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5V @ 3.5A for automotive - best way?

I'm trying to make a little project work here... trying to use seven

5x7 dot-matrix displays to make a little "digital readout" that I can display text on based on feedback from various things in a car (like temperature, etc).

The 5x7 displays are of course multiplexed and have a 1/10th drive current of 100ma. I am using shift registers to control the state of each row, then sequentially updating the rows to produce the display. That's all fine.

Problem is, given 7 rows, 35 dots per row, at 100mA per dot, that's a potential max current of 3.5A! The duty cycle doesn't help me because I drive the rows one after another, so if all elements were on, there's a potential 3.5A constant current requirement, thats a lot.

Can anyone suggest solutions for getting such a drive current from a

12V automotive source? The problem is, I do not want the display to flicker in brightness when the vehicle is started, so I really want to use a regulated source, also if I just ran the segments off the 12V, the power dissipation in the resistors becomes huge and I am trying to keep this small. I have looked at the 1084 regulator which can do 5A with up to 30V input, but this is a linear regulator.

I am a bit of a noob when it comes to circuit design, but am I correct in believing the power dissipation of the regulator is the voltage drop from Vi to Vo times the current? If so, and if you assume 13.8V in, then that's around 30 watts which seems to be a ton. I have used switchers in the past but the problem is the cost and number of additional components (board space is tight).

Am I missing some obvious answer that would solve my problems here? Would you guys go a different route in this design other than a traditional linear or switching regulator, and if so, what would you use?

Thanks!

CJ

Reply to
ferrari.secret.santa
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snipped-for-privacy@gmail.com skrev:

Yes, but the page is down at the moment:

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If it doesn't come up soon, look for SEPIC SMPS

--
Hilsen Mikkel Lund
"Sund fornuft, har aldrig stoppet en tosse"
Jokeren i "Mænds ruin"
Reply to
Mikkel Lund

schreef in bericht news: snipped-for-privacy@k21g2000cwa.googlegroups.com...

To get 5V/3.5A from 12V automotive requires a switcher. Try Maxim and/or Lineair, they have the right chips and application notes. You can also experiment with the 2 transistor black regulator. Just google.

But before starting to build a power supply like this you better get some questions answered first. Do you really need that amount of current? Do the displays require it and do you need those displays? You're not making an extra tail- or brake light, do you? Are you sure the displays can handle that amount of power? 3.5A at 5V gives 17.5W. They may become pretty hot. I'd half the maximum current and try out that first. Maybe other displays give better results.

petrus bitbyter

Reply to
petrus bitbyter

Mikkel Lund wrote:

I'm trying to figure out exactly what on that page you are point at. You can get the Google cache here (sans images):

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*-Internet-Explorer+sucks (Props to the guy for telling it like it is and for not pandering to users of defective software.)

Reply to
JeffM

I thought I'd check Rat Shack, but geez - they don't even give an output spec on this little gem:

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But there should be something - what kind of voltage do you need at this

3.5A?

Good Luck! Rich

Reply to
Rich Grise

you can make a buck type switching regulator quite small these days, if you go SMD of course, one small 8pin controll chip and a small mosfet and with a high switch frequency a small inductor, thats probably the way to go, but dont forget the high voltages surges that can apear on car electrics.

however 245 LEDs at 15ma each is going to be a heck of a lot of light, I doubt if you seriously need 3.5 amps if its inside the car, or is it some sort of external message board ?

Colin =^.^=

Reply to
colin

Thanks for the info. It sounds like a switcher is the way to go. Is there a preferred source for these? Pricing them out on Digikey (just for reference) they are much more than linears. The least expensive part they have is a 5A/5V switcher at $3.75 or so in 100's. That's a BIG step up from something like a 7805. The part I used before (7856 IIRC) required a decently large inductor, some caps and diode, and even going all SMT it was of reasonable size, certainly very hard to shoehorn into the board size I have available.

On the wattage, the displays I would like to use are red and blue (not together). The Vf of the red is around 2.6 and around 4 on the blue, so of the ~17 watts going to the LED's, around 8 are being dissipated in 35 resistors for the red variant, and 3.5 through the resistors in the blue. I grant that even at 9 watts we're talking about a lot of light, but driving at 100mA (the max current for 1/10th duty) is the "worst case". I can go dimmer in software easily with PWM, but it will only ever be as bright as the hardware allows. Bear in mind that in a convertible in full sunlight in July, its a different animal than on a moonless night in an enclosed car :). Also, the display will rarely ever use all segments, but of course it's possible, so I want to design for that. I would imagine generally around 1/3 of the elements would ever be on simultaneously.

I was thinking of ways I could split up the segments so instead of a

3.5A rail, I could have four .85A segments or something, but those solutions seem inelegant and however you slice it, there would be 30W going *somewhere* which is going to get a bit toasty (then again, thats a max design level, that would rarely ever be reached and if so would be for a few seconds at most).
Reply to
ferrari.secret.santa

Try something like this,

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You can get the parts at Digikey. Very reasonable prices. Brian Ellis

Reply to
Brian Ellis

Not really - it's apples/oranges. A 7805 can handle 1 amp, so you need a pass element, and you need a good heat sink. You indicate size is a problem, so that rules out the

7805/pass element/sink combo in any event.

Ed

Reply to
ehsjr

Well, there are other linears that can handle, say, 50% of the current but are about 10% of the price, that's all I'm saying. It just seems that switchers are proportionally expensive compared to linears, but I guess thats because they are better, especially as the currents go up :) The main issue would likely be 30W of power being burned off - thats a lot. On the space, its a constraint on the board, but I have a reasonable (3"Wx1.25"Dx.75"H amount of room inside the electronics housing that could be utilized for a heat sink. Even given that, it seems that an appropriate heat sink to allow 3.5A from a 1084 TO220 package is going to be relatively large and that's at the extreme upper end of what TO220 heatsinks can do, no?

Reply to
ferrari.secret.santa

schreef in bericht news: snipped-for-privacy@k21g2000cwa.googlegroups.com...

Actually some types can handle 5A but you will need a huge heatsink. These types are not cheap either. Look for LT1083-5 or LT1084-5. Digikey does not sell them, Farnell does.

petrus bitbyter

Reply to
petrus bitbyter

You could cut that ~ 30 watts in half by feeding the

1084 through a 1.5 ohm power resistor. Still the same total heat, but ~half of it moved out of the 1084. But it still grates. You could start at ~60 degrees C in the summer in a closed passenger compartment, and the 30 watts would raise it above that.

Since the 3.5 amps is brief, you could Rube Goldberg it if you want - a linear reg feeding some supercaps with a slow charge.

O +5 from regulator | +------------->|---+ | | +---[R]---+--->|---+ | | [C] [Load] | | Gnd --------+--------+

It would be an interesting experiment, and your car and money. Then you could let us know how you made out. Or you could use a switcher.

Ed

Reply to
ehsjr

Point taken.

Sometimes there is no need to re-invent the wheel. I realized I was looking for a solution when a solution was already in front of me. The choices were exotic designs, or using my existing micro to build a PWM current regulator, or just buy a chip that has all the necessary functionality built in.

Switcher it will be. Also, the issue I had previously with a switcher was it was an older part, with a lower frequency, which (of course) meant a larger inductor. With a more modern part, these ancillary parts can be much smaller, so my fear of a bunch of physically large extra parts for the switcher goes away when you read the datasheets on the newer switchers.

Thanks gents

Reply to
ferrari.secret.santa

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