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LT3741 Current Regulation Accuracy

A customer asks a question about the LT3741: “how (do) you calculate the accuracy or tolerance of the current it is controlling”?

 

ANSWER:

Let’s limit the discussion to the VCTRL = 1.5V case because you will have the best current regulation accuracy when VCTRL = 1.5V.

 

The LT3741 regulates the voltage across the resistor sensing the current (the SENSE+ to SENSE- voltage) as shown in the image below of the Full Range SENSE+ to SENSE- line item from the LT3741 datasheet ELECTRICAL CHARACTERISTS table:

LT3741 DS EC table excerpt_1.png

I asked the design group and they said the Full Range SENSE+ to SENSE- line item is at a fixed duty (12VIN and constant VOUT), so if your application is a fixed duty your current regulation accuracy will be according to Full Range SENSE+ to SENSE- line item shown above.

 

If your application is NOT a fixed duty, there will be UP TO +/-1% of error IN ADDITION to the Full Range SENSE+ to SENSE current regulation error indicated above, DEPENDING ON YOUR DUTY CYCLE VARIATION.  According to the design group, the +/- 1% variation in current regulation accuracy shown by the Current Regulation Accuracy CTRL = 1.5V, VIN = 12V graph from the datasheet TYPICAL PERFORMANCE CHARACTERISTICS shown below is due to a full sweep of duty cycle. 

LT3741 DS current reg accuracy_1.png

The effect of an increase in current regulation error with duty cycle range is shown in the datasheet VIN vs VOUT series of graphs shown below:

LT3741 DS VIN vs VOUT_5.png

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  • Hi ,

    Thank you for the detailed explanation above. Can you provide guidance for acceptable accuracy at lower ranges of VCTRL1? One such operating point is:

    • VCTRL1 = 0.3V
    • Vout = 2.6 V
    • Iout = 5 A

    In general we are looking to drive a commanded constant current from a 24V supply through approximately 1.8 ohms. We have had good performance with the LT3471 in the past but recently came across a component variation that had a higher DC current error than expected when we commanded currents less than 10 A.

    Cheers,

    Nate

  • I think the LT3741 tries to regulate an average voltage of 50mV across the SENSE resistor when the CTRL voltage is at the top of its range.  The amplifier that monitors the SENSE resistor voltage probably has some offset voltage that limits accuracy - I'm guessing that the offset voltage is a few mV.  When you regulate a lower current by decreasing the CTRL voltage, that offset voltage does not change much even though the voltage you are sensing is getting lower due to the lower CTRL voltage and consequently current regulation accuracy is worse at lower CTRL voltages compared to higher ones.

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  • I think the LT3741 tries to regulate an average voltage of 50mV across the SENSE resistor when the CTRL voltage is at the top of its range.  The amplifier that monitors the SENSE resistor voltage probably has some offset voltage that limits accuracy - I'm guessing that the offset voltage is a few mV.  When you regulate a lower current by decreasing the CTRL voltage, that offset voltage does not change much even though the voltage you are sensing is getting lower due to the lower CTRL voltage and consequently current regulation accuracy is worse at lower CTRL voltages compared to higher ones.

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