ECN0044: Current rating of backlight LED inductor (L1704)

classic Classic list List threaded Threaded
4 messages Options
Reply | Threaded
Open this post in threaded view
|

ECN0044: Current rating of backlight LED inductor (L1704)

Werner Almesberger
We have a new ECN: the inductor (L1704) of the boost converter feeding
the backlight LED has an odd specification that makes it hard to source.

We can either lower the current rating (see the ECN for why this should
be okay) or increase the footprint (which would also lower the
resistance and save a bit of power.)

I've tentatively changed the current rating of L1704 to 600 mA to let
the BOM system pick up a suitable part.

Further details in
http://svn.openmoko.org/trunk/gta02-core/docs/ecn/ecn0044.txt

Comments ?


By the way, the BOM system now supports not component parameters
that are identical to some specified value but also parameters
that are below or above some value. E.g., one can now specify the
voltage rating of a capacitor as 20 V and the BOM system will
automatically pick a 25 V part, if available.

The rules for this are defined in gta02-core.sub E.g., in the
above example, C1764 has a parameter "V" with a value of 20V.
This gets translated by the rule  FN=*V -> V=>=$FN  to V=>=20V,
which in turn matches the V=25V of
TAIYO-YUDEN TMK316BJ106KL T=C   M=X5R   C=10uF  TOL=10% V=25V   FP=1206

This is much cleaner than relying on footprint sizes to force
selection of parts with the right parameters.

- Werner

_______________________________________________
gta02-core mailing list
[hidden email]
https://lists.openmoko.org/mailman/listinfo/gta02-core
Reply | Threaded
Open this post in threaded view
|

Re: ECN0044: Current rating of backlight LED inductor (L1704)

AlvieBoy
Werner Almesberger wrote:

> We have a new ECN: the inductor (L1704) of the boost converter feeding
> the backlight LED has an odd specification that makes it hard to source.
>
> We can either lower the current rating (see the ECN for why this should
> be okay) or increase the footprint (which would also lower the
> resistance and save a bit of power.)
>
> I've tentatively changed the current rating of L1704 to 600 mA to let
> the BOM system pick up a suitable part.
>
> Further details in
> http://svn.openmoko.org/trunk/gta02-core/docs/ecn/ecn0044.txt

Not sure about this one. Point is you'll have peak currents in inductor which are probably diverted to ground by capacitance in circuit components, and not
necessarily trigger the over-current protection. Those peaks might not hurt the inductor at all, even if its current is higher than its rating, due to their
transient characteristic.

But, I'd go for 1210 package. It's better be on the safe side, and we ought to have enough room for it. OR, we can (can we?) lay out both, use the smaller one,
and then if we run into problems, do a little of SMD soldering magic.

Álvaro


_______________________________________________
gta02-core mailing list
[hidden email]
https://lists.openmoko.org/mailman/listinfo/gta02-core
Reply | Threaded
Open this post in threaded view
|

Re: ECN0044: Current rating of backlight LED inductor (L1704)

Werner Almesberger
?lvaro Lopes wrote:
> But, I'd go for 1210 package. It's better be on the safe side, and we
> ought to have enough room for it. OR, we can (can we?) lay out both,
> use the smaller one,
> and then if we run into problems, do a little of SMD soldering magic.

The larger one is 1210, the smaller 0805. The gap between the pads
of 1210 is 2.0 mm. The long side of 0805 is 2.0 mm. So yes,
hand-soldering a 0805 onto a 1210 pad should work fine.

I've changed the footprint to 1210 and also added the 1210 footprint
to stdpass.fpd

Oh, and I did some rough calculation on the power saving by using an
inductor with lower resistance, but it's quite low, less than 1%.

- Werner

_______________________________________________
gta02-core mailing list
[hidden email]
https://lists.openmoko.org/mailman/listinfo/gta02-core
Reply | Threaded
Open this post in threaded view
|

Re: ECN0044: Current rating of backlight LED inductor (L1704)

Horst

> Oh, and I did some rough calculation on the power saving by using an
> inductor with lower resistance, but it's quite low, less than 1%.

and as to my knowledge there are magnetic losses, which are higher in
larger inductors, are to be considered as well. Not sure if it makes
any measurable difference anyway.

Horst


>
> - Werner
>
> _______________________________________________
> gta02-core mailing list
> [hidden email]
> https://lists.openmoko.org/mailman/listinfo/gta02-core

_______________________________________________
gta02-core mailing list
[hidden email]
https://lists.openmoko.org/mailman/listinfo/gta02-core