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'[EE]: LM3485 Hysteretic PFET Buck Controller.'
2002\08\01@234432 by Mike Singer

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part 1 710 bytes content-type:text/plain; (decoded 7bit)

Was: " More efficient way to drop voltage."

Russell McMahon wrote:
.
.
{Quote hidden}

Not cheaper (LM3485 is $.50 1K+), but
nevertheless, interesting. From LM3485 d
atasheet:
4.5V to 35V wide input range
1.242V to VIN adjustable output range
High Efficiency 93%
Maximum operating frequency > 1MHz

Look at attached "Typical Application Circuit".

Mike.


part 2 13930 bytes content-type:image/gif; (decode)


part 3 144 bytes
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2002\08\02@055748 by Russell McMahon

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> > It is unlikely that you could build an IC based switching
> > regulator for a component cost much cheaper than for
> > this one.
> .
>
>  Not cheaper (LM3485 is $.50 1K+), but
> nevertheless, interesting. From LM3485 d
> atasheet:
> 4.5V to 35V wide input range
> 1.242V to VIN adjustable output range
> High Efficiency 93%
> Maximum operating frequency > 1MHz


Nice looking IC
Details here         http://www.national.com/pf/LM/LM3485.html
Uses the FET Rdson as the current sense resistor which is a good trick.

Uses a PFET which is less fortunate but you can't have everything. Couple
this with eg a MDT2955 FET at about $US0.35 each ($NZ0.76 when I priced them
locally earlier today) in tube quantities and you have a substantial power
down-converter

FET         http://www.onsemi.com/productSummary/0,4317,MTD2955V,00.html

Note the LM3485 costs $22 one off from  NatSemi - they don't really want you
to buy small quantities.


       RM

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2002\08\02@092745 by Roman Black

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Hi Mike, it's not cheap at all, $0.50 x 1000.
In the circuit you showed you could probably replace
the LM3485 with a cheap 8 pin comparator chip, or
even a transistor and zener. With that many external components and FET
you have many options for a much cheaper circuit.
(once you are buying 1000 of them!) :o)
-Roman

Mike Singer wrote:

{Quote hidden}

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2002\08\02@095131 by A.J. Tufgar

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Thanks to the list I'm using just a fet, anything more would be
undoubtably more expensive.

Aaron

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2002\08\02@123733 by Matt Pobursky

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On Fri, 2 Aug 2002 21:30:08 +1200, Russell McMahon wrote:
>Note the LM3485 costs $22 one off from  NatSemi - they don't really
>want you to buy small quantities.

That's not the one off chip price -- that's the price for Qty. 1 of
their PS demo board for the chip.

Doing a quick 'net search I couldn't find any in stock anywhere. If
they are $.50USD@1K, then they are probably less than $1 in small
quantities when they become available from a distributor like Digikey.

Nice device. I'm going to request some samples and give them a spin.

Matt Pobursky
Maximum Performance Systems

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2002\08\02@165946 by Mike Singer

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Hi,Roman,
you are joking, aren't you?
The cost of the circuit is not the only thing, I'm obsessed
with. Technical parameters declared in the LM3485
datasheet mean something (input range, adjustable
output range, efficiency, maximum operating frequency...)

  Mike.

Roman Black wrote:
{Quote hidden}

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2002\08\02@173123 by Mike Singer

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Has anybody any suggestions of max regulated frequency?
If I replace bottom resistor of feedback chain by digital potentiometer and reduce output capacitor, what "max frequency" very aprox could I send to digital potentiometer, aiming at microstepping motor applications?
LM3485 can work at >1mhz.

  Thanks.
  Mike.

Matt Pobursky wrote:
{Quote hidden}

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2002\08\02@183143 by Russell McMahon

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>> $US22 / 1

>That's not the one off chip price -- that's the price for Qty
> 1 of their PS demo board for the chip.

True - my mistake (reading too fast and/or uptoo late :-) ).

A better choice MAY be the LTC1624 controller

       http://www.linear.com/prod/datasheet.html?datasheet=395

Similar functionality but

   - Drives an N Channel FET
       (implements a high side pump for gate drive supply)

-    Wider input and output voltage ranges

   - Able to implement other topologies.

They use an external current sense resistor but it is in the high side and
drops only about 160 mV (AFAIR) so this is fairly bearable.

N Channel FETs are generally cheaper, more available, have better Rdson and
are available in higher voltage ratings (not an issue here).

All these small pinout modern chips tend to have lower functionality
compared with many existing "full house" switching regulator ICs with more
pins and therefore access to more functionality. Despite this they look very
useful for simple dedicated applications. Even some VERY old SMPS ICs (20 +
years)(eg xx384x family and many more) will implement many of these
functions with a little design work - external component count may be
higher. If you want absolute minimum parts count then finding an IC with
internal pass FET and flyback diode adequately rated for your application is
the "way to go". Cost MAY be higher.

       RM

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2002\08\02@185448 by Matt Pobursky

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Mike,

Check the frequency response specification of the digital pot. If I
recall correctly (and I haven't looked recently), most digital pots
don't have a frequency response much above a few 100KHz. Also, check on
the settling time from when you give a digital control input until the
pot value settles to it's final value. I think these parameters are
going to be your limiting factors concerning speed.

A better way to do it might be to use a D/A converter in the PS
feedback loop if you want to adjust output voltage/current at a fast
rate.

Matt Pobursky
Maximum Performance Systems

On Sat, 3 Aug 2002 00:32:18 +0300, Mike Singer wrote:
>Has anybody any suggestions of max regulated frequency?
>If I replace bottom resistor of feedback chain by digital
>potentiometer and reduce output capacitor, what "max frequency" very
>aprox could I send to digital potentiometer, aiming at microstepping
>motor applications?
>LM3485 can work at >1mhz.

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2002\08\02@192013 by Matt Pobursky

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On Sat, 3 Aug 2002 10:30:35 +1200, Russell McMahon wrote:
>>That's not the one off chip price -- that's the price for Qty 1 of
>>their PS demo board for the chip.
>
>True - my mistake (reading too fast and/or uptoo late :-) ).

I did a double-take also. Then I reread it, I couldn't believe National
would have a $22/each switcher chip of ANY kind! ;-)

{Quote hidden}

OUCH. Without even looking I knew... $4.30 (Qty. 1-500) for that part.
Linear Tech makes great parts, but they are DAMNED proud of them.

>They use an external current sense resistor but it is in the high side
> and drops only about 160 mV (AFAIR) so this is fairly bearable.

Have you priced low ohm, precision current sense resistors (especially
SMD ones)? Another OUCH. In a high side application, taking advantage
of a FET's disadvantage - Rds(on) - is a good thing.

{Quote hidden}

I'm a big fan of TCA (total cost of assembly) since most all my designs
are destined to be built and tested in some quantity for some customer.
PC board space, component cost, assembly labor and test labor are all
factors in this. Generally, a bullet-proof switcher design that has
fewer parts but slightly higher parts cost will be the best solution
for all but the highest volume, cost sensitive applications.

For my personal hobby use, I could care less about parts cost most of
the time since I building one-offs and generally using free parts.

That's the great thing about engineering -- there are a large numbers
of solutions to a given problem based on your requirements and
perspective. None of them may be absolutely right or wrong, but some
will certainly be better than others... ;-)

Matt Pobursky
Maximum Performance Systems

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2002\08\03@024718 by Michael Rigby-Jones

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> -----Original Message-----
> From: Roman Black [SMTP:spam_OUTfastvidTakeThisOuTspamEZY.NET.AU]
> Sent: Friday, August 02, 2002 2:13 PM
> To:   .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject:      Re: [EE]: LM3485 Hysteretic PFET Buck Controller.
>
> Hi Mike, it's not cheap at all, $0.50 x 1000.
> In the circuit you showed you could probably replace
> the LM3485 with a cheap 8 pin comparator chip, or
> even a transistor and zener. With that many external components and FET
> you have many options for a much cheaper circuit.
> (once you are buying 1000 of them!) :o)
> -Roman
>
I wonder if you could achieve the 93% efficiency and the load regulation of
that IC though!

Regards

Mike

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2002\08\03@135911 by Roman Black

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Mike Singer wrote:
>
>  Has anybody any suggestions of max regulated frequency?
> If I replace bottom resistor of feedback chain by digital
> potentiometer and reduce output capacitor, what "max
> frequency" very aprox could I send to digital potentiometer,
> aiming at microstepping motor applications?
> LM3485 can work at >1mhz.


A good rule for PWM is 10:1. So if your PWM freq is
1MHz, (as above) you should not try to accurately
simulate changing waveforms over 100kHz. Even 10 PWM
cycles will cause significant ripple as you can't use
large filter caps. 50:1 is better.
-Roman

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2002\08\04@025139 by Mike Singer

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Thanks Roman and Matt,

I'll experiment with the chip when it'll be available to me. It seems I could reach 80-90% efficiency at 10khz (1:100). For a start I'll put open drains with resistors in place of a digital potentiometer.

  Mike.

> Mike Singer wrote:
> >  Has anybody any suggestions of max regulated frequency?
> > If I replace bottom resistor of feedback chain by digital
> > potentiometer and reduce output capacitor, what "max
> > frequency" very aprox could I send to digital potentiometer,
> > aiming at microstepping motor applications?
> > LM3485 can work at >1mhz.

Roman Black wrote:
> A good rule for PWM is 10:1. So if your PWM freq is
> 1MHz, (as above) you should not try to accurately
> simulate changing waveforms over 100kHz. Even 10 PWM
> cycles will cause significant ripple as you can't use
> large filter caps. 50:1 is better.
> -Roman

Matt Pobursky wrote:
{Quote hidden}

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