ATMega328 and LM2575 switching voltage regulator noise issue

Other Hardware General Electronics
1

Hi,

I have a problem with a circuit that I'm working on. I have an LM2575 adjustable switching voltage regulator in order to efficiently drop part of the circuit to ~8v. (the rest is 19v). Also, I have an ATMega328 which has a separate power source of 5v. The ground plane is shared. I've noticed that when the ATMega is attached but without any power source, there is noise on the 5v line. I presume that its because the atmega doesn't deal very well with the noise on the ground plane from the LM2575. Is this the case? Can anyone confirm this? Can anyone think of a work around? Please find schematics attached!

Many thanks.

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2

What is the separate power supply? Im kinda confused, you say the noise appears when there is no extra powet supply? Like 0 volts on the atmega?

3

AREF should not be connected to Vcc, only a 100nF.
I assume those resistors R6 and R7 are not really 10 ohm.

4

mart256:
What is the separate power supply? Im kinda confused, you say the noise appears when there is no extra powet supply? Like 0 volts on the atmega?

Exactly, even when the 5v power supply is not connected, I see voltage on the 5v line... This affects the CATs (led drivers) and not in a good way.

5

Peter_n:
AREF should not be connected to Vcc, only a 100nF.
I assume those resistors R6 and R7 are not really 10 ohm.

Are you sure about that? I followed guides that build a DIYduino (obviously none of them work in this way) that had it connected to Vcc. Do you think this could be what's affecting the 5v line?

R6 is meant to be 200ohm for the Power LED and R7 is meant to be 10Kohm for the RESET. Bad labelling.

6

Yes, I am sure.
When you select 'DEFAULT', the internal reference is already set to Vcc.
When you select INTERNAL, the AREF should not be connected to Vcc.
That error is also in these Arduino pages:

There are more errors in other pages, but it is impossible for us normal users to get that corrected.

This is not your problem with 5V or with ground noise.

7

AVcc to Vcc (+5)
But Aref just gets the cap to Gnd.

AVcc vs Aref (I've done that, too.)

8

Peter_n:
This is not your problem with 5V or with ground noise.

Do you mean that you think fixing this won't fix my problem with noise on 5V?

9

I think so, it won't fix it.
I just noticed in the schematics that AREF was connected to Vcc and I thought I better report it :roll_eyes:

10

on the schmetic :

the LM2575 circuit shows D1 as a common diode, not a schottsky.

on the PCB layout :

pin 1 appears to be your 19v and the inductor is on pin 1.

pin 2 is the 9v output.

could be that I do not have the correct data sheet or am reading your layout wrong.

11

dave-in-nj:
on the schmetic :

the LM2575 circuit shows D1 as a common diode, not a schottsky.

on the PCB layout :

pin 1 appears to be your 19v and the inductor is on pin 1.

pin 2 is the 9v output.

could be that I do not have the correct data sheet or am reading your layout wrong.

it's a schottsky diode.

the inductor is on pin two labelled L1. On 19v is a capacitor labelled C9.

12

aparaske:
the inductor is on pin two labelled L1. On 19v is a capacitor labelled C9.

not on the board layout that was posted.

L1 in on pin 1, the input.

LM2575.png

13

dave-in-nj:

aparaske:
the inductor is on pin two labelled L1. On 19v is a capacitor labelled C9.

not on the board layout that was posted.

L1 in on pin 1, the input.

Apologies for the labels. I didn't really bother with eagle to fix the labelling.

LM2575.png

14

What I find very bizarre is that when the atmega is not attached there's no noise on the 5v line.

No one can think of anything? Would you need more details?

15

with the multiple labels being wrong, it is hard to know what is where.

makes everything uncertain.

if the labels are wrong and whomever stuffed the board followed the labels, it is hard to know what might be connected.