How to control a dc-dc converter with an AVR???

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Hi,

in the past ive made DC-DC converters using the AVRs ADC and PWM, where they are connected directly to the Buck-Boost converter, and regulate from there. However, the problem is that the transient response was too slow, and the PWM frequency is too small, resulting in large converter components.

Is there some sort of dedicated DC-DC controller IC that we can connect directly to an AVR, which will handle the regulation??

The task: i want to make a high current variable power supply. So the chip needs to be able to accept some type of signal from the AVR, and regulate around that.

Also, ive looked at analog solutions such as a tl494, but im not really sure how i can connect it to an avr. One thought was, maybe i can use a digital potentiometer connected to the feedback loop of the 494, and then connect it on the i2c interface of the AVR. ant thoughts??

Thanks.

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

Last Edited: Fri. Jan 25, 2008 - 01:43 AM
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I would suggest (since I actually do it) take a pwm output, filter it, and sum it into the feedback node of a switchmode regulator chip. With proper choice of the resistors in the feedback/summing network you can make a very nice variable output supply.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Ok, sounds pretty simple.

However, isn't there any dedicated switchmode controllers that can connect directly to a microcontroller. There has to be somewhere??

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

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I haven't run across one, but I haven't really looked either. The tl494, due to it's flexible options for vref and error amps could accept a dc control voltage from a dac or pwm, but this does not appear to me to offer any clear advantage over the first summing scheme, which would work with almost any of the new high performance, low parts count switchers.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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tpappano wrote:
which would work with almost any of the new high performance, low parts count switchers.

Whats an example of one, or, a few of them. Which do you recommend for say a 10A power supply?

I got a ~30v rectified dc voltage which i want to use for the supply, and i want an output voltage range of around 3-20 volts.

- Tony B. Sydney, Australia.
tbaz2679@mail.usyd.edu.au

Status: Supporting the GNU

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It's funny that this question came up. On the way home today, for some reason, I thought of using an AVR to control a switcher.

One of the ideas that floated across my head was using the AVR to control a digital potentiometer, and using that in the voltage-divider on the feedback pin of the switcher.

I haven't spent enough thought to explore the ins and outs, but I thought I'd at least share it and get others' opinions on it.

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Quote:
Whats an example of one, or, a few of them. Which do you recommend for say a 10A power supply?

I got a ~30v rectified dc voltage which i want to use for the supply, and i want an output voltage range of around 3-20 volts.

I just plugged your requirements into National's Webench thing and got:

LM25116

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Ub3r wrote:
Hi,

in the past ive made DC-DC converters using the AVRs ADC and PWM, where they are connected directly to the Buck-Boost converter, and regulate from there. However, the problem is that the transient response was too slow, and the PWM frequency is too small, resulting in large converter components.

Is there some sort of dedicated DC-DC controller IC that we can connect directly to an AVR, which will handle the regulation??

Thanks.

Wouldn't the Tiny45 do this , it has a fast PWM , like the 15

But the code prob. gotta be tight , for the regulation loop

/Bingo

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That's what I like about the idea of using a digital pot - the feedback is all done in the converter, which is way faster than what you can do in code. Yet you still have control over the output voltage via the AVR.

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The solution with adding the PWM Signal to the feedback is probably the easiest. Except for a shutdown there will be probably only very few regulators with a digital interface. One calss are the ones for new PC processors to adjust the core voltage, but they are probably quite speciallized.
Using a AVR with PLL for the PWM will allow 64 MHz PMW base clock and 250 kHz for fast 8 Bit PWM. This may be an option to implement it in Software.

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There are several with digital interfaces. Linear Tech makes them. Here is the access page for their selection table:

http://www.linear.com/pc/viewCat...

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
There are several with digital interfaces. Linear Tech makes them. Here is the access page for their selection table:

http://www.linear.com/pc/viewCat... ... 1042,C1763

Jim

That link just shows one part, which seems intended for a max 3.6 volts out, and it requires a separate fet driver chip. From the schematic though, it may be able to work with higher Vouts.

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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Why not just use an external DAC and mix that with the DC/DC converter's feedback pin? I've seen that done on countless app notes and data sheets.

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I haven't used the LM25116 however a project at uni last semester used one of these for a switch mode DC-DC like you said, it worked fairly well, no real issues with it. Someone decided to be tricky and use a micro albeit a PIC (gasp) and had more trouble than it was worth with it

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http://www.atmel.com/dyn/product...

Has something like an AVR used as a regulator IIRC.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Remember that if you have a fast feed back you don't need 8 bit PWM 4-5 bit will do so now the pwm run 8-16 times faster.

Jens