Hey guys. Long time no see... again. I hate to come crawling back here after so long, as I've been trying to figure things out on my own these days. But I'm having trouble finding the voltage regulators I'm looking for. I need to build a 3-output power supply with the following characteristics:

Input: 6V-7.2V @ 4800mAh (Two li-ion batteries)
Output 1: 3.3V @ 1500mA
Output 2: 5V @ 1000mA
Output 3: 8V @ ~50mA (Needed for biasing an LCD display.)

The problem here, is that I need to make it as efficient as possible. So linear regulators are completely out of the question. They have to be switching regulators or DC-DC converters. Preferably switching regulators because they have a lower external part count. ops: What I've been trying to do is just find three regulators that meet or surpass my needs, and just use the "Typical Application Circuit" schematic found in the datasheet, since I have no clue how to just read the datasheet, throw numbers into a calculator and have it tell me the values of every single external part... but it’s just not that simple I guess. It's just really getting confusing to me. :cry: I’ve been looking through Digikey for weeks now, and none of the regulators that would suit my needs have a schematic that I could just follow without making any changes. Any chance someone can recommend a set of regulators or something? Maybe a schematic...? Thanks.

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You need a transformer with multiple taps. Normally, the highest current output is regulated, the others just follow. Transformer design is complex; I do it by using a lot more iron then necessary so I don't have to worry about saturation and packing factor. The core has to be low loss at the frequency of operation or it will get very hot.

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Hmm... do you really think all of that is necessary? It's all going to be powered off of a Lithium Ion battery pack, not a AC line power. And it needs to be compact, because it's going to be used for a portable device. That's why I just wanted to use a few voltage regulators to get the voltages I needed.

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You listed current needs in mA hours. For your output, you need to spec actual current draw (instantaneous).

to avoid a transformer, you could create 3 seperate switch mode supplies that operate off of 6-7.2V I like national's webbench as you can plug in voltage/amp requirements and get a complete circuit. Unless you want to become an expert on smps, that's probably the quickest way to go.

I would not use linear VRs due to their inefficiency.

 

Whoops. You're right. That was careless. ops: I didn't even mean to add the "h" there. :P It confuses me sometimes, is all. The whole mAh mA thing... >.> The ratings listed are the actual current draw (estimating slightly high, to be safe) of my project.

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Okay, I think I've got something figured out. First of all, I think I'm going to use this regulator for the 8 volt output. I plan to follow the schematic shown on the first page exacly. Does this look okay?

For the 3.3 and 5 volt outputs, I'm thinking about using a couple of these. One of the 3.3 volt versions, and one of the 5 volt versions. The only problem with that, is that I don't know if I can follow the "Typical Application" circuit "exactly" since it has a higher input voltage, and is shown using the 5 volt version. (I don't see a circuit for the 3.3 volt version.) So my question is, what would I have to change about that circuit (if anything) so that it can accept a voltage between ~6-8 volts instead? And would I use the same circuit with the 3.3 volt version? Thanks.

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When life gives you lemons... make a battery.

 

Whoa there. He is talking about dc input (batteries). No tranfsformers on dc please.
For the regulator, i was looking at some regulators that have adjustable outputs. These are changed by changing the resistors on the regulator. You could look into that. I have attached an app note regarding this.

Attached Files

appnote-adjustable_regulators.pdf (647.7 KB, 18 views)

 

The National Semi device LM2717 could almost fit the design parameters you listed. It's a dual DC/DC converter chip. However, it is designed to supply 1000mA @3.3V and 1500mA @ 5V. It is possible to swap the outputs so that the higher current one supplies 3.3V instead.

As for the third output, The output current is low enough that you can add a secondary winding to the 3.3V regulator (wound with approximately 1:1 turns ratio) so that it supplies 3V when rectified with a shottky diode. This will ride on top of the 5V rail to get 8V.

The LM2717 operates at a fairly high frequency range of 300-600Khz. You don't need much inductance. A small ferrite core with an air gap (to keep it from saturating) should be enough. I wouldn't worry too much with the transformer design. There are switcher design helper programs (SwitchCAD from Linear for example) to help with the design.

Attached Images

smps.gif (2.8 KB, 380 views)

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There is a problem with any forum (of which you are no doubt aware): not eveyone who replies knows what they are talking about! Motion has the right idea, but the schematic won't work. If there is no load on the 3.3 volt supply, the 8 volts becomes 5 volts. You DO need a transformer with multiple taps!

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I'm sorry... but how many transformers do you see inside and iPod? Or a cell phone? Or a portable CD player? They use regulators, and that's exactly what I intend to do. However, if you can provide me with a schematic using a transformer that I can find from Digikey, that's small and efficient, then I'll go with that in a heartbeat.

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When life gives you lemons... make a battery.

 

You probably find transformers in ALL of those (or at least inductors), however, you probably won't find suitable switchmode transformers at Digikey, they tend to be custom items.

But switchmode PSU's including transformers or inductors are essential to the battery life and small size of such devices.

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You're right. I forgot to specifically mention that the circuit needs a minimum load at 3.3V output to obtain 8V. I don't think it's a serious problem given the relatively low 50mA load requirement of the 8V rail and the high 1500mA requirement for the 3.3V rail.

The circuit I recommended is a practical one. I've used it on an actual commercial product. And I am not the first one to recommend it. There is similar design found in the Maxim-IC triple supply evaluation kit. See the ff. MAX782 datasheet.

http://pdfserv.maxim-ic.com/en/ds/MAX782-MAX782S.pdf

Maybe Electric Rain can just buy the kit to test the design himself. It supplies 3.3V @3A, 5.0V @3A and 15V bias output.

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Wait-a-minute... I'm confused now. ops: In your previous post, motion, you talked about the LM2717. But it seems as though you posted a schematic for a different chip... because it's only showing three pins. Is the chip in the schematic you posted nothing more than any 3.3V buck regulator (that can supply enough current)? Or is it the LM2717?

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Alright. I think I've got it. What would you guys think if I followed Figure 13 on this datasheet for the 3.3V and 5V, then followed the schematic on the first page of this datasheet for the 8V?

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When life gives you lemons... make a battery.

 

Yeah, I think it will work. You have to supply the 8V circuit from the 3.3V output. It ain't pretty but with just 50mA load, I think it's alright. At least you won't have to worry about sourcing transformers.

BTW, I think you mean figure 12 of the LTC1143 datasheet.

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