low impedance output driver, but short circuit proof? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: May 26, 2016, 07:20:58 pm
content: I'm using a LVDS line driver as a signal line driver, in order to get nice sharp edges and to be highly noise resistant (low impedance to VCC / GND)But, such low impedance makes that driver vulnerable to output shorts (to VBATT or GND).I was wondering about putting some series resistance in the VCC and GND feeds, but bypassing that resistance with some capacitance to maintain an low AC impedance??Is this a silly idea??

author: danadak
author_rank: Super Contributor
timestamp: May 26, 2016, 08:22:29 pm
content: Check the datasheet for your part, "normally" there is an outputshort circuit current limit designed into part.Regards, Dana.

author: bktemp
author_rank: Super Contributor
timestamp: May 26, 2016, 08:26:48 pm
content: Quote from: max_torque on May 26, 2016, 07:20:58 pmLVDS does not have a low impedance: LVDS drivers uses a current source (~3mA) followed by the output switches.

author: max_torque
author_rank: Super Contributor
timestamp: May 26, 2016, 09:10:30 pm
content: Sorry, it turns out i'm an idiot!I wrote "LVDS" driver because i'd been working with those today, but in this case i mean't "MOSFET GATE" driver!it's the IXYS IXD604http://ixapps.ixys.com/DataSheet/IXD_604.pdf

author: Marco
author_rank: Super Contributor
timestamp: May 26, 2016, 09:34:26 pm
content: What kind of noise, signal and transmission line are you working with that this even makes sense? A couple kHz signal over a long unshielded single conductor with earth return?

author: dannyf
author_rank: Super Contributor
timestamp: May 26, 2016, 10:15:56 pm
content: The easiest would bee to out a small resistor in thee output, preferably before the nfb pick-up.

author: max_torque
author_rank: Super Contributor
timestamp: May 27, 2016, 02:21:26 pm
content: Quote from: Marco on May 26, 2016, 09:34:26 pmThe signal is a speed control signal to a high power but rather "Noisy" brushless motor controller. Clamping the line hard to vcc or gnd keeps the duty cycle non distorted, even though there is a relatively high impedance on the end of the line. The loom is also a bit of an issue because the signal line runs alongside the power lines, and is not necessarily guaranteed to be shielded.By capacitively coupling the driver to it's power rails, i was wondering if that would maintain the sharp edges and AC noise immunity, but allow me to install a sensible DC resistance to prevent any shorts to power lines causing the driver to fail??

author: bktemp
author_rank: Super Contributor
timestamp: May 27, 2016, 02:37:01 pm
content: Having a driver with a low impedance does not help avoiding noise coupled into the cable, because of the inductance of the wire the rf impedance of the cable will be much higher than the driver's output impedance.Instead of using a oversized mosfet driver I would use a differential line driver and a twisted pair cable. It should rejecte the noise much better than any unshielded cable.Maybe you can use a CAN transceiver, it can tolerate shorts to GND or supply rails.

author: Marco
author_rank: Super Contributor
timestamp: May 28, 2016, 05:00:57 am
content: Sharp edges have high frequency components and at high enough frequencies the cable has high impedance, the driver can do little to preserve those edges. But if the cable is very short relative to the wavelength of the signal and the controller has a low pass filter on it's input the low impedance driver can help. A low impedance driver will suppress 50 Hz hum on a couple meter of wire, but it won't do much against the interference caused by snap off in the body diodes of MOSFETs in an electrical motor driver.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 28, 2016, 08:17:34 am
content: Quote from: max_torque on May 27, 2016, 02:21:26 pmIt sounds like you want an RS-232 transmitter.Tim

author: sarepairman2
author_rank: Frequent Contributor
timestamp: May 29, 2016, 02:39:52 pm
content: just run a wave guide