Praticing Safe Circuitry: Use a Transorb 1

[Auburnate]

I've got a circuit I'm trying to design and have a few questions. I'm interfacing a microcontroller to a device with an open collector output. I'm trying to design transient voltage suppression circuitry. I'm looking into using transorbs but I couldn't find any that clamp at a voltage level that is safe for the micro (less that 4.1V) What is the lowest clamping voltage for a SMT transorb? If there isn't a transorb that clamps at ~ 3.6 V , what kind of additional circuitry could I implement to protect the micro. I am controlling my pull up resistor with one of the GPIOs to conserve battery life when I am not reading the input. I was thinking of some transistor buffered circuit, but wasn't quite sure how to implement that.

Thanks for any help!

 

[MacGyverS2000]

Use a large resistor in series with the line, like 100k... there won't be an appreciable enough current through the pin to blow it.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[melone]

Is your open collector circuit "exposed to the outside world" or is it in an internal signal. Transorb's are great for massive power spikes, i.e. load dump in a automotive application, but aren't the best for very fast transients, i.e. ESD. Would an ESD diode (zener with fast response time, low capacitance, etc.) work in your application?

 

[itsmoked]

What are you hooking to the OC's pin?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Auburnate]

I am hooking the OC to a TI MSP430 microcontroller.

 

[itsmoked]

So you are going to control a TI MSP430's input pin with the O.C. output? I think not.

What I'm try to get at is it depends on what you are going to hook to the O.C. on what protection you need! If you are going to hook an LED to it you need NO protection.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Auburnate]

The circuit I wish to improve upon is located here:

http://base.google.com/base/a/1185036/D16510830683844946917

I am trying to read data from an open collector output. That's why I have the pull up.

The transorb is clamping at 5.6V This is higher than the 4.1V max voltage on a pin. Also, the pullup line data-bias is connected directly to the micro, leaving no isolation from a transient voltage. I am wondering if I need a diode on the pullup line or maybe a buffered transistor.

 

[melone]

The 69K should limit the current pretty well, right?

1.5V / 68K = ~22uA

Even on the pullup side, you are still talk a pretty small current.

1.5 / 10K = 150uA

If you are still concerned, do as macgyvers2000 suggested.

 

[Comcokid]

Your diagram downloads somewhat unreadable. However, I understand part of your issue. I too design with MSP430 micros, and their I/O lines are ESD sensitive. Transzorbs are intended for handling extremely large transients. What you need are more inline with ESD diode arrays. However, even the ESD protection diodes and diode arrays are typically 5.6 volt minimum.

I generally try to have a fair sized series resistor with the micro pin, fairly close to the micro. Closer to where the I/O arrives on the board, I use a ESD protection diode and/or small capacitor to ground(a 10 to 47 pF as I do a lot of RF), and/or a high value resistor to ground. Basically several methods to shunt ESD energy to ground, then long trace length (distributed inductance/capacitance), then a resistor to make sure current is limited to any internal protection clamping diode.

But, if you have only a very few I/O to protect, try a zener diode(s) to limit the voltage on the I/O line along with maybe one or more of the other techniques.