Simple DC-DC with wide Vin

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I need to come up with as simple a DC-DC converter as I can. There are a TON of chips out there that make nice simple converters, but I my problem is high/wide input voltage and space. It needs to be small and, hopefully, simple.

Requirements:
Vin: 20-75V (15V-75V would be even better)
Vout: 12V +/-5%
Iout: ~1A would be about all I would need. But 600mA should suffice if I can't get to 1A.
Ripple: Very low ripple isn't all that important. The AVR will be running behind a secondary 5V linear regulator.
Isolation: non-isolated. Design shares a common ground.

I could go linear using an LT3014B with an NPN pass transistor to handle current, but there's no way even a D2PAK transistor could dissipate enough power given my space requirements.

I've done some searching, but I might not be looking at all the options. Do you gurus have any additional thoughts?

Jim M., Rank amateur AVR guy.

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National has the LM557x series controller. IIRC the MOSFET is internal, and all you need to provide is a Schottky and the inductor.

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I like the TI PT5101A... wide input range

Imagecraft compiler user

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Have you considered using the Avr as the dc:dc controller?

Thats quite the wide input operating voltage in your spec.

oddbudman

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oddbudman wrote:
Have you considered using the Avr as the dc:dc controller?

Thats quite the wide input operating voltage in your spec.

oddbudman


Yes. I had considered it, but it's busy enough with other duties and I don't think it could run the DC-DC as a sufficiently high enough frequency to make the inductor as small as I'd like.

Jim M., Rank amateur AVR guy.

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The LM5575 looks interesting I'll investigate it a little more.
The PT5101A is for old designs and their newer non-isolated offerings only go up to 38V. But a nice simple solution. Something like that 3SIP is certainly simple.

Jim M., Rank amateur AVR guy.

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Look about LT1074,its a buck mode conveter.

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Quote:
but I my problem is high/wide input voltage and space. It needs to be small and, hopefully, simple.

You didn't mention low cost!

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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LDEVRIES wrote:
Quote:
but I my problem is high/wide input voltage and space. It needs to be small and, hopefully, simple.

You didn't mention low cost!

Well YEAH. I want the world! Bwaha ha ha.
The overall cost of the project needs to be kept down. (Don't they all). I was hoping to be able to supply current to run auxiliary devices LEDs, LCD. Things of that nature without much hassle. A 3 pin SIP device that could do 70V down to 12V would be great. I found this one http://www.dimensionengineering.com/SWADJHV.htm but it's $25. Perhaps I'll design the board to just be able to run the AVR and FET drivers (~10-12mA) and leave a provision on the board to install one of these if the customer is OK with the up-charge.

Jim M., Rank amateur AVR guy.

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High PWM frequency doesn't tie up the AVR, but rather how often you need to adjust the duty cycle. If you can get by with a free running ADC and a slow interrupt rate the CPU load would be minimal.

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dak664 wrote:
High PWM frequency doesn't tie up the AVR, but rather how often you need to adjust the duty cycle. If you can get by with a free running ADC and a slow interrupt rate the CPU load would be minimal.

I can't use a free running ADC. My application uses the ADC extensively. But, yes, I understand what you're saying.

Jim M., Rank amateur AVR guy.