Negative voltage one shot - 2 usec

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atferrari

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I need to implement a one-shot stage to generate a negative voltage pulse (0 to -3V) lasting around 2 usec, triggered by a TTL pin of a micro. Dual PSU (+/- 5V) available.

Been considering an LM311 connected between 0 and -5V. I got just one! of those comparators for the week end and I fear to destroy it in any uncontrolled test. Additionally I cannot see how to trigger it with a positive voltage and even more as a one shot.

Reading the datsheet got me even more confused.

Any ideas about this or any better/simpler?
 

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  • LM311.pdf
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I'm presuming you're wanting to generate the pulse externally?, and not just in the micro? - why not chose a suitable monostable, something like the TTL 74121 would do.

To trigger it from a positive supplied micro, use a PNP open-collector transistor, with a pull-down resistor to the -5V rail.
 
You can AC couple the trigger pulse, clamping the capacitor output to the minus supply with a diode (view the minus supply as a virtual ground point with circuit ground being the positive rail for the LM311).
I can show you a circuit, if you need that.
 
You can also use a PNP for the pulse level conversion as Nigel suggested.
One interesting non-inverting configuration for that is to use the PNP in the grounded base configuration, with the pulse applied to a series emitter resistor whose value is equal to the collector resistor (which goes to the -5V). That will generate a positive going pulse from -5V to ground for a positive pulse input.
 
For such a short pulse, I'd go with having a charged inductor with a flywheel clamping to the -3V (resistor/diode combo) being open circuited by your TTL pulse.
If u have a 'scope it's dead easy to evaluate this and size/wind the inductor.

Advantage is that no negative supply is needed.
 
I have not 121 handy around Nigel.
 

Yes, please do. Gracias Carl.
 
Hola mosaic,

Could you show the circuit. Not sure I follow you.
 
Here is a spice sim, running at 0.5mS clock intervals providing 2uSec negative going pulses.
 

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  • Negative Pulse gen.zip
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Below is the simulation of an RC circuit to convert a 3v to 0V pulse to a 0V to -3V pulse, which I believe is what you want.
All you need to generate is a 3V to 0V pulse from your micro.

What is the duty-cycle of the pulse?

 

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  • Pulse Level Shift.asc
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