What's the best way to sink 100mA in a TTL line

Thread Starter

redrover

Joined Dec 13, 2021
3
Hi all, I have a device that accepts TTL input to turn it on/off, but it is designed to assume 'on' if you have no input. To do this it has an internal 5V source with a 50 Ohm pull-up resistor, so that if you plug in a BNC cable (the 'gate drive') with no voltage the device will switch off. BUT to do this the device specifies that "the gate drive must be capable of sinking 100 mA to gate the module off (5V/50Ω)". My BNC cable with TTL high/low can't do this, and I assume there must be a simple solution here... any ideas?
 

dl324

Joined Mar 30, 2015
14,322
Welcome to AAC!

You could have the TTL control signal drive a transistor to pull the input low. What TTL family is generating the control signal?
 

Papabravo

Joined Feb 24, 2006
18,123
Hi all, I have a device that accepts TTL input to turn it on/off, but it is designed to assume 'on' if you have no input. To do this it has an internal 5V source with a 50 Ohm pull-up resistor, so that if you plug in a BNC cable (the 'gate drive') with no voltage the device will switch off. BUT to do this the device specifies that "the gate drive must be capable of sinking 100 mA to gate the module off (5V/50Ω)". My BNC cable with TTL high/low can't do this, and I assume there must be a simple solution here... any ideas?
There used to be (still are apparently) TTL compatible peripheral drivers that could do this. We used to use 75463 and similar devices.

https://www.ti.com/lit/ds/symlink/sn75463.pdf?ts=1639444463259&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FSN75463
 

MisterBill2

Joined Jan 23, 2018
11,621
A "50 ohm " cable does not mean a 50 ohm resistance across the cable, that is the transmission line impedance. So to sink that 100mA you need an NPN transistor across the other end of the cable, with the emitter to the shield and the collector to the center. Then applying enough positive bias current to the base will put the transistor into saturation and switch the device off. Or maybe switch "ON", as the explanation is not really clear. But just connecting a 50 ohm cable will not pull the input down to zero. That requires an active device, such as a transistor.
 

crutschow

Joined Mar 14, 2008
29,519
The best solution depends upon the frequency.
What is the maximum frequency you need to switch?
If it's no higher than about a MHz, and can you can tolerate a signal inversion, then you can likely use one small N-BJT or N-MOSFET transistor.
 

Thread Starter

redrover

Joined Dec 13, 2021
3
The best solution depends upon the frequency.
What is the maximum frequency you need to switch?
If it's no higher than about a MHz, and can you can tolerate a signal inversion, then you can likely use one small N-BJT or N-MOSFET transistor.
Our frequencies are around 1 MHz but usually a little slower.
 

crutschow

Joined Mar 14, 2008
29,519
What is the drive capability of your 3.3V pulse source?

Below is the LTspice simulation of a BJT driving a 50Ω transmission cable at a 1MHz pulse rate with a 50Ω termination to +5V.
The simulated cable has a 50ns delay which is around 30 ft. long for a typical coax cable.
Note that the 50Ω termination eliminates any cable reflections.

The 3.3V pulse source (V1) must be able to drive 10mA into the base of Q1.

Q1 is a high-speed switching transistor such as the 2N2369 or 2N5769, designed for low saturation delay when turned off.
Beware that a general-purpose transistor with longer saturation delay, such as the 2N3904, 2N2219, 2N2222, etc. will not work well (or at all).

1639453584079.png
 
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