LM25141-Q1: LM25143-Q1 Bode

Hi Xianfeng

Please upload your full schematic as I can see partial image only. R3443, R3444 are part of current sensing part. 

For measuring BODE, you need to add external feedback divider to measure the loop and test using conventional techniques. The values of feedback resistors do not matter, only the ratio of the feedback resistors matters, effectively the gain from Vout to FB. Please make sure that the Thevenin equivalent impedance of the resistive divider connected to the FB pin must be greater than 5 kΩ to detect the divider.

Otherwise, you can check the bode plot using the excel tool here . Please fill the excel tool and check the bode plot.

Hi ARPITA NATH,

Thank you for your reply!

Attached is the full schematic.

Hi Xianfeng,

Thanks for full schematic. As I have said, you need to install an external feedback divider and test the bode as per usual technique.

Thanks

Arpita

Hi Xianfeng,

DO you have any other query?

Regards

Arpita

Hi, Arpita. According to the LM5141-Q1 EVM User's Guide, the FB pin is connected to ground to get a 5V output. If I need to test the bode, what should I do? Still need to add external feedback divider to FB to get 5V output instead of connecting to ground? Thanks

Hi Siv,

the default setting of J9 [FB connection] is FB is shirted to GND to get 5V output in the EVM.

As explained above to measure bode, kindly disconnected the short between J9-pin 2 to 3 and place the external dividers. Otherwise, you can find experimental the bode plot given in EVM user guide or you can get the bode using the excel tool itself.

R20 (RFb2), R22 (Rfb1) can be calculated using the following eq [4] from datasheet or you can use the excel tool attached previously.

Arpita, thanks for your advice.

1) Also, I am wondering if the ''disconnect the short between J9-pin 2 to 3 and place the external dividers" action has different loop response (shown in Bode) compared to FB shirted to GND to get 5V output. I recently learned about Type II transconductance compensation for peak current mode, and I think Kfb is also a part of transfer function. Is Kfb the same for both two ways?

2) For loop compensation, I mentioned that in the LM25141-Q1 quick-start calculator excel, there is no Cc2 for compensation. Only series Rc1 and Cc1 are connected to COMP pin. Under which circumstance can we not use Cc2 for compensation.

Really appreciate for response.

Hi Shiv,

Do you have any other query regarding bode?

Regards

Arpita

Thanks for asking. There is only one question left. For EVM board under 2.2MHz and 5V Vout, could you please give me some advice on the loop stability test as now I am learning it? What can I do on EVM board? My thought is to change the C and R connected to the COMP pin and then do the bode test to see the phase/gain margin. Also, if you have some relevant materials, very appreciate it if you can tell me.

Hi Siv,

I believe the EVM default schematic is for 2.2MHz and 5V output only. The compensation network is already connected in COMP pin. 

To measure bode, follow the above suggestion. 

For other circuit conditions, you can use the excel tool to design compensation network accordingly and measure the bode.

For more learning on loop stability analysis, you can follow Simplifying Stability Checks (Rev. B),

                                                                Understanding and Simulation of Loop Stability Test for DC/DC Converters

Load transient may be another simple diagnostic tool available to instantly analyze the loop stability. https://www.ti.com/lit/an/snoa507/snoa507.pdf

Regards

Arpita