Nick_L

Hello

I want to construct a sawtooth generator by using a binary ripple carry counter and a D/A converter.The counter i am using is the CD4060BE and the D/A converter is the DAC0832.The sawtooth waveform i want to the output must be at about 40 KHz of frequency.
The main problem is that i update the DAC with the output of the counter but the final wavewform is not what i want.Either is not a sawtooth waveform or the waveform is of much lower frequency of 40 KHz!
Does anyone have any idea if there is a solution for my problem?Or any proposals to make of how to take the desired output by using this counter and this DAC?

Thanks!![/quote]

Nigel Goodwin

You would need to clock the 4060 a great deal faster to get a 40KHz output, what do you need the signal for?, and is there a specific reason to want to do it digitally?.

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Nick_L

I use a 4MHz crystal oscillator.I have tried a 10 MHz crystal as well but the results are about the same. The reason i want to to do it with this way is because the power supply is going to be provided by a 12V battery which is going to be charged by a photovoltaic system.And therefore i want to keep the power consumption as minimum as possible.On the other hand the circuit i have thought of is i believe very simple and the one that needs very little energy to work, compared to others that produce sawtooth waveforms.
Thanks for your interest!!

Nick_L

I forgot to tell that the signal is going to be used for the current control in a DC/DC converter.In general terms the whole project i want to make is a system that is tracking the maximum power point of a photovoltaic system when it is shaded.In order to do this i want a current controlled DC/DC converter and the method i have chosen needs a sawtooth waveform to provide stability to the system!!

Nigel Goodwin

Assuming you're using full 8 bit resolution?, you will need a 10.24MHz clock frequency to generate a 40KHz sawtooth. You just need to clock an 8 bit binary counter at that frequency, and feed the output to a DAC - a simple R2R resistor network would be fine, but your DAC0832 should be as well.

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jbeng

I see a couple of problems and both of them concern your counter:

First: The 4060 is a 14-bit binary counter. But, your lowest four bits of the counter's output (ie. Q0-3) are not available to use on that chip. If you set it up in the configuration you describe, the output from your DAC won't be a sawtooth waveform.

Second: Nigel calculated the clock frequency you'll require to get an output freq of 40KHz at 10.24MHz. Assuming you're regulating your power supply down to 5 volts, that freq is too high for a 4000-series Cmos chip to handle at that supply voltage. (My Motorola databook indicates a max input freq of 3.5MHz @ a supply of 5 volts, 8MHz @ 10 volts.)

I suggest using a 74HC393 for your counter, which contains two 4-bit counters. Cascade them to make an 8-bit counter. It will give you all the output bits required and (according to Motorola) it can be clocked at 22MHz when powered by 4.5volts. It will probably do even better at 5 volts. Keep in mind though, it's absolute maximum supply voltage is 7 volts.
Good luck,
Jeff

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