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[FAQ] LP2985: minimum slew rate for ON/OFF

Intellectual 2680 points

Replies: 5

Views: 231

Part Number: LP2985

Hi,

The datasheet states that the ON/OFF pin must be driven with a slew rate greater than 40mV/us. What is the potential concern if the slew rate is slower than this? Is there still a concern if ON/OFF is shorted to Vin and they both come up with a slew rate slower than 40mV/us?

  • Hi,

    Yes, this slew rate is required to ensure consistent and reliable startups and shutdowns. If the minimum 40 mV/us is not met, the state of the output may be undefined. One thing to keep in mind is that this slew rate is only required on the enable signal between the logic low and high thresholds for the ON/OFF pin (between 0.15 V and 1.6 V, respectively).

    If the expected input voltage has a slew rate less than 40 mV/us, ON/OFF should not be tied to Vin. One workaround is to add a voltage supervisor to the design to drive the ON/OFF pin. TI has a wide selection of supervisors available here: http://www.ti.com/power-management/supervisor-reset-ic/products.html

    Another option is to use an alternative part that does not have an enable slew rate requirement. Some low-voltage p2p alternatives include the LP5907, TLV755P, and TLV757P. Unfortunately, there are not many p2p mid-voltage LDOs in the SOT23 package due to its poor thermal performance. The TPS764 is an older device that does not have an enable slew rate requirement and is fully p2p compatible with the LP2985. However, if it is possible to slightly increase the package size to the more thermally efficient 6 pin WSON package, I suggest using a newer device that offers better performance like the TLV767 or the TPS7A26. More suitable LDOs in the WSON and other packages can be found on the TI website through the parametric search function here: http://www.ti.com/power-management/linear-regulators-ldo/products.html

    Thanks,

    Gerard

    Check out the LDO Basics e-book to learn all about the basic elements of incorporating LDOs into your power design:

  • In reply to Gerard Copeland:

    Hi Gerard,

    Thanks for your timely help. Customer will MP next week, so can't change the PCB. The Max input voltage is up to 10V, the only P2P solution is LP2985?

    Thanks,

    Cera

  • In reply to Cera Wei:

    Hi Cera,

    The TPS764 is a fully P2P alternative with a maximum recommended input voltage of 10 V. Please keep in mind that, with the 3.3 V fixed output version, the LDO will dissipate:

    Pd = (Vin - Vout) * Iout = (10 V - 3.3 V) * 0.15 A = 1.005 W.

    The SOT23 package typically has very poor thermal performance with thermal-to-junction thermal resistances on the order of 180 C/W. When dissipating this much power, the rise in junction temperature can be approximated as:

    delta(Tj) = Pd * Rja = 1.005 W * 180 C/W = 180.9 C

    The maximum recommended operating junction temperature for both the LP2985 and TPS764 is 125 C. If a 10 V input is expected in the application for an extended period of time, the PCB must be designed to minimize Rja and prevent the LDO from entering thermal shutdown. Copper ground and power planes local to the device on the topside, internal, and backside layers will act as heat sinks that help dissipate the heat generated in the pass transistor. Arrays of thermal vias near the device can further improve heat dissipation. A thermally optimized layout can reduce Rja by around 30% to 40%.

    Thanks,
    Gerard

    Check out the LDO Basics e-book to learn all about the basic elements of incorporating LDOs into your power design:

  • In reply to Gerard Copeland:

    Hi Gerard,

    Thanks for your reminder. The system typically works at 4.4V input and 10V is just the peak voltage, not consist for a long time. The max current is 0.12A. In that way, delta(Tj) would be 23.76C.

    TPS70933 is in the customer's device library, so we'd like to choose this part.

    Thanks,
    Cera
  • In reply to Cera Wei:

    Hi Cera,

    Although the TPS709 is pin compatible with the LP2985, it has a larger dropout voltage due to the integrated reverse current protection:

    The maximum dropout voltage at 120 mA can be calculated using Ohm's Law because, when an LDO operates in dropout, the pass transistor is biased in the triode region and acts as a resistor.

    Rds,on = Vdo @ 120 mA / 120 mA = Vdo @ 150 mA / 150 mA

    Vdo @ 120 mA = Vdo @ 150 mA / 150 mA * 120 mA = 1.4 V * 120 mA / 150 mA = 1.12 V

    When operating with 4.4 V input, some TPS709 units may still operate in dropout, resulting in poorer performance metrics including output accuracy and PSRR. A minimum input voltage of Vin = Vout + Vdo = 4.42 V will ensure that all TPS70933 units properly regulate the 3.3 V output. 

    Thanks,

    Gerard

    Check out the LDO Basics e-book to learn all about the basic elements of incorporating LDOs into your power design: