UCC27425 MOSFET Driver Spec Sheet Questions - Page 1
Forum section: electronics / beginners

author: axawire
author_rank: Contributor
timestamp: September 15, 2021, 12:57:13 am
content: Hi,In the spec sheet there is a diagram of the chip (see attachment) I have highlighted the part in question. Does this symbol just mean its a buffer and so acts as an amplifer of what comes in on pin 4 and puts it out pin 5 but the arrow going to it indicates its powered from pin 8 (ENBB) and so basically operates like a logical AND? Does this have a name for this symbol for searching because I couldn't find others that had a triangle with an arrow going to it on top like that when I used google image search and provided the image.Should I be providing the PWM signal on to pin 4 or should I be putting it on to pin ENBB and using pin 4 for the feedback monitoring?Also how do I pick the resistor between pin 5 and the MOSFET or should I be running it through a small transformer first to power the MOSFET?Spec sheet is here for reference:https://www.ti.com/general/docs/suppproductinfo.tsp?distId=10&gotoUrl=https%3A%2F%2Fwww.ti.com%2Flit%2Fgpn%2Fucc27425Thanks

author: jmelson
author_rank: Super Contributor
timestamp: September 15, 2021, 01:11:59 am
content: Pin 8 is the enable for the pin 5 driver, just as for the other one. Pin 6 is the power for both drivers. You should supply PWM to pin 4, if you turn off ENBB it will most likely leave pin 5 floating.Jon

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2021, 01:40:54 am
content: Nope. If you look at the pin functions table *IMMEDIATELY* below that figure in the datasheet, it *TELLS* you what the enable pins do: If pulled low they disable their output, forcing it low.Section 8 - Detailed Description - explains this in more detail with a block diagram that shows the logic, which should make it clear to anyone who isn't 'hard of thinking' even if English isn't their first language.@jmelson: A MOSFET driver with an enable that tristated its outputs would be a *very* niche product, verging on dumb!

author: axawire
author_rank: Contributor
timestamp: September 15, 2021, 02:45:27 am
content: I didn't read that far down because I didn't recognize the symbol. But now I know its a Tristate buffer from your reference about tristated, thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2021, 03:45:49 am
content: No, its *NOT* a tristate buffer.*RTFM*and *attempt* to comprehend it.Skimming datasheets quickly for information is a skill you will only develop with practice and experiance. For novices, the best option is to trudge through the datasheet line by line and figure by figure, marking up sections they don't understand, and if it still doesn't become clear after rereading the datasheet and googling unfamiliar terms and technologies, unless its a particularly complex datasheet for a really obscure 'niche' device, you'll always find members here willing to answerwell presented questions.

author: axawire
author_rank: Contributor
timestamp: September 15, 2021, 05:23:57 am
content: Are you sure its not a generic tristate symbol? It really looks like a generic tristate buffer symbol and when I look at the truth table in the spec sheet it looks a lot like the attached image and truth table for a generic tristate buffer found from googling.Also it looks like my initial sentence explaining my understanding of the operation of what was happening with this symbol was correct I just misspoke with the word powered instead of enabled and looking at the truth table of the tristate buffer and in the truth table in the spec statement its basically a logical AND.

author: Ian.M
author_rank: Super Contributor
timestamp: September 15, 2021, 07:30:32 am
content: The critical difference is a real Tristate buffer has a high impedance state (usually shortened to Hi-Z), Z in the truth table you just posted. That truth table only resembles an AND function if you take Z as 0, which would only be somewhat accurate with a pull-down on the output.For the simplified internals of this MOSFET driver, T.I. used the traditional symbol for a generic buffer or amplifier with control or enable signal, which, although commonly used for tristate buffers, is part of a system of circuit and logic symbols originating from an era before CMOS or totem-pole output TTL logic capable of being tristated came onto the market.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2021, 09:27:22 am
content: To clarify the above discussion: it would seem it is a schematic error. That symbology is traditionally used to mean a tristate buffer. But it seems it should indeed be an AND gate instead.Have also seen two-input triangles, seemingly suggesting a comparator or differential amplifier if not for the missing +/- labels; but where the function is actually AND with one input inverted.Tim