• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 407 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

UCD9248: UCD9248

MaWü
Prodigy 220 points
Part Number: UCD9248
Other Parts Discussed in Thread: UCD7242

Hi TI peers,

I have seen schematics using different componments/values for UCD9248 and wondering what is the "correct" one.

First point is regarding the UCD9248 current sensing pins:

#1 Series resistors connection to e.g. a UCD7242 (IMON pin): 100R or 1k ?

#2 Capacitor to AGND: 0.068uF or 0.01uF, add 4k99 in parallel or skip?

Second point is regarding UCD9248 differential voltage sensing pins (input filter), assuming a desired 1MHz switching frequency:

#3 Voltage divider R1/R2: 1K/787R or 750R/2k  or skipping R2 ?

#4 Parallel capacitor C2 (RC filter): 10.82nF or 560pF ?

Thx and regards

MW

  • 0
    TI__Genius 11095 points

    Hello, 

    1,2- This RC filter is required for anti-aliasing since the current is being sampled inside the controller. See pg 21 of the datasheet to select the RC filter value vs. your deisred response time constant and number of rails. 

    3- The reference DAC can go up to 1.6V, and the Error amplifier is capable up to 1.846V common mode. If your desired output voltage is less than this, you can skip the parallel resistor. 

    4- Similarly, this is used as an anti-aliasing filter .See pg 17 of the datasheet to eq. 4. Place this poll ~40% of fsw. 

  • 0 in reply to Matt Schurmann
    Prodigy 220 points

    Thx Matt,

    regarding 1,2 current sensing I got more questions. 

    In the datasheet the number of rails in formula (8) refers to all used  channel pins (A&B)?

    When selecting UCD7242 in the  "Fusion Digital Power Designer" GUI resp. under "edit full power stage in schematic" I can't see/edit the R value for the RC filter. I can see/edit Rcs2 which is parallel to Ccs. How does the GUI know the serial Rcs value (between pin CS and IMON)?

    Similar issue for voltage sensing: The UCD7242 datasheet recommends 10R (serial) for differential sensing at the load. But the GUI does not show this serial resistor. How is this covered by the GUI? 

    Regards MW

     

  • 0 in reply to MaWü
    TI__Genius 11095 points

    Hello, 

    1- Nrails refers to the number of voltage outputs (voltage rails, control loops) in use. The ADC sampling rate is changed by the number of rails. 

    2- I believe this is a mistake in the GUI. Actually you can see the RCS1 in an screenshot of an older version of the GUI, from the UCD9248 user's guide here. 

    www.ti.com/.../sluu490.pdf

    3- These 10 ohm resistors are optional. It is generally good practice to have remote sense lines which connect directly to the regulation point, and a separate connection local to the power stages. This keeps the voltage loop closed, for example, if the load is uninstalled. The local sense connection should have a higher impedance path back to the controller, to keep the DC regulation point appx at the remote point. 

  • 0 in reply to Matt Schurmann
    Prodigy 220 points

    Hello Matt,

    thanks a lot!

    Regarding #2 (current monitoring):

    The document "sluu490.pdf" makes it more clear.

    Regarding chapter 2.8 there is no Rcs in series for using UCD7242 but in parallel for adjusting the transresistance
    gain (VCS/IOUT).

    So the GUI displays the right circuit when changing from e.g. PTDxxx to UCD7248.

    What is not clear in chapter 2.8 is: The formula #28 and #29 refers then to 1 or 4 used/configured voltage rails?

    Both Ccs and Rcs should be placed as close as possible to UCD9248?

    Furthermore I saw applications where designers added a 100R in series (before the filter) on the current monitor lines. Wondering what will be the benefit of this?

    Thx & Regards

    MW

  • 0 in reply to MaWü
    TI__Genius 11095 points

    Hello, 

    Eq 28 is giving the response time for the fast OCP comparator. It is irrespective of the number of rails, since it does not go through the ADC. 

    Eq 29 seems to assume 25us would work for all cases, but you can get away with a smaller time constant for lower rail count systems

    The only trade-off for the value of this resistor is that it will cause output voltage offset. The error amplifier in UCD9248 has an offset current spec of +/- 5uA. so, for example, 100 Ohms can cause up to 500uV offset error. This is probably OK. But if you divide the input for a higher voltage output, it will also affect the voltage gain. 

  • 0 in reply to Matt Schurmann
    Prodigy 220 points

    Hello Matt,

    thanks for that.

    In the "sluu490" document under "Figure 2-2. Single Board VOUT Sense" there is a "Cdecouple" placed at the load resp. between the diff. sense lines.

    Is Cdecouple required and what value it should have?

    Best regards MW

  • 0 in reply to MaWü
    TI__Genius 11095 points

    In figure 2-2, the Cdecouple is meant to show that the remote sense line should be connected to the decoupling caps of the load to ensure the controller is regulating to the correct point on the board. The decoupling capacitor values are driven by the requirements of the load and the current transient requirements.