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OPA547: recommendation for DAC

May
Genius 9880 points
Part Number: OPA547
Other Parts Discussed in Thread: DAC7811, , DAC7800

Hi Team,

Good day.

Customer is looking to control the current limit feature of the opa547, using a DAC on the Ilim pin. Customer tried the DAC7811 12bit current output spi dac with ref connected to Ilim pin and Iout2 connected to 0v, Ifb and Iout2 not connected but according to them it only seems to only change the Ilim voltage between 2.88V and 1.16V.

They assume there is some internal resistance limiting the full swing I require of 4.75V to 0V. Is the DAC on the data sheet a current output or voltage output? Is there other DAC that is recommended for opa547?

Thank you and looking forward for your kind response.

Regards,

Maynard

  • 0
    TI__Mastermind 47716 points

    Hi Maynard,

    Q: Is the DAC on the data sheet a current output or voltage output?  Is there other DAC that is recommended for opa547?

    DAC I_limit controlled method requires that current output of DAC is able to source 150uA and 4.75Vdc at pin3, see Figure 3 of the OPA547's datasheet. 

    I am not the best person to recommend the DAC part. In Figure 14, DAC7800/1/2 family is recommended. 

     

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • 0
    TI__Guru* 93105 points

    Hi Maynard,

    It would appear from the DCA7811 datasheet that it should be able to provide the correct 0 to 150 uA IDAC current required to control the OPA547 ILIM current. The DAC7811 datasheet in Section 7.3 Feature Description states:

    Each 2R leg of the ladder is either switched to IOUT1 or the IOUT2 terminal. The IOUT1 terminal of the DAC is held at a virtual GND potential by the use of an external I/V converter op amp. The R-2R ladder is connected to an external reference input VREF that determines the DAC full-scale current. The R-2R ladder presents a code independent load impedance to the external reference of 10kΩ ±20%.

    If we examine a simplified circuit as seen below for the OPA547 ILIM circuit and the external IDAC we can see not only what the DAC output current needs to be, but also the voltage range that it must be able to provide that current. Taking into account the DAC's 10kΩ resistance it becomes evendient it is a current limiting factor in the circuit. When the voltage at the ILIM pin 3 gets within 10 mV of V- the maximum pin current that can flow is about 114 uA which sets the maximum OPA547 output current closer to 500 mA; not the 750 mA maximum.

    Likely the easiest way to resolve the issue to either add an op amp connected as an I-to-V converter at the DAC7811 output, or use a voltage output DAC. In those cases the 10kΩ R-2R ladder resistance is no longer acting as a source of voltage drop in that current loop. Alternately, I believe if V- for the DAC7811 is made more negative than the OPA547 V- by about -1.5 V, then the DAC output could sink the full 150 uA.

    The voltage output DAC is likely the easiest solution. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering