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INA240: Part recommendations

user4811888
Intellectual 530 points
Part Number: INA240
Other Parts Discussed in Thread: INA168, INA302, INA293, INA303, INA290, INA301, INA138, INA280, INA281, LMP8640

I would like to use this part to sense inductor current connected to switching point of half-bridge, in a high-frequency(up to MHz fsw) power converter. Would like to consult:

1.If the current ranges from 0A to 50A, how much shunt resistance do you recommend? And how to evaluate its output accuracy when current approaching to 0?

2.What is the special reason why this part has very good PWM rejection?

As in my understanding, if it is connected to switching point, good high-frequency CMRR OP-AMP is needed to deal with switching glitch. Does it mean this part has a very good high-frequency CMRR OP-AMP, along with well-matched resistance inside?

3. Do you have similar parts but with much higher bandwidth(up to MHz)?  Could I understand that the PWM rejection functions have bad impacts on dynamic performances?

  • 0
    TI__Mastermind 26650 points

    Hello engineer,

    Thanks for your questions.

    Let me preface by saying I highly recommend watching our free online training video which detail how quickly calculate error and shunt resistors. 

    training.ti.com/ti-precision-labs-current-sense-amplifiers

    1. To calculate shunt resistance you must first define some system-level parameters like the max allowed shunt power dissipated, desired full-scale output range of INA240 into an ADC, the supply voltage, the accuracy required at full-scale (50A), and the accuracy required at smallest current to be sensed. You mention 0A to 50A, is this bidirectional? Do you need to sense -50A to 50A? If you absolutely need to know when current is 0A, then you will need to provide a reference voltage for INA240. The device can generate a mid-supply reference by connecting one REF to Vs and the other REF pin to GND as shown in datasheet. All of these things need to be determined firstly.

    So if full-scale is 5V, supply (Vs) is 5V, and you need to measure -50A to + 50A, then the analysis is as follows. Configure device so it has a mid-supply reference voltage of 2.5V. Vout_max = Vs - swing-to-rail = 5V-200mV = 4.8V (see datasheet). To maximize dynamic range set 4.8V for when load is +50A. Thus Rshunt = (4.8V-2.5V)/Gain.

    Error at currents approaching zero are dominated by input offset (Vos = ±25 uV). So if load truly is 0A, then Vout = Vref ± Vos*Gain. Thus for a gain of 100, you could see as much as ±25uV/Rshunt absolute error.

    The best way to get accurate 0A measurements is to calibrate system when the load is off (0A). So I would have ADC read the differential output (Vout-to-Vref) when load is off, and then store this value into memory so subtract off all future measurements to correct for offset error.

    2. Device has good PWM rejection because of special proprietary circuitry. There is no need to think of the device as so. Yes it does contain an amplifier with well matched resistor network, but it is much more complex and includes an input biasing stage.

    3. There really are no similar parts with that level of BW unfortunately. The PWM rejection circuitry does not necessarily have an inverse correlation to BW.

    Sincerely,

    Peter

  • 0 in reply to Peter Iliya
    Intellectual 530 points

    Hi Peter,

    Got it, thanks very much for your kind answers! I will go through the training video and your calculations.

    BTW, if I don't need PWM rejection function, which part in this series has the highest level of BW? How much BW does it have?

  • 0 in reply to user4811888
    TI__Mastermind 26650 points

    No problem valued engineer.

    The PWM rejection is best for applications where input common-mode voltage (VCM) is rapidly switching up and down large amplitudes.

    You can search for larger BW among CSA (current sense amplifier) products using our parametric search tool:

    https://www.ti.com/amplifier-circuit/current-sense/analog-output/products.html#sort=p314;desc

    This shows the devices with best BW are INA281, INA293, INA280, INA290, LMP8640, INA138/INA168, INA303/INA302/INA301.

    If you are sensing 0A load currents or the load is PWM and falls to 0A, then you will want to choose a bidirectional device (includes REF pin), or use the INA138/INA168 and bias the output voltage into the linear region according to section 8.2.3 of datasheet. Keeping the output in linear range is important for all amplifiers to avoid saturation delays.

    Sincerely,

    Peter