question of boost circuits design and layout

Hi,

I need to design a voltage boost circuit from +15V or +5V to +24 V with less than 40mA output current. I use some DC/DC converter chips to realize this. After I test the prototype board, I found that the output pk-pk ripple/noise is very big which is around 4V. I used some LC filter circuits to get rid of this ripple/noise, the result is not very good. I doubt the ripple/noise is coulped from the switching power supply or surrounding environment.

BTW: The switching frequency of the DC/DC converter is about 1.2MHz. the measured peak ripple/noise is at the frequency of about 95Mhz.

does anybody have any experience to design this kind of boost circuit? and what I should take care of when doing the board layout of this circuits?

Thanks a lot.

Have you separate the power ground and analog ground?

No.
what's the difference between the power ground and analog ground? Could you pls explain it to me?

The power ground connect the output, and the anlog groud is control logic ground.
although tht output is 40mA, the peak current may be more than 3A

sunking said:

The power ground connect the output, and the anlog groud is control logic ground.
although tht output is 40mA, the peak current may be more than 3A

Click to expand...

Could you please give me more detailed explanation such as how to separate the power ground and analog ground. I just connect the return path of the input power supply to dc/dc converter with the gound pin of that converter. is that correct?

Thanks

hi arizona999
When the gate in digtial circuit turn on or off,there will be a pulling down or pushing up effect on power and ground.It is one source of noise,sunking is right.
Actually in all Mix-signal design I have seen,the power&ground of digtial&analog are separated.

Best regards
flyankh

flyankh said:

hi arizona999
When the gate in digtial circuit turn on or off,there will be a pulling down or pushing up effect on power and ground.It is one source of noise,sunking is right.
Actually in all Mix-signal design I have seen,the power&ground of digtial&analog are separated.

Best regards
flyankh

Click to expand...

I know what you are saying. But in my case, the ground of the dc/dc converter is the same as the ground of its output circuits. Could you pls tell me how to isolate the ground of DC/Dc converter from its output driving circuits?

Thanks.

I know in some applications the optoisolation works well. But it does not work in my case.

Look at the way analog and digital grounds are separated: First make two seperate ground planes for power and circuit, and then connect them with just one small trace. This puts them at same DC potential, but forms a high impedance from noisy currents to flow into your sensitive circuit side.