WEBENCH® Tools/LM5022: LM5022 maximum output current

Hi Adjivas,

Thanks for considering the LM5022 in your design. I've attached my responses below: 

Q: Is a output of 34.8V @ 1.17A or 34.8V @ 0.5A possible?

The application section discusses how to design a circuit to generate a 40V rail that can output 0.5A as an example. 

I've attached some reference designs within your power requirements (20-40W) below that you can use to help you get started.

http://www.ti.com/tool/PMP20497

http://www.ti.com/tool/PMP20070

Q: How can I calculate maximum current output available?

To calculate the maximum output current, you will need to know the maximum input current and desired efficiency. Then, you can calculate the output current since n*P_in = P_out, where n = efficiency, P_in is input power, and P_out is output power. 

The maximum output current available is not dependent on the controller, instead- it is determined by the ratings of your components like your inductor, FET and diode.  

Additional notes:

In your schematic, your input supply is connected backwards. 

Please let me know if I answered your questions.

Thanks,

Richard

Hi adjivas,

I still think your input source (BT) is connected backwards. Please flip the source so that the positive sign is connected to the positive terminal of the 390uF capacitor. 

As for selection of power components, I think you should refer to the guide I listed below, which will help you modify the components value as needed.

http://www.ti.com/lit/an/slva372c/slva372c.pdf

The LM5022 datasheet has section 8.2.2.10, which can be used to help you calculate the control loop compensation values.

I hope this helps!

Thanks,

Richard

Hi adjivas,

Yes, you have understood how to select the inductor correctly and you can use equation 53,54,55 to select compensation components. However, you will need to iterate through designs until you have reached your desired result. 

According to the calculations, your inductor value is very large, so I would recommend increasing the switching frequency (modifying the RT resistor) to reduce that value to minimize cost and size constraints. In the first link (SLVA372C), I would advise looking at the references section so you have more resources to help you design your boost converter.

Let me know if you have any questions. 

Thanks,

Richard

Hi adjivas,

Here are my responses to your questions:

1) Should I not change the value of A?

Your value of 'A' will depend on the gain of the power stage at the cross-over frequency. So the first step is to determine the cross-over frequency. Since you have a wide input range, I would recommend first determining the Right-half plane zero (RHPZ) frequency, and then making your cross-over 1/5 of it or smaller. 

Once you do this, you should take the negative of that gain and convert that value in dB to a linear gain. Please see steps 1->3 on page 24 of the datasheet for further insight. 

2) Should I also adapt the Rfb1/Rfb2's resistance values?

Your value of Rfb2 should be in between 10k Ohm and 100k Ohm for practicality. Once you selected a Rfb2 value, your value of Rfb1 should be selected such that the voltage seen at the FB pin is equal to the voltage reference (1.25V). Your values from Webench are fine.

3) What Flfp's frequency should I choice?

F_lpf is dependent on your power stage and selection of power components. Please refer to equation 46. 

4) What Fp1's frequency should I choice?

I recommend following the datasheet's recommendation, which is to be 1/5 of the switching frequency. 

5) Inductor Selection (Reduced to 27uH)

Inductance still seems a bit high, but probably ok for your switching frequency. You can reduce it to minimize cost, size, power dissipation if you increased your switching frequency- but your switching losses will increase.

6) I'm to suppose the fsw variable is exprimed in Hz?

Fsw variable is supposed to be chosen as part of the input variables, and is in Hz. 

Thanks,

Richard