Does a switching regulator effect the output of a buck converter with a fixed output voltage of +5 V like the L7805?

[FKamm.2]

I'm trying to design an voltage regulator on a motor control board. I want an output voltage of 5 VDC and the input voltage is 24 VDC. Because of heat production i was thinking about using a switching regulator like the L7983 followed by an L7805. But I'm unsure if the frequency of the switching regulator will effect the output signal of the L7805. I need the 5V for the microcontroller and the current sense amplifier.

Or is there an easier way?

Thank you very much

[FKamm.2]

oh, yes that was a drawing mistake.

thank you very much! i dont have a contact person for these questions in my company, sorry for the detailed questions.

Wouldnt it be better to supply the microcontroller after the pi-filter because of electromagnetic interference?

So that the path of the pulsed signal is as short as possible?

Do you think the 1000 kHz are a good compromise? A higher frequency will lead to a smaller ripple voltage, but a frequency

choosen too high is bad because of the electromagnetic interference.

Regards,

Fabian

[Peter BENSCH]

The supply of the mcu should be taken before the filter, because otherwise you will have the ripple caused by the switching currents of the mcu fully on the analogue supply. This is exactly the case you want to avoid, right?

You can think of your circuit like this:

1. the output voltage at pin 1 (VOUT) supplies the digital section, e.g. the mcu
2. the +5V at C16 supplies the analog circuitry

BTW: an mcu with 5V supply? Sounds like an old one.

Regards

/Peter

[FKamm.2]

Yes, that's what i want to avoid. okay, thanks for the explanation.

I'm using the AT90USB1286.

Regards

Fabian

[Peter BENSCH]

I forgot to answer your last question: 1MHz could well be a sensible frequency, but of course it also has disadvantages. EMI is an issue that can be dealt with by a very well thought-out layout (current paths GND and input/output voltage, especially the nets VIN and LX).

Another issue is the choice of passive components, as this also has a great influence on the EMI generated. Here, L2 should be emphasised, whose construction contributes significantly, which is why a magnetically shielded inductors should be used.

Of course, you can also use a lower switching frequency such as 200kHz, but this requires larger component values for L2, C4 and L4, which in turn increase their mechanical size.

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

[Harvey White]

There's a replacement for the 7805 regulator, which is a 3 pin switching regulator to which you can add a filter. Check with Digikey, who seems to stock them. They're about 2-3 USD each, and can tolerate a higher input voltage. Haven't use them in a design yet.