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Any innovative ideas for this LED driving problem?

I am sure many of you are familiar with the newer higher-power SMT
LED's that are out. They generally require larger copper pads on the
PCB to dissipate heat.

I am working on an application where I need a lot of brightness, and
very small size. I also need to have my design be dual color (for
example red and green). My solution was to put the LEDs in strings of
three, side by side but oppositely biased to one another. So that if V
+ is on the left and GND on the right, its red, if V+ is on the right
and GND on the left, its green. Since red and green would never be on
together, this allows me to use the same copper for power dissipation
on both strings.

I was originally using an H-bridge to swap the polarity. But the
design has turned into 6 strings of 3 for red and 6 stings of 3 for
green. Six h-bridges... I am trying to keep the board as small as
possible and I am running out of room doing this with discrete
components.

Of course they make motor drivers that are H-bridges built in, but
they are expensive and overkill for what I need. If I had my wish
come true, it would be for a small chip with 5 or so pins... power and
ground in, a control line, and to outputs. If control is low, output
1 is V+ and output 2 is ground, and if control is high, output 1 is
ground and output 2 is V+. Does such a beast exist?

If not, is there any other suggestions on how to tackle this? I could
do away with the h-bridge idea and just parallel the strings of LEDs,
mass the cathodes (since that's where heat dissipation is needed) and
seperate the anodes and control via transistors switched from the
microcontroller, but I am not sure if that is the best way.

As a non-professional designer, does anyone have alternate suggestions
or something I am missing or perhaps a more elegant solution?

Thank you!
 
J

John Larkin

On Thu, 29 Nov 2007 09:21:21 -0800 (PST),
I am sure many of you are familiar with the newer higher-power SMT
LED's that are out. They generally require larger copper pads on the
PCB to dissipate heat.

I am working on an application where I need a lot of brightness, and
very small size. I also need to have my design be dual color (for
example red and green). My solution was to put the LEDs in strings of
three, side by side but oppositely biased to one another. So that if V
+ is on the left and GND on the right, its red, if V+ is on the right
and GND on the left, its green. Since red and green would never be on
together, this allows me to use the same copper for power dissipation
on both strings.

I was originally using an H-bridge to swap the polarity. But the
design has turned into 6 strings of 3 for red and 6 stings of 3 for
green. Six h-bridges... I am trying to keep the board as small as
possible and I am running out of room doing this with discrete
components.

Of course they make motor drivers that are H-bridges built in, but
they are expensive and overkill for what I need. If I had my wish
come true, it would be for a small chip with 5 or so pins... power and
ground in, a control line, and to outputs. If control is low, output
1 is V+ and output 2 is ground, and if control is high, output 1 is
ground and output 2 is V+. Does such a beast exist?

If not, is there any other suggestions on how to tackle this? I could
do away with the h-bridge idea and just parallel the strings of LEDs,
mass the cathodes (since that's where heat dissipation is needed) and
seperate the anodes and control via transistors switched from the
microcontroller, but I am not sure if that is the best way.

As a non-professional designer, does anyone have alternate suggestions
or something I am missing or perhaps a more elegant solution?

Thank you!

How much voltage? How much current?

John
 
How much voltage? How much current?

John

John,

The LED's are in strings of 3, and have an individual Vf of ~2.65V and
a forward current of 150mA each. So each string would be around 7.95V
and 150mA. There are 6 strings so a total current draw of 900mA. I
do need each string of three to be able to be individually turned on
and off seperately from the other 5, which adds a bit of complexity.
I am using an Atmel Mega8 microcontroller so I have around 15 outputs
to work with after the ones I am using elsewhere are accounted for.

This project will run on 12V and I am trying to keep this all on a PCB
of around 4 square inches. Not a ton of room to work with, although I
don't mind moving the power solution to a sub-board if necessary (and
if that helps me keep this from being a 4-layer board).

Thanks for any insight you can provide!

-Rina
 
T

T

[email protected]>,
[email protected] says...
I am sure many of you are familiar with the newer higher-power SMT
LED's that are out. They generally require larger copper pads on the
PCB to dissipate heat.

I am working on an application where I need a lot of brightness, and
very small size. I also need to have my design be dual color (for
example red and green). My solution was to put the LEDs in strings of
three, side by side but oppositely biased to one another. So that if V
+ is on the left and GND on the right, its red, if V+ is on the right
and GND on the left, its green. Since red and green would never be on
together, this allows me to use the same copper for power dissipation
on both strings.

What you are describing sounds very much like this 8x8 matrix:

http://www.sparkfun.com/commerce/product_info.php?products_id=682

It's a common cathod setup, so with three wires you can trigger a cell
to go either red, green, or a yellow-orange color if both are set high.
 
J

James Arthur

I am sure many of you are familiar with the newer higher-power SMT
LED's that are out. They generally require larger copper pads on the
PCB to dissipate heat.

I am working on an application where I need a lot of brightness, and
very small size. I also need to have my design be dual color (for
example red and green). My solution was to put the LEDs in strings of
three, side by side but oppositely biased to one another. So that if V
+ is on the left and GND on the right, its red, if V+ is on the right
and GND on the left, its green. Since red and green would never be on
together, this allows me to use the same copper for power dissipation
on both strings.

I was originally using an H-bridge to swap the polarity. But the
design has turned into 6 strings of 3 for red and 6 stings of 3 for
green. Six h-bridges... I am trying to keep the board as small as
possible and I am running out of room doing this with discrete
components.

Of course they make motor drivers that are H-bridges built in, but
they are expensive and overkill for what I need. If I had my wish
come true, it would be for a small chip with 5 or so pins... power and
ground in, a control line, and to outputs. If control is low, output
1 is V+ and output 2 is ground, and if control is high, output 1 is
ground and output 2 is V+. Does such a beast exist?

If not, is there any other suggestions on how to tackle this? I could
do away with the h-bridge idea and just parallel the strings of LEDs,
mass the cathodes (since that's where heat dissipation is needed) and
seperate the anodes and control via transistors switched from the
microcontroller, but I am not sure if that is the best way.

As a non-professional designer, does anyone have alternate suggestions
or something I am missing or perhaps a more elegant solution?

Thank you!

Why not just connect like this...

(View in Courier or fixed font)

V+
-+-
|
Red LEDs | Green LEDs
+---------+-----+-----+---------+
| | | |
--- --- --- ---
\ / ~~> \ / ~~> \ / ~~> \ / ~~>
----- ----- ----- -----
| | | |
| | | |
--- --- --- ---
\ / ~~> \ / ~~> \ / ~~> \ / ~~>
----- ----- ----- -----
| | | |
| | | |
--- --- --- ---
\ / ~~> \ / ~~> \ / ~~> \ / ~~>
----- ----- ----- -----
| | | |
+----+----+ +----+----+
| |
| |
| |
| |

....and drive them with just 2 open collector transistors / MOSFETs +
current limiting?

Best,
James Arthur

They can still share they same heat sinks for the same reason as
before: they're not bot on at the same time.
 
F

Frank Buss

James said:
Why not just connect like this...

Connecting LEDs in series is no problem with a constant current source, if
it can provide high enough voltage, but don't connect LEDs in parallel,
because LEDs are very non-linear. E.g. if some LEDs on one string have a
bit lower resistance, they'll get a bit hotter and then the resistance
falls again, which is no problem with the constant current source, but the
other strings gets less current and will be darker.
 
J

James Arthur

Connecting LEDs in series is no problem with a constant current source, if
it can provide high enough voltage, but don't connect LEDs in parallel,

Yes, of course--I was trying to sketch the concept, simplified, which
is why I didn't show the drivers or the limiting.

I assumed his LEDs have electrically isolated thermal pads however,
which they might not.

Cheers,
James Arthur
 
J

James Arthur

John,

The LED's are in strings of 3, and have an individual Vf of ~2.65V and
a forward current of 150mA each. So each string would be around 7.95V
and 150mA. There are 6 strings so a total current draw of 900mA. I
do need each string of three to be able to be individually turned on
and off seperately from the other 5, which adds a bit of complexity.
I am using an Atmel Mega8 microcontroller so I have around 15 outputs
to work with after the ones I am using elsewhere are accounted for.

This project will run on 12V and I am trying to keep this all on a PCB
of around 4 square inches. Not a ton of room to work with, although I
don't mind moving the power solution to a sub-board if necessary (and
if that helps me keep this from being a 4-layer board).

Thanks for any insight you can provide!

-Rina

That's about 7 watts max. dissipation in the LEDs, plus any current
limiting losses, smd, on a small board. Yikes!

You could use fewer, bigger LEDs, and/or pipe the heat out with
thermal vias to a "real" heatsink on the backside of the board.

But we don't know your constraints, optical requirements, or other
degrees of freedom.

For driving your existing scheme there are H-bridge ICs like the
Vishay/Siliconix Si9986--that's an 8-pin part. Possibly some MOSFET
gate drivers could handle 150mA continuous.

Cheers,
James Arthur
 
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