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DC DC Buck Regulator and current sensing using INA138 Current sensor

BALKRISHNA TULSYAN
Intellectual 850 points
Other Parts Discussed in Thread: INA138

Hi,

i am using 100 khz as a switching frequency for buck regulator.
As per INA138 datasheet: Gain can be between 1 and 100 and output load resistance(or the gain) will affect the measurement bandwidth.

Moreover Electrical characteristic states that:
if RL = 5 Kohm , F = 800 khz
if RL = 125 Kohm, F =32 khz

Does it mean that for 100 khz switching frequency, maximum gain that can be selected is 8 and thus max output load resistance of INA138 is limited to 40 kohm ???


That means there is a direct relation of gain of the INA138 with the switching frequency of the buck regulator. Am i right or i am missing something???

Thank you in advance.

  • 0
    TI__Mastermind 31580 points
    Hi Amit,

    This post is in the wrong forum, I will move it to the correct forum and one of my colleagues will answer it.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Intellectual 850 points
    Which forum is a right place to post such question. i would appreciate if someone helps early.
    Thank you in advance..!!
  • 0
    TI__Genius 15760 points
    Hi Amit,

    You are correct. I performed same calculations using Figure 1 (Gain vs Frequency) and Voltage Gain - RL Table from Frigure 9 of the datasheet and I have same results as you.

    For 100kHz, the maximum gain that can be selected is 18dB (8 V/V), thus Rl should be set to 40kohm.

    Regards.
  • 0 in reply to Mayrim Verdejo
    Intellectual 850 points
    Thank you so much for the confirmation Mayrim Verdejo .
    To be in a safe side i have selected 39k instead of 40k for 100khz operation, thus gain is 7.8(v/v)

    Thank you again..!!
  • 0 in reply to BALKRISHNA TULSYAN
    Expert 2305 points
    Amit - did the INA138 end up working at the bandwidth you need? I am working on a similar circuit (high-side current sense for a buck regulator). I was originally using a similar current monitor from Diodes Inc, although what caused problems was not strictly bandwidth, but a "delay" between when my current begins to ramp up (when my switch turned on) and when the output began to react. At about 50 KHz I would begin to see a significant delay between the actual current and the monitor output.

    The INA138 is rated for a higher bandwidth, but also more explicitly shows increased bandwidth at lower gain. In my application I would be OK with a gain of 1 but I wonder how you made out at 100KHz with a gain of 7.8?

    Thanks,
    Dave
  • 0 in reply to David Dean
    Intellectual 850 points

    Hi Dave,

    I have NOT done the practical interface of the sensor with the buck, as i stuck in the working of Full bridge MOSFET driver (Heating issue) used in the buck. It's all theoretically calculated based on the datasheet of INA138. You can find out the details in the Electrical Characteristics section of the datasheet.

    FREQUENCY RESPONSE
    Bandwidth ROUT = 5kΩ 800 800 kHz
    ROUT = 125kΩ 32 32 kHz

    From the above parameters, ROUT comes out to be 40 kΩ for 100 kHz Bandwidth operation. Thus gain = 8

    Since standard ROUT available in market is 39 kΩ, I opted for this value where gain comes out to be 7.8

    PS: I got some urgent work, so i left this project in between but will work on this soon.