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TPS2549: LM5088 Buck based design for TPS2549

Rohit Kashyap1
Prodigy 30 points
Part Number: TPS2549
Other Parts Discussed in Thread: LMR14030, LM5088, TPS2546

Hello Community,

Hope the lockdown's going good for all!

I was designing a USB charge control IC based on the TPS2549 IC.

The datasheet's application note mentions a design procedure for CS pin feature available with the LMR14030 Buck IC.

However in my application I am using the LM5088 Buck IC.

I have problem in computing the exact values for R-FA & R-FB for which the V-CS > 2..5V condition can be met appropriately.

The O/P voltage for LM5088 can be set via the following equation:

RFB2/RFB1 = (VOUT / 1,205) -1

The V-REF is 1.2V

The VOUT is 5.7V,  RFB2 = 6.49K & RFB1 = 1.74K. Now RFB1 = RG = 1.74K for TPS2549 calculations. RFB2 = R-FA + R-FB

The calculations are below assuming Wire resistance = contact resistance = 0R.

Calculation for TPS2549

Sr. No. Parameters Calculation Equation Calculated Value Assembly Value
1 V-DC (V) without load   5.7  
2 R-G (kOhm)   1.74  
3 R-Total (Ohm) R-Total = R-Internal (0.42) + R-Wire (0)+ R-Contact (0) 0.42  
4 G-CS (mA/A)   0.075  
5 R-FA (kOhm) R-FA = R-Total / G-CS 5.6 5.6kOhm
6 V-FB (V)   1.2  
7 R-FB(kOhm) R-FB = ((V-DC)/(V-FB)/(R-G))-R-G - R-FA 0.925 0.931 kOhm
8 V-CS (V) V-CS = ((V-FB)/(R-G))*(R-G + R-FB) 1.84 Ensure V-CS > 2.5
9 Maximum Ios at 27K (A)   2  
10 V-DC Maximum (V) V-DC Maximum = 5 + Ios Max * G-CS * R-FA 5.84  
11 C-OUT (uF)     2 * 47uF
12 C-Comp (nF) C-Comp > 3 * G-CS * C-OUT >21.15 22nF

The criteria for V-CS is > 2.5V which is needed for this calculations.

I tried using some combinations of equations to get RG, RFA & RFB values in some range and try out but I am getting negative values for RFB.

Has anyone tried using the LM5088 with the TPS2549? Any suggestions on what can be done here to get deliverable results?

  • 0
    TI__Mastermind 30000 points

    Hi Rohit,

    First, I would suggest having an output voltage closer to 5V instead of 5.7V which may be a little high. Also, having a wire resistance of 0 defeats the purpose of this feature. This feature is intended for applications that have a large cable connected to it internal such as an automotive infotainment system. If you are creating a charger does not have a long connection on VBUS, then there is no need for this feature and you can use the TPS2549 without CS functionality or the TPS2546.

    If your system does require this feature, then recommend updating your wire resistance, and following Equation 1 and Equation 2 in the TPS2549 datasheet. 

    I have not seen a system that uses the LM5088

  • 0 in reply to Adam Mc Gaffin
    Prodigy 30 points

    Hello Adam, 

    Greetings of the day. Hope you are doing fine.

    With reference to the 5.7V, I actually have a Power OR circuit implemented. The LM5088 Buck (5.7V, 8A) is in parallel with  5V 100mA LDO (NCP781). The Power OR circuit is implemented via two schottky diodes, 

    The diode on the 5.7V line has a forward drop of 0.5V so effectively the buck output is ~5.2V.


    Now with reference to considering 0 Ohm wire resistance, if you check the TPS2549  datasheet, in the application note itself there is consideration of only the internal resistance of the IC itself and not the wire length.

    I do have the calculated values of wire length, gauge, resistance per unit length, etc. ready but I followed the calculation method in datasheet first and tried to come to values of all parameters that met the conditions.

    But unfortunately the results I got didn't work out with reference to the minimum values set for few parameters.

    I then tried to do the calculations with reference  to one meter cable length and then two meters to see if that could meet my requirements, but that didn't help either. I did eventually switch to the TPS2546 IC.

    But in my application, I am communicating with a Android Tablet connected to my system. The USB Cable length would at maximum be around 2 meters and I definitely need to use the CS feature. 
    Your further insights into this topic would be really helpful.

    Please let me know if any other information is required from my end to get this problem resolved.

    Also, you mentioned, that you haven't seen anyone use the LM5088. I currently need a buck converter with maximum input voltage in the range of 50-70V. The output of the Buck will be set at 5.7V, upto 8A of O/P current capacity.

    What are the common DC-DC converters from TI generally recommended for these specifications? 

  • 0 in reply to Rohit Kashyap1
    TI__Mastermind 30000 points

    Hello,

    I cannot comment on the DCDC that you will need. I would recommend using the WEBENCH tool to find a component close to your requirements, and then you can post an E2E question with that part number if you have any specific questions

    https://www.ti.com/design-resources/design-tools-simulation/webench-power-designer.html 

    As far as the TPS2546 and TPS2549, if you are wanting to use the cable compensation functionality, then you will need to use the TPS2549 as the TPS2546 does not have this functionality. 

  • 0 in reply to Adam Mc Gaffin
    Prodigy 30 points

    Hello Adam,

    Hope you are doing good.

    I selected LM5088 via on WEBENCH only. No worries on that front.

    Regarding selection between TPS2546 & TPS2549, the functionality is almost same expect for the CS pin. I currently moved ahead in my design with the TPS2546 but would definitely like to know where I made mistakes while designing with the TPS2549 so that in future designs I can use it.

    Thanks for your time.

    Regards,

    Rohit Kashyap.

  • 0 in reply to Rohit Kashyap1
    TI__Mastermind 30000 points

    Hi Rohit, 

    Thank you for the update. Based on your description, I believe that the TPS2546 is the correct device for you. 

    As far as evaluating the TPS2549, I believe this was the main limitation you had

    I do have the calculated values of wire length, gauge, resistance per unit length, etc. ready but I followed the calculation method in datasheet first and tried to come to values of all parameters that met the conditions.

    The customers that I have seen implement this device with the cable compensation functionality know the values for all of these items. I believe this was your main limitation.