Input protection for OEM MCU module application - Page 1
Forum section: electronics / projects

author: CM800
author_rank: Frequent Contributor
timestamp: August 06, 2018, 01:55:09 pm
content: Hi All,I'm spinning up a circuit board for a commercial product at the moment.The product is a sub-module circuit board for integrating into their own electronics.Should I put some level of protection on the board for the product, in terms of ESD or such?Bare in mind this will be an OEM product, most customers will likely take 1,000-10,000pcs annually.As it's cost sensitive, i would rather minimize un-needed protection.The product will sit between a small brushless motor drive, and a customer product, connected to each by some small pitch connectors.What is everyone's thoughts?Currently, I've put this together:RX/TX Protection:Output Protection:Input Protection:Microcontroller:

author: max_torque
author_rank: Super Contributor
timestamp: August 07, 2018, 05:19:08 pm
content: for protecting serial busses i use a capacitively bridged large value resistor current limiter. Obvs capacitor size depends on baud rate.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 07, 2018, 06:02:39 pm
content: IMHO/AFAIK, it's very rare to seeanyextra protection on a typical submodule, which is integrated as a part of electronic assembly during professional assembly step, and unless there is some specific threat to protect against, I wouldn't bother. One reason for extra protection would be the usage of extra sensitive components such as RF transistors. If the module is going to be replaced by the end user on the field, would also consider some level of protection - in cost sensitive applications, I tend to use simple series resistors in GPIO, doubling as series termination, and RC circuits in inputs-only - it's still better than nothing at all, which is the norm in low cost (think about Arduino or Raspberry).

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2018, 06:51:28 pm
content: If your motor runs at say 12V and an input gets 12V, it will bump up your 3.3V rail and destroy the MCU or burn up the 100R resistor. It's not ideal to dump it to the rails with a low value resistor. What is VREF and are your diodes 1N4148 reversed?If it's cost-sensitive, I rely on the MCU's internal substrate diodes for clamping, if it has them. Must first have 1-5nF capacitors to GND at the connectors to attenuate ESD voltage hits and for EMC.So the input series-resistor is as large as reasonable (so no troubles with MCU pullup value for logic "1"), and physically large enough to not arc over during ESD event. Like 10k-50k ohm. This will tolerate a long-term voltage overload (i.e. 12-24V).

author: CM800
author_rank: Frequent Contributor
timestamp: August 08, 2018, 09:26:50 am
content: thanks for the help everyone.So, looking into the numbers, I'm thinking a 6.8k input series resistor would be best.@ 6.8k, with 24V to ground, you will get 3.5mA, which should be safe-ish.The MCU I/O, with internal 25k minimum pull up resistor gives me a 0.99V logic low voltage.This leads me to choosing 0.8V, which, gives me a resistor value of 8k, rounded down to 6.8k for safety / assurance.Does this sound like a good choice?Input - >Capacitor to ground -> 6.8K resistor - > MCUQuote from: floobydust on August 07, 2018, 06:51:28 pmgood catch on the diode

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2018, 07:43:45 pm
content: I think I would back up and do it differently... take my suggestions lightly as it depends on the I/O specs I don't know, and if the board/connectors are handled by customers or just installers, how much abuse the board will see. You have to know the MCU really well to design to lowest cost.Injection current spec forSTM32F030x(Table 45) doesn't look great with "NA" as the pin +ve current rating. Since the MCU has some 5V tolerant inputs, the diodes are not there? But Fig. 23 shows them. If in doubt, you can design the PCB to have protection diodes and just do not populate them if you find the MCU tolerant after your testing.For the digital inputs, look at your expected incoming voltages. 12V is a "1" but what about 10v or 3V? Or is it a only switch to GND on the input? Then I use a series diode.Use RPU lowest value (over temperature, not spec'd) 25k ohms Table 46. For a Schmitt trigger MCU input min. is around 2.3V (Fig. 18,19) but it's different for 5V FT pins2N7002may not be the best. RDS on 3.8-5.3 ohms is high and VGSth 2.5V means it is not well suited to 3.3V drive.2N7002Elooks better, and more discussion here:https://www.eevblog.com/forum/projects/finding-the-perfect-n-channel-mosfet-for-3-3v-cpuFor on/off switching I use a large gate resistor like 10k as I saw mosfets short and backfeed +12V into the MCU output, which smoked and basically destroyed the whole product, there's no fuse.. But 10k is too large to do PWM, if you end up needing that ability.