LM5155: poor load capacity when vin is low

Hello, 

Thanks for reaching out. Can you please share the schematic with the modifications made? Looking at the Bode plot in the Excel sheet you shared, it looks like the converter is not stable with the compensation values selected. 

Kind regards, 

EM 

Hi EM,

I have changed the Rcomp from 470k to 11k, and the details shown in attachment, the result in excel shows stable. But the inductor current shown as below, Vin=3.7V，Iin=14.96A,Vout=17.4V，Iout=1.2A. So is there any other problem? Could you kindly share your comments?

LM5155.xlsx

Hi, 

Thanks for your reply. You can try with a phase margin in this Excel file at least 45deg above the point where the gain is crossing the 0dB axis. A good starting point could be the suggested values (4.5k and 41nF). For CHF, since it is not mandatory on the schematic, I would try first with 100pF . Other than the Excel, do you also have a schematic or are you modifying an LM5155EVM-BST? Are there other additional components on your board other than those on the Excel?

Kind regards, 

EM

HI EM,

①I have changed the R13 and R15 in EVM to get Vout=20V，and this is correct

②have short VCC、VIN、BIAS together in EVM to enable 3V input voltage

③I have replaced the current sample R6 from 8mohm to 3mohm

④Change inductor to 74435581000，for the size is too big， I use flying leads to connect inductor with EVM's pad

⑤I have changed the value 4.5k and 41nF you recommend today， and after I test it， but it doesn't help.

Hello, 

Thanks for your feedback. If you have a higher Vin do you still see this problem? You can try to slow down a bit more the compensation and try with 3k and 70nF. Other than this, can you also please share the plots of SW node voltage and Vout?  The EVM also has a resistor divider on the UVLO pin, which you should update if you want to use this feature. Please keep in mind as well that the VCC UVLO threshold is also close to 3V, therefore you need to have sense connection to your power supply, otherwise you might have a lower voltage at the input of your converter due to losses cause by such a high input current. One more comment: in this case the duty cycle is quite high, therefore it would be helpful to add some additional slope compensation (R4 in the schematic of the EVM).

Kind regards, 

EM

Hi EM,

Thank you very much! It can support 3.5V input and 20V/2.6A output, when lower input voltage--3.15V, the output can cover 20V/2A. But don’t work in the condition shows above so long time for the EVM will be so hot, 20s is OK.

The detail modify shows as below:

• Change C19 to 33nF;
• Change C24 to 220nF;
• Change R17 to 2Kohm;
• Change R15 to 2.49Kohm;
• Change R11 to 24Kohm;
• Change Q1 to CSD17576Q5B(I will also deliver 3pcs to you)
• Change R6 to 2mohm;
• Short BIAS with VCC(J8 in EVM).