Lack of information on datasheet - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 06:33:38 pm
content: I plan on using the NCP718ASN500T1G for an upcoming project, datasheet available here:https://www.onsemi.com/pub/Collateral/NCP718-D.PDFThe EN pin is active high but they don't mention if it can be directly connected to Vin to activate, or if I need a resistor. some of the LDOs require resistor and some are cmos so they don't need that. I had a look on their website and they have an evaluation board but they don't provide any schematic on that and the pictures are way too blurry to make anything out. Can anyone confirm if I can connect the EN directly to VCC and it wouldn't kill the chip?

author: dmills
author_rank: Super Contributor
timestamp: May 09, 2019, 08:31:29 pm
content: Second line of the 'Abs max values table' says max is Vin + 0.6V so direct connection would appear just permissible as long as there is not significant inductance between the junction and Vin....Regards, Dan.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 08:37:35 pm
content: the problem is not the voltage in this case, if it's using BJTs instead of fets for the "ENABLE LOGIC" section, a resistor is mandatory to limit the current.The "EN input current" is listed as 100nA which hints at it being a FET (just leakage level current) rather than a bjt but I want to be sure before designing the product with a fault. I contacted them directly so waiting for their answer as well.TI always specifies these sort of things + provides "typical application" circuit diagrams. even layout guidelines for a LOT of their chips. I wish all manufacturers released detailed and comprehensive datasheets like that.

author: ajb
author_rank: Super Contributor
timestamp: May 09, 2019, 09:03:16 pm
content: If it needed a resistor it would give you the information you needed to calculate that resistor value (such as an input voltage characteristic plus a maximum input current rating). Also, the En pin current is given at a specified En voltage of 5.5V, so that confirms that it's not a naked BJT base like you're worried about. I'm not sure that any modern regulator out there *requires* a resistor, but pull-up/pull-down resistors are often shown anyway, because that might be what you need functionally. Also you often see dividers on the input to provide UVLO in conjunction with the input hysteresis. Obviously if you have a device with a max input voltage less than its max Vin you might see a resistor plus a zener, but that's not applicable here.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 09:07:19 pm
content: yes I have confirmed this device does not require a resistor, but it's not the case for all "modern" LDOs.TI specifies a MINIMUM pullup resistor of 100k with recommended value of 300k on a few of their LDOs with modern specs

author: ajb
author_rank: Super Contributor
timestamp: May 09, 2019, 09:42:07 pm
content: Quote from: OM222O on May 09, 2019, 09:07:19 pmExample?

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 10:36:47 pm
content: Quote from: ajb on May 09, 2019, 09:42:07 pmI can't find the exact one from TI again as I searched through dozens of chips before selecting the one I wanted, but back tracking my parametric search, I found this one from NJR which is an active part AND stocked in mouser:https://www.mouser.co.uk/datasheet/2/294/NJM2870_E-48496.pdfthey specify 0-300k pullup but then add the note to select the correct resistor in order to limit the current, while ensuring the EN voltage stays above the threshold over the temperature range. I will update again if I found the TI part which specified minimum of 100k.

author: dmills
author_rank: Super Contributor
timestamp: May 09, 2019, 10:57:07 pm
content: That's not how I read that NJR part datasheet.I read the resistor as being optional to reduce power consumption, not required.Regards, Dan.

author: amyk
author_rank: Super Contributor
timestamp: May 10, 2019, 02:47:54 am
content: Look at figure 6 "Current to Enable Pin vs. Temperature", which specifies "Ven = Vin". Regardless of whether it's a BJT internally, the intent is that the pin is voltage-driven, not current-driven.It's definitely not a "naked BJT base", as otherwise they would specify in the maximum ratings the amount of current it can take.

author: dmills
author_rank: Super Contributor
timestamp: May 10, 2019, 11:15:46 am
content: How many revisions to that datasheet!Section 7.4 covers the enable input which is zenner clamped and internally pulled up by a weak pullup, the intention being to leave it open to enable or pull to ground to shutdown.Annoyingly different, and just another reminder to ALWAYS read the datasheet and not assume.Regards, Dan.

author: mikerj
author_rank: Super Contributor
timestamp: May 10, 2019, 12:04:06 pm
content: On the TI part the EN is a logic level input which plenty of regulators have. It's far more useful than something that has its enable switching levels related to the supply rail voltage.