Forums

Voltage to PWM

Started by George Herold November 11, 2016
Hi all,  I want to make a V to PWM circuit.  (For a servoed heater.)  
I drew a triangle wave into a comparator, and that seems simple enough.  
Though it will take a few IC for the triangle wave.  (I guess I could 
use an opamp and dual comparator.)    
So I went searching for V-PWM IC's.  Most are for SMPS controll, which 
seem overly complicated for what I want.  I also want 
a fairly low frequency ~10-100 Hz, so I can roll off the edges.  
I did find the LTC6992 
http://cds.linear.com/docs/en/datasheet/69921234fc.pdf
which looks fine.  
Are there any other IC's I should look at?  
The 555?  

Thanks,

George H. 
On 11/11/2016 12:20 PM, George Herold wrote:
> Hi all, I want to make a V to PWM circuit. (For a servoed heater.) > I drew a triangle wave into a comparator, and that seems simple enough. > Though it will take a few IC for the triangle wave. (I guess I could > use an opamp and dual comparator.) > So I went searching for V-PWM IC's. Most are for SMPS controll, which > seem overly complicated for what I want. I also want > a fairly low frequency ~10-100 Hz, so I can roll off the edges. > I did find the LTC6992 > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > which looks fine. > Are there any other IC's I should look at? > The 555?
If you make a 555 sawtooth generator with a current source instead of the top resistor, it PWMs reasonably linearly. I assume this is inside a feedback loop, and merely intended to linearize the heater characteristic, so +-10% or so is not a problem. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
George Herold wrote...
> > Hi all, I want to make a V to PWM circuit. > (For a servoed heater.) [snip] > So I went searching for V-PWM IC's. [snip] > I did find the LTC6992 > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > which looks fine.
I'm using that IC for my test bed, see thread "Achieving 98% efficiency in a boost converter" and the links to my schematics and PCB layout. -- Thanks, - Win
On Friday, November 11, 2016 at 12:27:36 PM UTC-5, Phil Hobbs wrote:
> On 11/11/2016 12:20 PM, George Herold wrote: > > Hi all, I want to make a V to PWM circuit. (For a servoed heater.) > > I drew a triangle wave into a comparator, and that seems simple enough. > > Though it will take a few IC for the triangle wave. (I guess I could > > use an opamp and dual comparator.) > > So I went searching for V-PWM IC's. Most are for SMPS controll, which > > seem overly complicated for what I want. I also want > > a fairly low frequency ~10-100 Hz, so I can roll off the edges. > > I did find the LTC6992 > > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > > which looks fine. > > Are there any other IC's I should look at? > > The 555? > > > If you make a 555 sawtooth generator with a current source instead of > the top resistor, it PWMs reasonably linearly. I assume this is inside > a feedback loop, and merely intended to linearize the heater > characteristic, so +-10% or so is not a problem.
Yeah inside a loop.. we've got a bunch of 555's in stock. I mostly forgotten the little I knew of 555's, what's the simplest current source? A two transistor current mirror? (I've never used on of those.) I'll probably get the LTC chip... laziness wins over creativity. (creativity being constrained by time.) George H.
> > Cheers > > Phil Hobbs > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
On Friday, November 11, 2016 at 12:58:02 PM UTC-5, Winfield Hill wrote:
> George Herold wrote... > > > > Hi all, I want to make a V to PWM circuit. > > (For a servoed heater.) [snip] > > So I went searching for V-PWM IC's. [snip] > > I did find the LTC6992 > > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > > which looks fine. > > I'm using that IC for my test bed, see thread > "Achieving 98% efficiency in a boost converter" > and the links to my schematics and PCB layout. > > > -- > Thanks, > - Win
Thanks.. I didn't notice, but I didn't know of the IC before today. I'll order a few. George h.
On 11/11/2016 01:19 PM, George Herold wrote:
> On Friday, November 11, 2016 at 12:27:36 PM UTC-5, Phil Hobbs wrote: >> On 11/11/2016 12:20 PM, George Herold wrote: >>> Hi all, I want to make a V to PWM circuit. (For a servoed heater.) >>> I drew a triangle wave into a comparator, and that seems simple enough. >>> Though it will take a few IC for the triangle wave. (I guess I could >>> use an opamp and dual comparator.) >>> So I went searching for V-PWM IC's. Most are for SMPS controll, which >>> seem overly complicated for what I want. I also want >>> a fairly low frequency ~10-100 Hz, so I can roll off the edges. >>> I did find the LTC6992 >>> http://cds.linear.com/docs/en/datasheet/69921234fc.pdf >a>> Are there any other IC's I should look at? >>> The 555? >> >> >> If you make a 555 sawtooth generator with a current source instead of >> the top resistor, it PWMs reasonably linearly. I assume this is inside >> a feedback loop, and merely intended to linearize the heater >> characteristic, so +-10% or so is not a problem. > Yeah inside a loop.. we've got a bunch of 555's in stock. > > I mostly forgotten the little I knew of 555's, > what's the simplest current source? A two transistor current mirror? > (I've never used on of those.)
I've just used a PNP transistor with an appropriate resistor between the positive supply and emitter, collector to the capacitor on the threshold/trigger/discharge pins, and a voltage divider to the base. Something like this works fine: http://www.circuitstoday.com/wp-content/uploads/2009/09/555-ramp-generator.jpg
George Herold <gherold@teachspin.com> writes:

> Hi all, I want to make a V to PWM circuit. (For a servoed heater.) > I drew a triangle wave into a comparator, and that seems simple enough. > Though it will take a few IC for the triangle wave. (I guess I could > use an opamp and dual comparator.) > So I went searching for V-PWM IC's. Most are for SMPS controll, which > seem overly complicated for what I want. I also want > a fairly low frequency ~10-100 Hz, so I can roll off the edges. > I did find the LTC6992 > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > which looks fine. > Are there any other IC's I should look at? > The 555? >
You can do something like this http://ee.devereux.me.uk/ratio-coded-adc.png (ignore any actual part numbers). This arrangement can be accurate to ppm, not that it sounds like you need it :) -- John Devereux
On Friday, November 11, 2016 at 2:16:59 PM UTC-5, John Devereux wrote:
> George Herold <gherold@teachspin.com> writes: > > > Hi all, I want to make a V to PWM circuit. (For a servoed heater.) > > I drew a triangle wave into a comparator, and that seems simple enough. > > Though it will take a few IC for the triangle wave. (I guess I could > > use an opamp and dual comparator.) > > So I went searching for V-PWM IC's. Most are for SMPS controll, which > > seem overly complicated for what I want. I also want > > a fairly low frequency ~10-100 Hz, so I can roll off the edges. > > I did find the LTC6992 > > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > > which looks fine. > > Are there any other IC's I should look at? > > The 555? > > > > You can do something like this > > http://ee.devereux.me.uk/ratio-coded-adc.png > > (ignore any actual part numbers). > > This arrangement can be accurate to ppm, not that it sounds like you > need it :) > > > -- > > John Devereux
Thanks John.. I'll have to play a bit to make sure I understand how it works. That's a schmitt trigger inverter on the output? Oh, a schmitt trigger with two outputs and you are using the non-inverting one. George H.
George Herold <gherold@teachspin.com> writes:

> On Friday, November 11, 2016 at 2:16:59 PM UTC-5, John Devereux wrote: >> George Herold <gherold@teachspin.com> writes: >> >> > Hi all, I want to make a V to PWM circuit. (For a servoed heater.) >> > I drew a triangle wave into a comparator, and that seems simple enough. >> > Though it will take a few IC for the triangle wave. (I guess I could >> > use an opamp and dual comparator.) >> > So I went searching for V-PWM IC's. Most are for SMPS controll, which >> > seem overly complicated for what I want. I also want >> > a fairly low frequency ~10-100 Hz, so I can roll off the edges. >> > I did find the LTC6992 >> > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf >> > which looks fine. >> > Are there any other IC's I should look at? >> > The 555? >> > >> >> You can do something like this >> >> http://ee.devereux.me.uk/ratio-coded-adc.png >> >> (ignore any actual part numbers). >> >> This arrangement can be accurate to ppm, not that it sounds like you >> need it :) >> >> >> -- >> >> John Devereux > > Thanks John.. I'll have to play a bit to make sure I understand how it works. > That's a schmitt trigger inverter on the output? Oh, a schmitt > trigger with two > outputs and you are using the non-inverting one.
Yes, you only need a single-output schmitt of course. The opamp output is a triangle wave with the turn-around at the schmitt thresholds. The key is that the integrator ensures the averaged PWM output, i.e. the duty, exactly equals the input voltage. Sort of like a delta-sigma without a clock. -- John Devereux
Hard to beat a classic comparator.
http://seventransistorlabs.com/tmoranwms/Circuits_2010/Pulse_Width_Modulator.png

The TimerBlox thingys are great if you're absolutely cramped on space, but 
geez, they're so expensive.  They're also low voltage, which bites for power 
switching applications.

And as Phil noted, it's easily linearized with a CCS (which can be diode 
gated, like this one,
http://seventransistorlabs.com/tmoranwms/Circuits_2010/Pulse_Width_Modulator.png 
but you only need the one current source/sink if you wire it for sawtooth 
wave instead), which gets it as good as most anything you'd need.

I'm also building a more elaborate PWM generator at the moment, though it's 
intended for a different kind of power control.


Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

"George Herold" <gherold@teachspin.com> wrote in message 
news:ec5443f6-caeb-49d3-aa4b-1d81c3b0bef8@googlegroups.com...
> Hi all, I want to make a V to PWM circuit. (For a servoed heater.) > I drew a triangle wave into a comparator, and that seems simple enough. > Though it will take a few IC for the triangle wave. (I guess I could > use an opamp and dual comparator.) > So I went searching for V-PWM IC's. Most are for SMPS controll, which > seem overly complicated for what I want. I also want > a fairly low frequency ~10-100 Hz, so I can roll off the edges. > I did find the LTC6992 > http://cds.linear.com/docs/en/datasheet/69921234fc.pdf > which looks fine. > Are there any other IC's I should look at? > The 555? > > Thanks, > > George H.