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LM22676: Difference in UVLO and Enable Threshold specs

michael yee
Prodigy 100 points
Part Number: LM22676

I'm trying to use the UVLO feature. This appears to be only accessed using the ENABLE pin. The datasheet specification for the pin reads:

VEN Enable Threshold Voltage V 1.6V typical

VENHYST Enable Voltage Hysteresis 0.6 V

However in the text of the datasheet (page 9):

The LM22676 also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the regulator from turning on when the input voltage is not great enough to properly bias the internal circuitry. The rising threshold is 4.3 V (typ) while the falling threshold is 3.9 V (typ).

Why is there this discrepancy? 1.6V vs 3.9V?

Thanks in advance,

Mike

  • 0
    TI__Genius 11040 points
    Hi Mike,

    The device will turn on and off based on the voltage at two pins, VIN and ENABLE.

    You are correct that VEN Enable Threshold is 1.6V typical and the enable voltage hysteresis is 0.6V. The datasheet references this functionality as Precision Enable (Section 7.3.1).

    On Page 9, the datasheet is references that an input voltage of 4.3V (typical rising) or 3.9V (typical falling) is necessary to turn on the device. This threshold is based on the input voltage being large enough to properly bias the internal circuitry. The datasheet references this functionality as UVLO (Section 7.3.1).

    Please let me know if you have any additional questions or if anything is still unclear.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 100 points

    I'm completely confused. Attached is our circuit. R60, R61 pertain to the Figure 10 (there's a little bit more loading the Standby line, but it basically grounds the net). I sweep the voltage at V24VDC (actually a battery voltage in the real application).  I monitor the ENABLE line, the LM22676 turns on at 2.13V with a rising voltage, turns off at 2.12V for a descending voltage. I don't see a 1.6V, I don't see any voltage near 4V.

  • 0 in reply to michael yee
    TI__Genius 11040 points
    Hi Mike,

    Are you measuring the voltage at EN5VDC? Based on the datasheet, I expect the device to typically turn off at EN5VDC=ENABLE=1.6V, and typically turn on at EN5VDC=ENABLE=2.1V.

    Based on your comment above, can we assume that R65 is effectively grounded? Can you try removing R65 and verify if the ENABLE start up and shut down behavior remains the same?

    Thank you,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 100 points
    I'm measuring EN directly while increasing V24VDC. I've removed R65 with exactly the same results (2.13V for ON 2.12V for OFF).

    I would like to ask my question a different way. Is the Precision Enable (EN) the same pin that the UVLO uses to determine ON/OFF behavior? If so, how does it know what range to use (1.6V-2.2V, or 3.9V-4.3V)? Is there another pin that defines its use?

    Thanks,
    Mike
  • 0 in reply to michael yee
    TI__Genius 11040 points
    Hi Mike,

    The UVLO is determined by the VIN pin (3.9V-4.3V). Both VIN>4.3V and EN>2.2V must be true in order to turn on the device.

    The datasheet suggests using a resistor divider between VIN, EN, and ground in order to determine the turn off/turn on VIN values desired for a given application.

    For example, if VIN is 12V but EN is 1V, the device will remain off; if EN is 2.5V but VIN is 2V, the device will remain off.

    Please let me know if this is still unclear.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 100 points

    Hi Katelyn,

    I'm sorry, I'm still confused. The specs say that Vin must be > 4.7V. There is no spec of 3.9V anywhere (just in the text).

    It appears this section of text "The rising threshold is 4.3 V (typ) while the falling threshold is 3.9V" is erroneously copied into many datasheets. I've googled the phrase, it shows up a lot of places. I cannot find the source where it is appropriate.

    Now for my biggest problem, where is the hysteresis? When I measure it, it is very low (0.01V). I think the threshold values for the EN pin are always valid while the Vin > 4.7V.

    Is it possible for me to call you?

    I'm at 408 242 3460. I'm in a hurry for this resolution.

    Thanks,

    Mike

  • 0 in reply to michael yee
    Prodigy 100 points
    I'm suspecting the hysteresis has disappeared because of noise on the ground plane in the area of this switcher. I'm looking into that now.
  • 0 in reply to michael yee
    TI__Genius 11040 points
    Hi Mike,

    I can give you a call now.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 100 points
    I was able to verify the hysteresis spec by lifting the EN pin and driving from an external supply (1.6 - 2.2V). This verifies there is noise from the layout. So the remaining issue is the spec that defines 3.9 - 4.3V for EN. But I'm assuming this is an error.

    Thanks so much for your help,
    Mike
  • 0 in reply to michael yee
    TI__Genius 11040 points

    Hi Mike,

    I verified both the Enable thresholds and Vin UVLO thresholds on the bench.  I used one of our EVMS, TPS22671 EVM, which is part of the same family as the TPS22676 and should have the same EN and VIN thresholds.

    First I tested the Enable thresholds.  I used two separate power supplies for EN and VIN.

    • Test Conditions:
      • VIN=6V
      • VOUT=3.3V
      • No Load
    • Device begins switching @ EN=2.159V (rising)
    • Device stops switching @ EN=1.579V (falling)

    Second I tested the VIN thresholds.  Again I used two separate power supplies for EN and VIN.

    • Test Conditions:
      • EN=2.5V
      • VOUT=3.3V
      • No Load
    • Device begins switching @ VIN=4.29V (rising)
    • Device stops switching @ VIN=3.95V(falling)

    The VIN threshold refers to the point where the device begins switching and is no longer in 'shut down' mode. 

    I hope this clears up any confusion, and please let me know if you have any additional questions.

    Best Regards,

    Katelyn Wiggenhorn

  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 100 points
    Hi Katelyn,
    Thanks for running these tests!!! I would have never suspected that the device ran down to 3.95V!!! It would have been great if the Vin was spec'd. It would have saved our discussion, but it was a pleasure to work with you!

    Mike