Hi,

In LTspice, I have a voltage. I want to convert it to a PWM. This is only for LTspice - it doesn't have to be a something that can be realised.

Tperiod is 10ms.

So if the measured voltage is 2.3V, and my reference is 3.3V, then I want a PWM to have a duty of 69% (2.3V/3.3V). So Ton 6.9ms.

The start of the PWM must be on 0, 10ms, 20ms, 30ms, 40ms etc. The reason I say this is, I used a Sawtooth and a comparator to do it, but the PWM doesn't align to those times and I need it to.

 

A simulation program analyses a circuit, it does not design the circuit. You must design or find a circuit for it.

 

hi m,
Look at this option.
E

 

hi m,
Look at this option.
E

I can't work out how to apply this to my scenario. I don't need to sweep. I need to set {period} based on a voltage.

How can I remove the sweep part? When I set sweep to 0, then it's just ON. And when I remove everything after the >, I get a sawtooth...

 

hi,
Post your schematic and if you have a LTS asc, also post that, we can check out what you are trying to do.
E

 

Use a comparator

 

dim is red. It is the control voltage.
I then use a Sawtooth function and a comparator, to work out how long the pulse should stay on for.
As shown, this works well.

BUT.

I need the purple line to turn on at the same time as the green line. It MUST be aligned to 80ms, 100ms, 120ms etc. The sawtooth is causing it to be shifted.

 

hi,
Is this more in line with your query.?

E

 

.... or how about this?

 

The LTspice Control Library has such a function, along with quite a few other useful bits.

https://github.com/kanedahiroshi/LTspiceControlLibrary

 

@Alec_t Spectacular. Thanks.



Two more questions.
1. Based on the datasheet https://www.mouser.com/datasheet/2/149/FL5150-964157.pdf, how can I ignore the DIM voltage below 40mV; so the BV is 0 if dim<40mV?

2. Using your BV design idea, how would I convert my clunky sawtooth + PWL 10% steps to a BV? I want a PWM that changes duty by 10% every 200ms.

 

I want a PWM that changes duty by 10% every 200ms.

What happens when it gets to 100%? Stay at 100%? Or back to 0% and start the cycle again?

 

What happens when it gets to 100%? Stay at 100%? Or back to 0% and start the cycle again?

Happy for it to stay at 100%. I'll set .tran accordingly stop time accordingly.
But either is fine. Whatever is easier.

I fixed Q1
V=9*((time/period-int(time/period))<if(v(dim,gnd_a)<40m,0,v(dim,gnd_a)/2.5))

 

I think if you use that Q1 equation for your BV, in conjunction with your stepped PWL source as the Dim voltage, that should answer Question 2.

 

Yeah nice. This works
V=3.3*((time/period-int(time/period))<v(step)/3.3)

Any ideas on how to avoid that? Have it so that it automatically steps down based on time?

 

Oh I got it!
V=3.3*((time/period-int(time/period))<int(time/200m)/10)