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Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
'evening all, I'm looking for a solution to a PWM problem:
I'd like a constant output frequency, up to 8MHz, with a 50% duty cycle, varied to one or other end (e.g. 5% to 95%) by a lower frequency analog signal (up to 1MHz).
As soon as I thought PWM, TL494 came to mind, but of course, that's 500kHz tops, so out of the question.
What chips are out there that are suitable up to these frequencies, with this depth of modulation?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I was looking for a PWM chip that would operate at 1MHz for a project at work, and never found any I liked. I ended up making my own: I discovered a cheap simple way to do it that I can't tell you about But I doubt even that would work at 8MHz.
Anyway, if it's for a power electronic application, you won't be able to use PWM at such high frequencies. A Class-E or Class-DE power stage only has proper soft switching at one duty cycle.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
So this one works up to 40Mhz:
I troubled myself to find one, dont know if there are any similar ICs. You can program the waveform, duty cycle, and frequency by your wish, IC may be harder to get but maxim may send you sample if you ask nicely.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Assuming you've already got the 8 MHz oscillator all you need is a monostable with a time of say 10 to 120 ns. I think that this can be as simple as a variable pull-up resistor of 1 to 11 kOhm on a schmitt gate coupled by a capacitor of about 10pF.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Assuming you've already got the 8 MHz oscillator all you need is a monostable with a time of say 10 to 120 ns. I think that this can be as simple as a variable pull-up resistor of 1 to 11 kOhm on a schmitt gate coupled by a capacitor of about 10pF.
With that approach you can simply make an astable out of 74HC14 and control duty cycle using diodes. Output signal quality would probably be like your monostable method.
Using specialised monostable ICs can help, but I think its best idea to stick to special ICs like one I put datasheet for.
I discovered a cheap simple way to do it that I can't tell you about
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
Max038 from DigiKey is USD$23.00. "High frequency waveform generator up to 20MHz".. we are talking about the same thing, methinks. Worth the investment? Where can I find them more affordably, knowing me I'll probably burn at least two before I get anything working
Whatever the implementation of the pwm (discrete, IC, etc), it'll be going into a commercial product, where later I'll have the capital to buy ICs at a rapacious cost. Can't buy 10 at that price now though... =P
The output waveform doesn't need to be perfect. Even a quasi-sinewave would do. The PWM will be going to discrete gatedrivers then power transistors, so not too much worry there.
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High frequency PWM
