Help with part of a schematic - Page 1
Forum section: electronics / projects

author: mayor
author_rank: Regular Contributor
timestamp: June 30, 2022, 04:54:48 pm
content: Hello,in this circuit:https://www.waveshare.com/w/upload/8/87/E-Paper-Driver-HAT-Schematic.pdfI am wondering why not drive the gate of the mosfet directly (threshold voltage is manageable)?

author: inse
author_rank: Super Contributor
timestamp: June 30, 2022, 05:29:43 pm
content: Depends on the supply of the driving circuit, if it is not supplied, the MOSFET could be turned on by the current through the protection diodes of the IC or uC.Decoupling via a transistor solves this issue.

author: mayor
author_rank: Regular Contributor
timestamp: June 30, 2022, 06:18:56 pm
content: Quote from: inse on June 30, 2022, 05:29:43 pmSo to clarify, if the gate is actively driven with a GPIO? Wouldn't that be fine?

author: moffy
author_rank: Super Contributor
timestamp: June 30, 2022, 10:50:05 pm
content: There is a bit of logic going on between the VCC_EN and EPD_RST as well as just enabling the supply.

author: mayor
author_rank: Regular Contributor
timestamp: July 01, 2022, 12:08:03 am
content: Quote from: moffy on June 30, 2022, 10:50:05 pmYeah but even then, the transistor isn't necessary for that bit, right?

author: antenna
author_rank: Frequent Contributor
timestamp: July 01, 2022, 12:22:03 am
content: i didnt look at the datasheet, but here is a simple reason I would have that transistor there... Most mosfets are not fully "on" at 4 or 5v whatever Vgs.. To make sure it is all the way on and off at max current capability, its best to drive the gate as close to the rail voltages as you can. If you don't, there could be some extra Rds you dont want and it will get hot.

author: moffy
author_rank: Super Contributor
timestamp: July 01, 2022, 01:50:50 am
content: Quote from: mayor on July 01, 2022, 12:08:03 amBut the transistor makes life easier and better, for the other reasons mentioned as well.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 01, 2022, 02:43:59 am
content: Another factor: this is a board intended for a RPi. Its GPIOs are 3.3V, but the PMOS switches VCC_INT which is 5V here if I'm not mistaken.Meaning that if the gate was directly connected to a GPIO, it may not be able to switch off the PMOS properly. There is a pull up to VCC_INT, but it will do nothing when the RPi is actively controlling the GPIO. It will keep the PMOS switched off when the GPIO is in high impedance though.The fact that you'd find 5V on the GPIO through a 47k resistor would be harmless per se for the GPIO itself when it's in high impedance.I don't know enough about the RPi GPIOs, and the library/language used here, to know if you can properly put them in high impedance programmatically. If so, a program could just set the GPIO to output/low level to switch the PMOS on, and set it to high impedance to switch the PMOS off, instead of setting it to a high level (which again would probably not work well.)

author: Daniel Perez LW1ECP
author_rank: Newbie
timestamp: July 01, 2022, 03:26:44 am
content: I began writing this:I see Q31's source terminal is connected to 5V from RPI, and U1 is fed with 5V. As U1's EPD RST swings between 0V and 5V, then it's ok to drive the MOS without Q32 and resistors. But you need to change something in software to reverse states, because Q32 is an inverting stage.BUT I was wrong! I was missing the VCC ENable line: it feeds the EPD RST to U2 to enable the regulator, so you can't invert state in software, or else U2 will work the opposite.In a nutshell: leave the schematic as is.

author: mayor
author_rank: Regular Contributor
timestamp: July 01, 2022, 11:23:18 am
content: Quote from: SiliconWizard on July 01, 2022, 02:43:59 amAh yes! Thank you! I was abstracting that fact away as I intend to use 3.3 V throughout in a reworked circuit.Quote from: Daniel Perez LW1ECP on July 01, 2022, 03:26:44 amYes, true, and it's necessary here. I wasn't clear on my intentions, as I can do away with that part.Thanks all.

author: mayor
author_rank: Regular Contributor
timestamp: July 01, 2022, 02:38:48 pm
content: So, coming back to a different part of the circuit...I'm now trying to understand the RST part. The RPI controls EPD_RST' (3V3), which sets EPD_RST (5V) through the level shifter.I get confused because RST is used to enable 3V3_OUT, which the level shifter depends on for Output Enable, so it's really not clear how changing RST' can even change RST if its initial state is low.Your help appreciated once more :-)

author: inse
author_rank: Super Contributor
timestamp: July 01, 2022, 05:20:20 pm
content: And the Vcc supply of the level shifter is also controlled by himself...

author: mayor
author_rank: Regular Contributor
timestamp: July 01, 2022, 05:23:00 pm
content: Quote from: inse on July 01, 2022, 05:20:20 pmYes, hence the confusion! I thought there might be a mistake and that the gate of the transistor might actually be controlled by RST', but no, I traced the working board, and it's according to this schematic.

author: inse
author_rank: Super Contributor
timestamp: July 01, 2022, 05:36:23 pm
content: Is this the product?https://www.waveshare.com/product/raspberry-pi/e-paper-driver-hat.htm?___SID=U

author: mayor
author_rank: Regular Contributor
timestamp: July 01, 2022, 08:39:26 pm
content: Quote from: inse on July 01, 2022, 05:36:23 pmYes!

author: mikerj
author_rank: Super Contributor
timestamp: July 01, 2022, 08:42:09 pm
content: Quote from: inse on July 01, 2022, 05:20:20 pmLooks like a great recipe for appearing work until it doesn't.

author: inse
author_rank: Super Contributor
timestamp: July 01, 2022, 09:50:16 pm
content: I keep thinking about this comment