This thread has been locked.
If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.
LP2985: Lp2985N slew rate requirement for EN
Part Number: LP2985
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?
Kyle Van Renterghem
LDO Product Line
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
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to Gerard Copeland:
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?
In reply to Cera Wei:
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.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.