Why PWM generate circuit can not adjust the duty cycle to 10%?

Hello,

I have already answered you on this one in the Analogue group where you also asked.

The LM311 is NOT an op amp - it's a comparator with an OPEN COLLECTOR output.

It can only sink current not source it. You need a pull up resistor to 5V (I'm assuming that is the value of your Vcc). The resistor should be a low value compared with the load ie which is 10k in your case so the pull up should be 1k.

If EWB does not correctly simulate the LM311 I suggest you use a simulator that works in future (try LTspice which is a free download at www.linear.com).

Hello,

I have already answered you on this one in the Analogue group where you also asked.

The LM311 is NOT an op amp - it's a comparator with an OPEN COLLECTOR output.

It can only sink current not source it. You need a pull up resistor to 5V (I'm assuming that is the value of your Vcc). The resistor should be a low value compared with the load ie which is 10k in your case so the pull up should be 1k.

If EWB does not correctly simulate the LM311 I suggest you use a simulator that works in future (try LTspice which is a free download at www.linear.com).

OK,

the LM311 is not an op amp but it is a comparator, but when you close in a reaction loop a comparator with open collector it appears just like an op amp, then it doesn't need to sink or source high current to uP.

In order to solve this problem is necessary to focalize on the time of charge and time of discharge of the capacitor, this is the real core of the ocsillator.

This time is depending by the resistance value, if you are using a 100K potentiometer, the R7 must be 1K if you want to reach 1% and 99%.

If you will use a R7 of 100 ohm, you can reach 0,1% and 99,9 %.

I also believe the voltage reference circuit should be improved.

Alberto.

Alberto - you are not quite right about open collector outputs. They can only sink current and not source it. If you refer to the last diagram in Jason's post you see he has resistors of 100k and 1k in series to the 5V supply. When the pot is set to the lower extreme the output of the comparator is trying to charge the capacitor though a 10k resistor - but the only path for current into the capacitor is though the 100k, the 1k and the 10k resistor - that's why the ciruit doesn't work properly. All that is needed is 1k from the output of the comp. to the 5V supply and he will get his 10/90 (approximately) range in duty cycle

Reducing the feedback fixed resistor to 1k won't help at all.

Hi Michael,

Yes, you are completly right about the comparator with open collector, expecially with that polarization, 100K resistence are too high for that device, but the timing problem around the duty cyle is depending by R7 value, and this is not my opinion.

The frequency (1/T) of the oscillator is coming from the constant time of charge and the constant time of discharge of the capacitors.
The comparator's output switch from a state to its complement when the voltage on the capacitor reaches the same value of the voltage of other comparator input.

So:

T = C * R (during the charge) + C * R (during the discharge)

T= (C * (Rpot(x)+R7)) + (C * (Rpot(1-x)+R7))

where:
T = 1/Foscillator
C = C4 = 1[nF]
Rpot = 100[Kohm]
R7 = 10[Kohm]
X = potentiometer position ( from 0 to 1, 0% to 100%)

How you can see, when X=0 and x=1 the only influence on the shorter half periods is due to R7 value.

Hello Alberto,

We can improve your formula to show the effect of the open collector output with 101k pull up:

T = C * R (during the charge) + C * R (during the discharge) - your formula

actually there is an additional constant:

T = K(C * R (during the charge) + C * R (during the discharge)) - K is determined by the values of the bias and feedback on the positive input. Actually even this is not totally accurate because the diodes have an effect but we wont go into that here.

If we now put in the effect of the OC stage:

     T=  K(C * (Rpot(x)+R7+101k)) + (C * (Rpot(1-x)+R7))

and the charge period range is:

          K(C* 111k) to K(C*211k)

the discharge period is:

     K(C* 100k) to K(C*10k)

so the PW range is 111/(111+100) to 211/(211+10) = 52.6% to 95.5%

I can't make any sense out of the EWB simulation.

I've simulated it with LTSpice and used an LTC1841 comaprator (still has open collector style output).

First with 100k pull up and second with 1k pull up.

In case the pictures are illegible (they look too small as I'm doing this) I have  attached the jpeg files.

Attachments:

Yes, I absolutely agree with you.

I was only trying to explain why it can't regulate < 10%.

Think to use the right op amp or comparator without open collector, with that configuration the low limit and high limit of duty cycle is depending from R7 value.

Hi, Alberto & Michael,

Thanks for you help.

You show me how the circuit really work and give me very useful advices. At the same time you show me a very professional discuss. As you said, I decrease the value of the pull up resister from 100k to 10k, the duty cycle can be adjusted to 10%. And, I change the R7 from 10k to 1k, the duty cycle can be adjusted to 1%.

By the way, Michael, thanks for recommending the LTspice to me, It's powerful.

Jason