LM63625EVM: LM63625EVM

Hi Gangadhar,

Please let me take a look and get back to you tomorrow.

Thanks,

Youhao

Hi Gangadhar,

I read the datasheets and it should work.  Thank you for the scope picture, but I cannot read the time scale. Is the noise repetitive at the switching frequency, or it is at a lower frequency?  Also, did you use the short ground lead probe to measure the output ripple? Note that the long ground lead of a regular probe would pick up noise and produce unrealistic  high voltage ripples. Please use the probe like below when monitor the output voltage ripples.

Thanks,

Youhao

Hi youhao

Thank you very much for the response.The noise is repeatative at switching frequency as well as lower frequency. I have used the following voltage probe for ripple measurement. Basically I have measured the output voltage directly across output capacitor using tip of the probe, still the same problem. The time scale for the above scope picture is 500 ns/div.

Regards

Gangadhar

May I bother you to use the short ground lead probe as I shown earlier and share the scope picture? 

Thank you very much for your response youhao. I used small wires to mesaure ripple, now its working good .

Another issue youhao, i wanted to use LM63625 EVM board for my choice of switching frequency. It is generally operating at 2.1MHz frequency. But when iam trying to operate it at 730kHz(my choice) it is not working properly, the output is coming as 0V. RT pin directly connected to GND will give me 2.1MHz operating frequency. But when i connect suitable resistor(through hole/SMD) to operate it at my switching frequency between RT pin and GND, output is not coming. Can I know what is the problem for this issue?

Hi,

What do you mean by through hole/SMD?  I am afraid you did not get the resistor placed correctly.  I took a look at the EVM layout and I cannot tell how could you make the modification to your 730kHz:  the pin is connected to GND underneath the IC and there is no way to cut the trace to insert an resistor.  I am afraid your rework may damaged the IC.  

From the 2.1MHz operation you should have confidence that the IC works well.  You may need to create your own PCB and enable the 730kHz, and you will do it anyway, so it is the time to move on to build your own circuit.  Please follow the EVM layout.

Thanks,

Youhao

Thank you for your new suggestion to make a new PCB layout youhao.

What I did was I lifted the RT pin of IC and then connected Resistor between RT pin and GND. It was not working for 730kHz. Is it possible in this way youhao?

According to datasheet, RT=15770/fsw is the formula given and for 730kHz, I got 21.6kohm resistor and i connected that resistor (Surface mounted Device) between RT pin and GND. So is this approach is not possible in this evaluation board?

Trust me that the device will support programming frequency by inserting a resistor from RT to GND, but I am afraid you damaged the IC pin when you lifted it up. 

You should be able to make it happen in a new PCB.  If you still want to verify it on EVM before moving forward, you may cut the trace by removing the IC, then soldered a new IC down and place the resistor across the RT pin and GND pin.  This is a very delicate work, and make sure your soldering will not go over your cut to reconnect the pin to the under pad.  Also make sure your cut would not be too deep to cut into the inner layers. 

Thanks,

Youhao