Does FTDI output need buffer - Page 1
Forum section: electronics / projects

author: ReneFromHolland
author_rank: Newbie
timestamp: September 28, 2020, 12:49:31 pm
content: Hello. Beginner designer here, big fan of Dave, and blessed with the saying 'If you suck at doing it, only means you are capable of doing so' i'm designing the new electronics for our company's equipment.Currently I'm designing a project that uses an FTDI230XS, datasheet:https://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT230X.pdf. I'm building a galvanic isolated power & IO (serial) for our 150-250mA measurement device. Galvanic isolation is not directly needed as in, can do without, but i figured it's the most elegant way to deal with unclean USB power issues and has added bonus for as in reducing noise - we known this because of battery-powered wireless equipment i built before, safety issues and supporting remote/outside locations. Of course a transformer won't magically filter all USB woes, especially considering we're also looking at a 80pF capacitative coupling.. But this all for introduction.I intend to use pin14/CBUS3, in default configuration which i leave as such configured as ~SLEEP~FTDI actually recommends to use this signal to power down/up the device, in case the host suspends.The thing is, i intend to have it 2 functions: fist: power a reasonable bright (15mA) LED, second: pull the sync from my galvanic transformer down via a Schottky.The datasheet claims a 22mA absolute maximum rating to source. That's uncomfortably close to driving a bright LED. I'm less worried about the pull-down function because that's low signal (we're pulling down the oscillator on a DCP020505).So, the question is: Is it wise to buffer the output pin or can i get away just letting the chip do all the work, saving components.Since a picture says more than 1000 words, find attached.(Alternatively, a consideration is to put the LED on the side after the transformer, with the bonus of actually verifying there is power. However, this might lead to grounding issues with the casing/little more wiring, nothing that overly bothers me, but rather keep it simple, although that is simple too. Please help me think.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2020, 12:59:51 pm
content: Do you really need a full brightness LED? It'll just about outshine the sun, remember. That is, be readily visible in daylight. Probably 1mA is fine, or even still a bit on the high side, for indoor use.Yes, a buffer is a good idea to deal with the current rating, if not strictly required in this case it would seem. A simple example is an open drain inverting buffer, otherwise known as a transistor. BSS84 would be a common pick, source to VCC, gate to nSLEEP, drain to resistor to LED to GND.Tim

author: ReneFromHolland
author_rank: Newbie
timestamp: September 28, 2020, 01:02:39 pm
content: Yes, the equipment is portable and intended to be used outdoor as well as indoor.

author: Ian.M
author_rank: Super Contributor
timestamp: September 28, 2020, 02:48:08 pm
content: You *MUST* buffer, not because of the FTDI FT230X, which when configured for 16mA CBUS I/O drive strength will certainly be good to sink at least 12mA, plenty for a high efficiency LED to be obnoxiously bright, but for the T.I.DCP020505as its datasheet has this to say about the sync pin:Quote from: DCP02505 datasheetYou'd have a hard time meeting the <3pF capacitive loading and leakage current requirements with a Schottky diode. The internal RC oscillator runs at a nominal 800KHz using a 75uA current source and a 34pF timing capacitor. I calculate 1uA leakage current would drop the frequency by about 10KHz. T.I. recommend a specific NPN BJT (Infineon BFR180) to drive the SYNC pin for shutdown purposes. You'll also need an inverter to provide it with base drive (unless you can find an open drain buffer with <<3pF output capacitance).

author: ReneFromHolland
author_rank: Newbie
timestamp: September 28, 2020, 04:40:53 pm
content: Thanks, that's good advice. The Schottky was a remnant of another project where it was hacked in and seemed to work fine, but i see now why that's not a proper solution if disturbs the oscillator. 3pF is a number to take serious. Best not take the risk running the transformer out of spec since they have enough losses already. Glad i asked.