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}>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.
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}>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.
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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