• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 908 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5088: Minimum value of current sense resistors Rs between pins CS and CSG

Joakim Östh
Prodigy 60 points
Part Number: LM5088

Hi!

I use the LM5088-2 controller in an application to convert nominally 48V to 12V, 3A or less in normal operation. In a previous design I used 20mR for the current sense resistors Rs between pins CS and CSG and the converter worked as expected.

In a later design I change the Rs value to zero ohm (0R) and it seems that some prototypes not work from beginning (3 of 10 not start) and also that the controller fails after a while on some boards.

I think the reason may be that I used 0R for Rs instead of a small value? According to datasheet the ramp signals is dependent on the Rs value so maybe the controller get stuck or oscillate with to small value? I don't find any information about the minimum voltage required between CS and CSG pins on the LM5088, but I see in the datasheet it is connected to an OP amp so I think it must be a minimum value for correct operation?

What is the minimum value of Rs I can use? (In webench designer the maximum current I can select is 10A). Basic connection is shown below, difference between the two cases is only I change Rs from 20mR (working good) to 0R (mostly working, but fail after a while).

Any information about the minimum value of Rs and if the reason some units not work or stop working after a while is the results of using 0R for Rs.

Best regards

Joakim

  • 0
    TI__Mastermind 40040 points
    Hello Jokeosth,

    You are correct, a value is required to generate an acceptable ramp presented to the PWM comparator. Without it, you will have instability as you are seeing. I thin the min value will depend on layout, I thing 10A is above the max you can run this device, and we set the Rsense as shown in the Datasheet; the result would be the minimum value I would use for this device? I suspect the value will come to the WB result of 20mR.

    Hope this helps?


    David.
  • 0 in reply to David Baba
    Prodigy 60 points

    Hi David.

    Thanks for help. When I use WB I get 6mR for 10A. However, your answer partly solve my problem. I really need to know what is the minimum? The internal OP that compare CS and CSG must have some limits. But I can't find it in the datasheet. Can you find out some more information about the limits?
    (By the way the EN pin is connected to R1122 in the schematic, but it disappeared in the figure. So this is not the issue some prototypes not started.)

    For additional information I can say that in normal operation it will be about 2 to 3A of load current. But in case of overload in other circuits I want the other circuits overcurrent limit to start before the LM5088. Hence I need a quite large maximum current of about 15A for the LM5088. Hence the interest in the minimum Rs I can use safely with the LM5088 controller.

    Best regards
    Joakim

  • 0 in reply to Joakim Östh
    TI__Mastermind 40040 points
    There is an additional problem to contend with, as the amount of voltage you have on the current sense will be minimal, you will need a certain amount to maintain stability. The other issue is, if you make the Rsense close to 0R, you will in effect be programming the loop with a fixed ramp, which is used for Slope compensation. This being fixed will transition your loop to a double pole system (voltage mode) and will render the loop unstable. As mentioned, you will need to have some current sense signal there sized again the slope compensation to keep the loop as a current mode control, you can try to set the Rsense to a value to greater than 10A, but when select very low Rsense, you will start getting an under damped loop response and a stability issue to deal with, this is assuming your layout is good enough to not corrupt the very little signal you have on the current sense input, which was my initial concern.

    Hope this helps?

    David.