bus. The bus voltage is 16V and is being regulated to 3.3V by a TS2950 to run the Attiny2313. The current draw from the circuit is between 0.68mA to 2 mA.

When sending commands on the DALI bus, a transistor shorts the power supply for 417 us (+/- 10%) to create a bit, but right now, when the power supply shorts, the microcontroller ofc gets powered off and stops sending the rest of the message.

My initial thought was just adding a fairly large capacitor right next to the TS2950, to supply current when the DALI voltage drops to 0V.

However when I add different size capacitors, the time for the 3.3V supply to drop, does not change. Here are some measurements from a USB oscilloscope.

Here is the measurement with no capacitor (not even the stabilizer for the TS2950)

no capacitor

The next measurement is 10uF


Which makes no improvement at all, except stabilize the output of the TS2950

Next is a 33uF


Which improves a bit again, but the next is 220uF


Which does not increase as much as I expected and I don't understand why. Is this even possible with a capacitor and if so, how?

  • 3
    \$\begingroup\$ When your power supply stops, does it drain the capacitor also? Might want to put a diode or something in there to stop what used to be sources from draining the cap the other way. Hook just a cap to the power supply, let it charge then turn the supply off. Does the cap drain? \$\endgroup\$
    – DKNguyen
    Commented Dec 13, 2021 at 2:18
  • \$\begingroup\$ I just assumed that the TS2950 regulator stopped current from going "the other way". Would you add the diode before the capacitor, or where are you thinking? Do you need my schematic? \$\endgroup\$
    – jonas
    Commented Dec 13, 2021 at 2:21
  • \$\begingroup\$ It depends on the regulator. Pass transistors can have parasitic body diodes, etc. There are also things like ESD clamp diodes with funny paths. A diode before the capacitor so current can flow from the supply/regulator, whatever, into the cap but disallow current from flowing from the cap into the supply/regulator. \$\endgroup\$
    – DKNguyen
    Commented Dec 13, 2021 at 2:22
  • \$\begingroup\$ The TS2950 datasheet does seem to imply the pass transistor should stop current though, but better check with a diode just to be sure. \$\endgroup\$
    – DKNguyen
    Commented Dec 13, 2021 at 2:24
  • 3
    \$\begingroup\$ Answered here: electronics.stackexchange.com/questions/560511/… \$\endgroup\$
    – Kartman
    Commented Dec 13, 2021 at 5:16


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