Voltage to PWM chip (similar to class D)?

[George Herold]

On Friday, May 30, 2014 1:00:16 PM UTC-4, Joerg wrote:

dagmargoodboat@yahoo.com wrote:

On Friday, May 30, 2014 12:45:25 PM UTC-4, Joerg wrote:



Digital is ok but 1000:1 granularity at a MHz or at least half a MHz

would require a fat processor with really good timer resources.



Okay, that narrows the field--too fast for a 555!



It's going to be hard to get good linearity from any consumer
part, and it's hard to imagine who'd need it other than class-D
audio.

That's the reason for starting this thread: Class D. They use PWM in th
hundreds of MHZ in order to get away with small magnetics yet it's quite
linear.

The DC stability is the issue, most audio ICs barely have any to write
home about.

Well I don't really know what I'm suggesting,
but can you patch up the DC response of an audio IC?
(maybe someone already did that?)
George H.

--

Regards, Joerg



http://www.analogconsultants.com/

 

[Tim Wescott]

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.

http://www.c-for-dummies.com/

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

 

[Reinhardt Behm]

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.

http://www.c-for-dummies.com/

But generating a PWM at 1MHz with a resolution of 0.1% mean you need a base
frequency for your counter of 1GHz. I don't know any CPU that could do that.

--
Reinhardt Behm

 

[John Larkin]

On Fri, 30 May 2014 09:32:27 -0600, Reinhardt Behm <rbehm@hushmail.com> wrote:

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.

http://www.c-for-dummies.com/

But generating a PWM at 1MHz with a resolution of 0.1% mean you need a base
frequency for your counter of 1GHz. I don't know any CPU that could do that.

Someone, TI I think, has a ARM with nanosecond-resolution PWM outputs. But Joerg
seems to want an analog input.

Digital delay generators generally use a clocked coarse width generator with
analog interpolation down to picoseconds. Too complex for this app.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation

 

[Tim Wescott]

On Fri, 30 May 2014 09:32:27 -0600, Reinhardt Behm wrote:

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and
can handle PWM frequencies in the 50-1000kHz range? Similar to a class
D driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A
uC is not suitable either because it should be simple and I need very
fine control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.

http://www.c-for-dummies.com/

But generating a PWM at 1MHz with a resolution of 0.1% mean you need a
base frequency for your counter of 1GHz. I don't know any CPU that could
do that.

Some MCUs follow the PWM section with a selectable delay line, which
gives you sub-clock PWM resolution.

But often you can stand higher error in the higher frequencies than you
can at DC. In that case you can put a sigma-delta modulator between your
drive number and your PWM. I do this often in control loops, because it
works well, it's easy, and it's cheap.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Maynard A. Philbrook Jr.]

In article <5387D833.9040002@electrooptical.net>,
hobbs@electrooptical.net says...

On 5/29/2014 5:20 PM, Joerg wrote:
Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.


How about a CMOS 555 with a current source to charge the capacitor?

Cheers

Phil Hobbs

Yeah, have a tube of those 1 cell voltage capable Cmos versions. They
don't handle the upper voltage well but they sure do work well in one
cell applications.

Jamie

 

[Tim Wescott]

On Fri, 30 May 2014 08:51:00 -0700, John Larkin wrote:

On Fri, 30 May 2014 09:32:27 -0600, Reinhardt Behm <rbehm@hushmail.com
wrote:

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and
can handle PWM frequencies in the 50-1000kHz range? Similar to a
class D driver but has to go down to DC. The changes in control would
be restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A
uC is not suitable either because it should be simple and I need very
fine control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

Hey! Microprocessors are simple! You just need to learn some
valuable skills, that's all.

http://www.c-for-dummies.com/

But generating a PWM at 1MHz with a resolution of 0.1% mean you need a
base frequency for your counter of 1GHz. I don't know any CPU that could
do that.

Someone, TI I think, has a ARM with nanosecond-resolution PWM outputs.
But Joerg seems to want an analog input.

Digital delay generators generally use a clocked coarse width generator
with analog interpolation down to picoseconds. Too complex for this app.

TI also sells a DSP-ish chip that has a PWM output with a final delay-
line stage that gives you finer clock resolution.

I know Joerg wants an analog input. _I_ want to tweak Joerg a bit!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tim Wescott]

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

A 555 or other teeny thing wrapped with integrating feedback, to hold the
average at precisely what you want? It kinda violates your "one chip"
desire, but at least it can be done with a minimum of small parts.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Lasse Langwadt Christense]

Den fredag den 30. maj 2014 19.18.22 UTC+2 skrev Joerg:

Lasse Langwadt Christensen wrote:

Den fredag den 30. maj 2014 18.34.37 UTC+2 skrev Joerg:

Lasse Langwadt Christensen wrote:



Den fredag den 30. maj 2014 16.15.46 UTC+2 skrev Joerg:

John Larkin wrote:

On Thu, 29 May 2014 18:07:45 -0700, Joerg <invalid@invalid.invalid> wrote:

John Larkin wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg <invalid@invalid.invalid

wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can

handle PWM frequencies in the 50-1000kHz range? Similar to a class D

driver but has to go down to DC. The changes in control would be

restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with

555-style timers or switcher chips. I am looking for better 1% and

ideally a lot better, including nonlinearity, drift, warts and all. A uC

is not suitable either because it should be simple and I need very fine

control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

How about a sawtooth or triangle waveform and a comparator. Close a

feedback loop around that, with a PWM to DC converter; the PWM-DC part

can be made very linear.

That's what I wanted to avoid for real estate reasons. But if I hafta

I'll do it.

If you want 0.1% accuracy, you might be able to do it open-loop, with a very

linear ramp, but it will be hard at that frequency. If you only want 0.1%

resolution ("granularity"?) it's not so bad.

Can you use delta-sigma? There are some integrated d-s modulators around.

I could but then I'd have to build a one-shot that stretches the pulses

into a very precise length and that's almost the same kind of challenge

as building my own PWM generator (needs too much space).

why would you need a one-shot? the output is clocked

something like the AD7401, if you want to be sure a flipflop on the output





It's possible but when assuming a master clock of 20MHz going in and I'd



want, say, a 1k granularity that would result in an effective PWM of



only 20kHz. Unless I am understanding something wrong in the datasheet.





with DS is hard to talk about pwm frequency, it has noise shaping so the

switching noise gets pushed to higher frequencies. i.e if you are exactly midrange the "pwm frequency" would be 10MHz



with a brick wall reconstructions filter you would normally gain 3dB every

time you double the sampling frequency, with a second order deltasigma you

gain ~15dB, first order ~9dB





http://www.analog.com/static/imported-files/data_sheets/AD7401.pdf



What I'd essentially need is a class D audio modulator but without the



DC cut-off. Unfortunately that's as rare as it is on audio CODECs where



only a few such as the AD1939 can go down to DC without onerous offset



issues or huge drift.





if you can live with the higher switching frequency I think deltasigma

would do that





I can't live with a switching frequency that gets much past a MHz in

certain areas. It will cause large losses in the attached power

electronics. I'd like the PWM to be at least somewhat constant in

frequency. It isn't critical though, if it varies even 50% that would be

ok. But not a lot more.

so clock it at 2MHz, I still think it'll be close


-Lasse


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[Lasse Langwadt Christense]

Den fredag den 30. maj 2014 22.19.20 UTC+2 skrev Tim Wescott:

On Fri, 30 May 2014 15:37:47 -0400, rickman wrote:



On 5/30/2014 3:00 PM, Joerg wrote:

rickman wrote:

On 5/30/2014 10:15 AM, Joerg wrote:

John Larkin wrote:

On Thu, 29 May 2014 18:07:45 -0700, Joerg <invalid@invalid.invalid

wrote:



John Larkin wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg

invalid@invalid.invalid> wrote:



Folks,



Does anyone know an IC that can turn a control voltage into PWM

and can handle PWM frequencies in the 50-1000kHz range? Similar

to a class D driver but has to go down to DC. The changes in

control would be restricted to the audio spectrum below 15kHz.



The LTC6992 does this nicely but isn't precise enough. Same with

555-style timers or switcher chips. I am looking for better 1%

and ideally a lot better, including nonlinearity, drift, warts

and all. A uC is not suitable either because it should be simple

and I need very fine control granularity, down to around 0.1%.



Can't use short-lived consumer chips for radios and TV sets and

such.

How about a sawtooth or triangle waveform and a comparator. Close

a feedback loop around that, with a PWM to DC converter; the

PWM-DC part can be made very linear.



That's what I wanted to avoid for real estate reasons. But if I

hafta I'll do it.



If you want 0.1% accuracy, you might be able to do it open-loop,

with a very linear ramp, but it will be hard at that frequency. If

you only want 0.1%

resolution ("granularity"?) it's not so bad.



Can you use delta-sigma? There are some integrated d-s modulators

around.





I could but then I'd have to build a one-shot that stretches the

pulses into a very precise length and that's almost the same kind of

challenge as building my own PWM generator (needs too much space).



Can the pulse width be dithered or does each pulse width have to

adjust to within 0.1% of the needed value? I think a digital solution

could do this if you just need the average pulse width to match.





Dither is a problem. If it's dithering between 2-3 consecutive pulses,

maybe, but it would be a serious compromise.



This does not mean I am ruling anything out at this point, it's just

that I'd first like to see if there is a class-D chip that can be

pressed into service. Thise have very clean PWM, just not down to DC

because offset voltages and stuff aren't important for audio.



I'm not clear on your answer. To make sure you understand what I am

suggesting... You can use a slower counter clock if you dither the

pulse width. In reality you would need to use some feedback similar to

a sigma-delta circuit. The pulse width would only vary from ideal by a

fraction of a count at any one time and the error could be accumulated

and used to add a delta on the next pulse width. Another plus is that

the feedback loop can be internal to the digital circuit since the error

source is the low order bits that are being truncated.



I think John Larkin has picked up on this judging from his post. The

question is whether the small instantaneous deviations will make a

difference. Similar to sigma-delta the "noise" would be at a high

frequency.



Depending on who you ask, it's not "like" a sigma-delta -- it _is_ a

sigma-delta. Just because _most_ sigma-delta converters use 1-bit D/A or

A/D conversion doesn't mean they all do.

exactly, sigma-delta is about getting from one resolution to another while shaping the quantization error towards higher frequencies

the reason for 1 bit is that then linearity comes for "free"

-Lasse

 

[Joerg]

Lasse Langwadt Christensen wrote:

Den fredag den 30. maj 2014 16.15.46 UTC+2 skrev Joerg:
John Larkin wrote:

On Thu, 29 May 2014 18:07:45 -0700, Joerg <invalid@invalid.invalid> wrote:
John Larkin wrote:
On Thu, 29 May 2014 14:20:26 -0700, Joerg <invalid@invalid.invalid
wrote:
Folks,
Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.
The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.
Can't use short-lived consumer chips for radios and TV sets and such.
How about a sawtooth or triangle waveform and a comparator. Close a
feedback loop around that, with a PWM to DC converter; the PWM-DC part
can be made very linear.
That's what I wanted to avoid for real estate reasons. But if I hafta
I'll do it.
If you want 0.1% accuracy, you might be able to do it open-loop, with a very
linear ramp, but it will be hard at that frequency. If you only want 0.1%
resolution ("granularity"?) it's not so bad.
Can you use delta-sigma? There are some integrated d-s modulators around.


I could but then I'd have to build a one-shot that stretches the pulses

into a very precise length and that's almost the same kind of challenge

as building my own PWM generator (needs too much space).


why would you need a one-shot? the output is clocked

something like the AD7401, if you want to be sure a flipflop on the output

It's possible but when assuming a master clock of 20MHz going in and I'd
want, say, a 1k granularity that would result in an effective PWM of
only 20kHz. Unless I am understanding something wrong in the datasheet.

http://www.analog.com/static/imported-files/data_sheets/AD7401.pdf

What I'd essentially need is a class D audio modulator but without the
DC cut-off. Unfortunately that's as rare as it is on audio CODECs where
only a few such as the AD1939 can go down to DC without onerous offset
issues or huge drift.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Tim Wescott wrote:

On Fri, 30 May 2014 09:32:27 -0600, Reinhardt Behm wrote:

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and
can handle PWM frequencies in the 50-1000kHz range? Similar to a class
D driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A
uC is not suitable either because it should be simple and I need very
fine control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.
Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.

http://www.c-for-dummies.com/
But generating a PWM at 1MHz with a resolution of 0.1% mean you need a
base frequency for your counter of 1GHz. I don't know any CPU that could
do that.

Some MCUs follow the PWM section with a selectable delay line, which
gives you sub-clock PWM resolution.

If that's the usual silicon delay line deal I don't want it. Took my
lumps there, or rather a client did and when they called me in all I
could do is rip it all out and design an analog solution. The digital
delay section had the noise performance of a pressure-assist loo during
the flush

But often you can stand higher error in the higher frequencies than you
can at DC. In that case you can put a sigma-delta modulator between your
drive number and your PWM. I do this often in control loops, because it
works well, it's easy, and it's cheap.

Which IC do you use for generating the PWM? Creating a number would be
as easy as dropping in an ADC. Most sigma-delta chiops I've seen use a
fairly low clock frequency and you don't get the granularity needed at a
sufficnetly hig PWM frequency. I could drop the upper frequency to
500kHz if hard-pressed but not a lot lower.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Lasse Langwadt Christensen wrote:

Den fredag den 30. maj 2014 17.32.27 UTC+2 skrev Reinhardt Behm:
Tim Wescott wrote:



On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:
Folks,
Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.
The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.
Can't use short-lived consumer chips for radios and TV sets and such.
Hey! Microprocessors are simple! You just need to learn some valuable
skills, that's all.
http://www.c-for-dummies.com/


But generating a PWM at 1MHz with a resolution of 0.1% mean you need a base

frequency for your counter of 1GHz. I don't know any CPU that could do that.


he did say the input was limited to 15KHz, so 2*15K*1000 >= 30MHz

plenty of cpus that can do that

But their timers won't run faster than MLCK and that seriously limits
the granularity if the PWM has to pipe out at a MHz. I'd need almost a
Giggeehoitz. Not that it can't be done but that's a really fat CPU.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Tim Wescott wrote:

On Fri, 30 May 2014 08:51:00 -0700, John Larkin wrote:

On Fri, 30 May 2014 09:32:27 -0600, Reinhardt Behm <rbehm@hushmail.com
wrote:

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and
can handle PWM frequencies in the 50-1000kHz range? Similar to a
class D driver but has to go down to DC. The changes in control would
be restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A
uC is not suitable either because it should be simple and I need very
fine control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.
Hey! Microprocessors are simple! You just need to learn some
valuable skills, that's all.

http://www.c-for-dummies.com/
But generating a PWM at 1MHz with a resolution of 0.1% mean you need a
base frequency for your counter of 1GHz. I don't know any CPU that could
do that.
Someone, TI I think, has a ARM with nanosecond-resolution PWM outputs.
But Joerg seems to want an analog input.

Digital is ok but 1000:1 granularity at a MHz or at least half a MHz
would require a fat processor with really good timer resources.

Digital delay generators generally use a clocked coarse width generator
with analog interpolation down to picoseconds. Too complex for this app.

Not too complex but all the phase noise measurements I did on digital
delay lines so far almost made me throw up.

TI also sells a DSP-ish chip that has a PWM output with a final delay-
line stage that gives you finer clock resolution.

I know Joerg wants an analog input. _I_ want to tweak Joerg a bit!

My wife's already trying that since a few decades

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Tim Wescott wrote:

On Thu, 29 May 2014 14:20:26 -0700, Joerg wrote:

Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

A 555 or other teeny thing wrapped with integrating feedback, to hold the
average at precisely what you want? It kinda violates your "one chip"
desire, but at least it can be done with a minimum of small parts.

I am going to do something like that (but probably not with a 555) if
nothing single-chip comes up. That's the reason for this thread, to see
if there isn't anything out there. I mean, every class-D amp must have a
super-linear PWM generator. It's just that most have the power stages
built in (would be ok, can be left idle) and have lousy or no DC
performance (would not be ok).

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Maynard A. Philbrook Jr.]

In article <a697758c-89a9-446b-9ebe-eb1f2be82d9e@googlegroups.com>,
whit3rd@gmail.com says...

On Thursday, May 29, 2014 6:07:45 PM UTC-7, Joerg wrote:
John Larkin wrote:

How about a sawtooth or triangle waveform and a comparator. Close a
feedback loop around that, with a PWM to DC converter; the PWM-DC part
can be made very linear.

That's what I wanted to avoid for real estate reasons.

It's only a dual comparator and a transistor! The first comparator is set
up as a Schmitt trigger with thresholds at +Vs and -Vs; its output drives
a resistor to the base, capacitor from collector to base, emitter to V-.
That makes a triangle wave. The second comparator takes signal in
on one input, and triangle wave on the other, and you're done.

yes, a fixed frequency PWM 0..100% generator, use that myself.

Jamie

 

[Joerg]

whit3rd wrote:

On Thursday, May 29, 2014 6:07:45 PM UTC-7, Joerg wrote:
John Larkin wrote:

How about a sawtooth or triangle waveform and a comparator. Close a
feedback loop around that, with a PWM to DC converter; the PWM-DC part
can be made very linear.

That's what I wanted to avoid for real estate reasons.

It's only a dual comparator and a transistor! The first comparator is set
up as a Schmitt trigger with thresholds at +Vs and -Vs; its output drives
a resistor to the base, capacitor from collector to base, emitter to V-.
That makes a triangle wave. The second comparator takes signal in
on one input, and triangle wave on the other, and you're done.

Sure, I know how to do it analog. But it's not quite as simple as it
looks. Offsets, drift, regulators for super-stable rails et cetera. I
thought there's got to be a solution-in-a-can because of all the class D
amps these days. But the problem seems to be DC because audio doesn't
need that. So maybe there isn't a suitable IC.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

dagmargoodboat@yahoo.com wrote:

On Thursday, May 29, 2014 9:10:45 PM UTC-4, Joerg wrote:
Phil Hobbs wrote:
On 5/29/2014 5:20 PM, Joerg wrote:
Folks,

Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.

The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.

Can't use short-lived consumer chips for radios and TV sets and such.

How about a CMOS 555 with a current source to charge the capacitor?

That could be done but would gradually lead me to the circuit complexity
(size) of a comparator solution. Because I'd have to control the current
source over a wide range and very precisely.

The current source would be fixed, not variable "over a wide range."
That's trivial.

Then the comparator in the IC must do the job and be good, and they
normally aren't.

555 comparator drift might be a problem, but there might be precision
versions without it.

That's the trade-off.

The 555 is cheap and small enough that it's almost worth using just
for the internal logic...

Yes, or go with an analog solution like whit3rd mentioned. But then
you'll have to make sure the offsets and all that are handled which
makes it all not as trivial and low in real estate as it looks.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

dagmargoodboat@yahoo.com wrote:

On Friday, May 30, 2014 12:45:25 PM UTC-4, Joerg wrote:

Digital is ok but 1000:1 granularity at a MHz or at least half a MHz
would require a fat processor with really good timer resources.

Okay, that narrows the field--too fast for a 555!

It's going to be hard to get good linearity from any consumer
part, and it's hard to imagine who'd need it other than class-D
audio.

That's the reason for starting this thread: Class D. They use PWM in the
hundreds of MHZ in order to get away with small magnetics yet it's quite
linear.

The DC stability is the issue, most audio ICs barely have any to write
home about.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

Lasse Langwadt Christensen wrote:

Den fredag den 30. maj 2014 18.34.37 UTC+2 skrev Joerg:
Lasse Langwadt Christensen wrote:

Den fredag den 30. maj 2014 16.15.46 UTC+2 skrev Joerg:
John Larkin wrote:
On Thu, 29 May 2014 18:07:45 -0700, Joerg <invalid@invalid.invalid> wrote:
John Larkin wrote:
On Thu, 29 May 2014 14:20:26 -0700, Joerg <invalid@invalid.invalid
wrote:
Folks,
Does anyone know an IC that can turn a control voltage into PWM and can
handle PWM frequencies in the 50-1000kHz range? Similar to a class D
driver but has to go down to DC. The changes in control would be
restricted to the audio spectrum below 15kHz.
The LTC6992 does this nicely but isn't precise enough. Same with
555-style timers or switcher chips. I am looking for better 1% and
ideally a lot better, including nonlinearity, drift, warts and all. A uC
is not suitable either because it should be simple and I need very fine
control granularity, down to around 0.1%.
Can't use short-lived consumer chips for radios and TV sets and such.
How about a sawtooth or triangle waveform and a comparator. Close a
feedback loop around that, with a PWM to DC converter; the PWM-DC part
can be made very linear.
That's what I wanted to avoid for real estate reasons. But if I hafta
I'll do it.
If you want 0.1% accuracy, you might be able to do it open-loop, with a very
linear ramp, but it will be hard at that frequency. If you only want 0.1%
resolution ("granularity"?) it's not so bad.
Can you use delta-sigma? There are some integrated d-s modulators around.
I could but then I'd have to build a one-shot that stretches the pulses
into a very precise length and that's almost the same kind of challenge
as building my own PWM generator (needs too much space).
why would you need a one-shot? the output is clocked
something like the AD7401, if you want to be sure a flipflop on the output


It's possible but when assuming a master clock of 20MHz going in and I'd

want, say, a 1k granularity that would result in an effective PWM of

only 20kHz. Unless I am understanding something wrong in the datasheet.


with DS is hard to talk about pwm frequency, it has noise shaping so the
switching noise gets pushed to higher frequencies. i.e if you are exactly midrange the "pwm frequency" would be 10MHz

with a brick wall reconstructions filter you would normally gain 3dB every
time you double the sampling frequency, with a second order deltasigma you
gain ~15dB, first order ~9dB


http://www.analog.com/static/imported-files/data_sheets/AD7401.pdf

What I'd essentially need is a class D audio modulator but without the

DC cut-off. Unfortunately that's as rare as it is on audio CODECs where

only a few such as the AD1939 can go down to DC without onerous offset

issues or huge drift.


if you can live with the higher switching frequency I think deltasigma
would do that

I can't live with a switching frequency that gets much past a MHz in
certain areas. It will cause large losses in the attached power
electronics. I'd like the PWM to be at least somewhat constant in
frequency. It isn't critical though, if it varies even 50% that would be
ok. But not a lot more.

--
Regards, Joerg

http://www.analogconsultants.com/