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ISOW7741DFMEVM: Consumption of extra quiescent current form the isopower

Deekshith Pejawar C
Prodigy 20 points
Part Number: ISOW7741DFMEVM

Tool/software:

While characterizing the Isopower circuit at no load, there is current consumption of 17mA which is more than the one mentioned in the datasheet.



As per datasheet at no load with digital isolator not enabled the supply current is 4.1mA but in our case we obtain current as mentioned below.

S.no Case Supply voltage at TP1 (VDD_PWR) Input voltage(VDD) Input current (I_VDD_PWR) Output voltage(VISO_OUT)
1 TP1 shorted to ENPWR 4.3 4.2745 23.109 4.9475
4.5 4.4746 22.523 4.9487
5 4.9062 21.822 4.9479
2 External supply to ENPWR 4.3 4.2916 17.2849 4.9477
4.5 4.4912 16.6959 4.9484
5 4.9095 15.586 4.9495

Tet setup:

  1. R17 and R18 are removed to isolate LM317MDCY.
  2. U3 removed.
  3. R26 is removed and TP1 shorted with ENPWR (Pin 8).
  4. VIO is not powered to make sure isolator part is not enabled.
  5. The isopower power part is enabled by Powering it through TP1.
  6. the above table represent the result.
  7. Output is measured at J5.

Do provide the reason for higher current consumption at no load.

  • 0
    TI__Guru 55575 points

    Hi Deekshith,

    Welcome to TI E2E forum and thank you for reaching out.

    Sorry, it seems like you are interpreting the table differently. Please allow me to clarify.
    As you can see in the functional block diagram of device in section 9.2 (also copied below), the power supply feedback channel is part of the I/O channels and for the feedback channel to work both VIO and VISOIN need to powered along with VDD and VISOOUT. The only valid supply configurations are also listed in Table 9-2, also copied below.

    From the supply configuration, powering VDD and not powering VIO is not a valid configuration. This can lead to an invalid voltage at VISOOUT and possibly damage the device due to no feedback control.

    The possible reason why the device still generating a valid output voltage in your case is probably because you powered ENPWR (referred as EN/FLT in datasheet) pin from VDD or external power. This pin should ideally be tied to VIO through a pull-up resistor as you cannot power EN/FLT through VDD and then control VDD.

    When VIO is unpowered but EN is powered either through VDD or external power, VIO accidentally gets powered through internal protection diodes. Please see below figure for more details where there is a diode between EN/FLT (listed as INx by mistake) pin and VIO pin. This is the reason why the device powered-up VIO accidentally through EN/FLT pin.

    Coming back to your original question, the screenshot you have shared of currents is for I/O channels. This doesn't include the current consumed by DC/DC converter. i.e., the table you shared refers to current at VIO and VISOIN, provided separately. The additional current you see is the current going into VDD for operating the DC/DC converter.

    Let me know if this makes sense, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 20 points

    Hi Koteshwar,
    As per your instruction I enable the isolator part as well by providing 3V to TP8 (VDD_LOGIC), I measured the input current of the converter part still the current reads around 18mA at no load.

    VDD_LOGIC (in V) Input supply VDD_PWR (in V) Input current at no load I_VDD_PWR (in mA) Input Voltage VDD (V) Output voltage VISO_OUT(V)
    3 4.3 18.4137 4.2804 4.9496
    4.5 16.7631 4.4819 4.9485

    circuit reference:

    U1,U2 and U3 are isolated.

    Can you confirm on the current consumption at VDD_PWR? as we require lesser current consumption on no load condition. 

  • 0 in reply to Deekshith Pejawar C
    Guru 298560 points

    The VDD (IDD) consumption is specified in section 7.10. There is no value for an idle device (with a load of 0 mA), so there is nothing for you to compare against. 17 mA sounds plausible.

  • 0 in reply to Clemens Ladisch
    Prodigy 80 points

    Hello Clemens Ladisch,

    These current numbers from section 7.10 are okay. But, how do we get the details on quiescent current of the module?

  • 0 in reply to Vijetha Jain
    Guru 298560 points

    The datasheet does not specify this detail. You have only your measurement.

  • 0 in reply to Clemens Ladisch
    Prodigy 80 points

    How do we ensure it is the right value? Usually datasheet specifies either the no load or Iq value

  • 0 in reply to Vijetha Jain
    TI__Genius 13475 points

    Hi Vijetha, 

    Please give us some time to look into this.

    Koteshwar will respond to you on Wednesday.

    Regards,
    Aaditya Vittal

  • 0 in reply to Vijetha Jain
    TI__Guru 55575 points

    Hi Vijetha,

    The datasheet doesn't include device current consumption when there is no load at the VISOOUT pin but the current of 15mA to 23mA hat you are observing are the expected values. The efficiency at no-load or light-loads is usually poor, that is why the current consumption at no-load is relatively higher.

    Let me know if this answers your question, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 80 points

    Yes, thank you Koteshwar!