> > Mike Singer wrote:
> >    There are a lot of very chip DC/DC converters
> > for this voltages: for CPU on PC motherboard, for
> > example.
.
.
> It is unlikely that you could build an IC based switching
> regulator for a component cost much cheaper than for
> this one.
.

> > It is unlikely that you could build an IC based switching
> > regulator for a component cost much cheaper
>
>  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.
>
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>
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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:
>
> > > It is unlikely that you could build an IC based switching
> > > regulator for a component cost much cheaper
> >
> >  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.
> >

> 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

>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.

>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.

> 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.