Induction Selection for DC-DC Step Up...

J

Jaye Gallagher

Guest
Hi there,

I'm designing a power supply to provide a regulated 3.3V at around 150mA
from a single AA cell. I'm using the MAX1706, which contains both a DC-DC
converter, and a low drop-out regulator, however I'm troubled by selecting
the required inductor for the circuit.

The suggested part is the Sumida CDR74B,and I'm trying to work out whether I
can substitute the Meggit Magnetics 3631B which I can actually obtain here
in Sydney. The part must be 22uH and handle up to 950mA.

Data sheets for the above are available at:

Sumida CDR74B: http://www.sumida.com/products/pdf/CDR74B.pdf
Meggit 3631B: http://www.farnell.com/datasheets/31236.pdf

All the specs for the Meggit seem fine, except the frequency stuff --
there's no indication of the SRF for the Meggit part, which is 18MHz for the
Sumida. Also, the inductance measurements are taken at different frequencies
for each part: 1 kHz for the Meggit and 2.52MHz for the Sumida. Does any of
this stuff make a difference? If so, how?

Really, I'm all at sea with this resonant frequency stuff, but I do know
that the circuit switches at around 300kHz.

Any clues as to whether I can substitute the two? And why, or why not?

Thanks,
Jaye.
 
"Jaye Gallagher" <msjaye@removethis.msjaye.com> wrote in message
news:419c2006$0$31710$61c65585@un-2park-reader-01.sydney.pipenetworks.com.au...
Hi there,

I'm designing a power supply to provide a regulated 3.3V at around 150mA
from a single AA cell. I'm using the MAX1706, which contains both a DC-DC
converter, and a low drop-out regulator, however I'm troubled by selecting
the required inductor for the circuit.

The suggested part is the Sumida CDR74B,and I'm trying to work out whether
I
can substitute the Meggit Magnetics 3631B which I can actually obtain here
in Sydney. The part must be 22uH and handle up to 950mA.

Data sheets for the above are available at:

Sumida CDR74B: http://www.sumida.com/products/pdf/CDR74B.pdf
Meggit 3631B: http://www.farnell.com/datasheets/31236.pdf

All the specs for the Meggit seem fine, except the frequency stuff --
there's no indication of the SRF for the Meggit part, which is 18MHz for
the
Sumida. Also, the inductance measurements are taken at different
frequencies
for each part: 1 kHz for the Meggit and 2.52MHz for the Sumida. Does any
of
this stuff make a difference? If so, how?

Really, I'm all at sea with this resonant frequency stuff, but I do know
that the circuit switches at around 300kHz.

Any clues as to whether I can substitute the two? And why, or why not?
Click to expand...
I'd be inclined to just buy a couple of the parts and try them.

Leon
 
"Jaye Gallagher" <msjaye@removethis.msjaye.com> wrote in message
news:419c2006$0$31710$61c65585@un-2park-reader-01.sydney.pipenetworks.com.au...
Hi there,

I'm designing a power supply to provide a regulated 3.3V at around 150mA
from a single AA cell. I'm using the MAX1706, which contains both a DC-DC
converter, and a low drop-out regulator, however I'm troubled by selecting
the required inductor for the circuit.

The suggested part is the Sumida CDR74B,and I'm trying to work out whether
I
can substitute the Meggit Magnetics 3631B which I can actually obtain here
in Sydney. The part must be 22uH and handle up to 950mA.

Data sheets for the above are available at:

Sumida CDR74B: http://www.sumida.com/products/pdf/CDR74B.pdf
Meggit 3631B: http://www.farnell.com/datasheets/31236.pdf

All the specs for the Meggit seem fine, except the frequency stuff --
there's no indication of the SRF for the Meggit part, which is 18MHz for
the
Sumida. Also, the inductance measurements are taken at different
frequencies
for each part: 1 kHz for the Meggit and 2.52MHz for the Sumida. Does any
of
this stuff make a difference? If so, how?

Really, I'm all at sea with this resonant frequency stuff, but I do know
that the circuit switches at around 300kHz.

Any clues as to whether I can substitute the two? And why, or why not?

Thanks,
Jaye.
Click to expand...

Hi there.

Your suggested Meggit replacement part appears to have lower DC resistance
than the Sumida part. This is good. This should make for better efficiency
while making it possible to obtain your desired output current at even lower
Vin voltages. The core type appears correct (ferrite designed for power
circuits such as yours), so core AC losses should be minimal. The
inductance and current ratings appear to be similarly suitable. I would say
your Meggit part should make a suitable (and even superior) replacement to
the Sumida part. Inductor self resonant frequency is usually not a
significant consideration when selecting the inductor for these types of
boost converters.
 
John Crighton wrote:

Woops! Farnell again! Cripes they are expensive from
that place! The cost and tiny size counts me out.
$22 is too much money for me as a hobbyist to spend
on a tiny part. When my CRO probe slips between pins
of an IC, I don't mind loosing a few dollars but the size
of that device being so small, I would zap it for sure.
Have you looked into buying from overseas?
Click to expand...
Yep -- Digikey sells it for US$3.58... in lots of 100 only. Perhaps you
might attempt to order a few samples from the Maxim website. I've done so,
but they haven't yet arrived; apparently Maxim is pretty good about that
sort of thing.

The part may be tiny, but it certainly packs in a lot of functionality. :)

Jaye.
 
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