PWM on a LOW Pass Filter

[Suraj143]

Hi!

I want to send a PWM output to a low pass filter.

I want to know whats the relationship between them?

Thanks

 

[Mikebits]

Why do you want to do this? You would round the pulse edges.

 

[Suraj143]

Hi thanks for that.

I need to find a way to control a voltage regulator to get different voltages from a digital input.

 

[Suraj143]

I like to control DC range 1.2V-30V.The PWM part ok for me. I don't know the output part what to connect to the regulator.

 

[Pommie]

Have you looked at the Microchip Application Note? Clicky.

Mike.

 

[Someone Electro]

For the regulator it may be a better idea to wire together a binch of resistors to make a DAC. Only down side is to get 8 bit resolution you need 8 pins.

But if you have a scope you could just try out different values and see how clean the output is.

 

[Nigel Goodwin]

For the regulator it may be a better idea to wire together a binch of resistors to make a DAC. Only down side is to get 8 bit resolution you need 8 pins.

It gives no advantage, you have to buffer the R2R anyway, so it's as easy to use a LPF on a PWM output, and you get 10 bit resolution rather than 8 bit.

It's also a VERY widely used technique, used in countless millions of TV sets, for generating the varicap tuning voltage (with great precision), plus lower resolution channels for volume, brightness etc.

 

[Someone Electro]

Well i was meaning that the resistors are the resistor from the ADJ pin to ground and then use tristate pins to enable/disable resistors.

 

[Suraj143]

Hi thanks for the suggestions I read Mikes AP note as well & got some new ideas.

Forget about the 30V side lats say I want to adjust 1.2V-12V maximum.

Can anybody tell me how to connect the low pass filter to the LM317T?

 

[Someone Electro]

What if you set the LM to regulate to 1,2V but the resistor that comes to ground rather connect it to the PWM.

 

[Speakerguy]

A Sallen-Key equal component topology lowpass on the PWM output should give excellent results if placed several octaves below the PWM frequency.

I want to send a PWM output to a low pass filter.

I want to know whats the relationship between them?

PWM Duty Cycle * Vcc (5V, 3.3V, whatever) = DC voltage after lowpass. Pick a regulator with a control input pin OR scale the DC voltage by some gain factor (you can do this in the LPF at the same time and save an op amp) and connect it to the ADJ pin on a variable regulator.

 

[Someone Electro]

Or if you don't want opamps use a bunch of resistors, schematic for that in the LMs datasheet.

 

[Ubergeek63]

It will not work. The LM317 is actually regulating the 1.2V and uses the current through that resistor to raise itself up on another while sending it's own 5mA of operating current to it's output.

You need an additional reference to compare the output to and regulate it with an additional transistor. Switching in resistors would require 40V transistors since the 30V when you try to disable one would blow out anything less.

 

[mneary]

Switching resistors is easy, but you need to use the right part. If I was working from my own junkbox I would use 7406 or 7407.

OP is going to have to use a high voltage part anyway to control the adj pin of the LM317 from 0V (1.2V out) to 28.8V (30V out).

 

[mneary]

Or if you don't want opamps use a bunch of resistors, schematic for that in the LMs datasheet.

It will not work.

See the National data sheet: page 21, drawing 906302.

 

[Roff]

What is the frequency of your PWM signal? How much residual ripple from the PWM can you tolerate?

 

[Suraj143]

Hi I also earlier planned to use different resisters to ADJ pin via different transistors.
Like in the national datasheet.

But if I need 12 values I need 12 transistors? Oh no I don’t like that.

That’s why I need a LPF method driven by a PWM output.

The problem is still I don’t know how to connect the LPF to the LM317T.

What is the frequency of your PWM signal? How much residual ripple from the PWM can you tolerate?

Actually a good question. I don’t know what residual ripple is & never heard.

The frequency of course I have to calculate. What’s the best frequency will it suits?

Thanks

 

[Someone Electro]

No 12 of then will give you 12 bit accuracy that means 2^12 and that is 4096 different levels of output. This means you can control the output in 0,007V steps.

 

[Ubergeek63]

It will not work!!! You need to set your max voltage with the normal circuit (a resistor from out to sense and a resistor to GND) and then put a transistor in the GND line.

Then put a BIG cap from sense to GND, the value is determined by the response time you need.

 

[Suraj143]

It will not work!!! You need to set your max voltage with the normal circuit (a resistor from out to sense and a resistor to GND) and then put a transistor in the GND line.

Then put a BIG cap from sense to GND, the value is determined by the response time you need.

Hi I just draw a diagram can you tell me is it ok? Because I have never worked on a similar thing.

Thanks