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Bobb Fwed
12-11-2009, 05:55 AM
I am looking for a switching/buck power supply for a battery operated propeller project I am working on.
I am mostly looking for suggestions of models, but also wondering if there are any special considerations I need to make because of it.
I can't recall ever seeing a switching regulator on a board with a propeller on this site (forums or parallax pre-fab -- not that I've been looking for them), do switching regulators have too much noise to operate the PLL or something? Or maybe does it cause something to function oddly?

Requirements:
I can't imagine using more than 400mA on the whole project at any one time. Input voltage may be as high as 16V while the device is charging, but shouldn't ever drop below 5.5V (the Li-Ion batteries I am using will cut off at about that point, if not at a higher voltage). High efficiency (>85%) would be great, but not necessary (I've been running this project on a linear regulator without serious problems, but I am looking for extended life).
What should I be looking for in oscillator frequency? I have seen quite a few in the 150-250kHz range, but have seen some as high as 2MHz. I am guessing the higher frequency would produce less noise on the 3.3V line, but may cause regulator to generate additional heat and loose efficiency?

A lot of these switching regulators suggest a 100H inductor; I am not too familiar with inductors, can I increase the henry's to reduce the noise without altering the function of the regulator?

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver (http://obex.parallax.com/objects/488/) - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database (http://obex.parallax.com/objects/493/) - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library (http://obex.parallax.com/objects/543/) - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm (http://obex.parallax.com/objects/546/) - Fast communication between two propellers (1.37MB/s @100MHz)!

Beau Schwabe
12-11-2009, 06:37 AM
Bobb Fwed,

As long as you adequately filter the output with a capacitor that can keep up with the ripple, you shouldn't see any problems. IOW - the Propeller wouldn't know any different.


At the higher frequencies make sure that the external mosfet can keep up with switching that fast. If everything is integrated including the mosfet, then I wouldn't pay much attention to the frequency in this case. Higher frequency will produce less apparent ripple, but there is a sweet spot. If the frequency is too high, then the efficiency starts to do down, because of HEAT being dissipated within the regulator.


For the inductor, there is usually a chart to go by within the datasheet. I would follow that, since in this type of application the inductor's kick-back is what is used to boost the voltage. In other applications an inductor is used as a low-pass filter. That however is not the case with a boost regulator's primary inductor.

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Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Leon
12-11-2009, 06:38 AM
Modern switching regulators like those made by LT are very quiet, and run very cool. They are typically used on high-performance systems, and don't cause any problems if they are properly designed according to the manufacturer's recommendations. Don't change the inductor value unless you know what you are doing!

Have a look at the LTC3412. It operates at 4 MHz and uses a small inductor, typically 1-2 uH.

Leon

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Amateur radio callsign: G1HSM

Graham Stabler
12-11-2009, 06:46 AM
I can also vouch for Linear Technologies stuff, I have used them and more than abused them. I used to boost the voltage from a pair of nicad cells to drive pager motors in model aircraft and that could only be done because of their excellent efficiency. The datasheets are generally excellent and you are best to follow their PCB layouts if you are running at higher frequencies as per the 3412 Leon suggests.

Low ESR caps and Inductors are also the name of the game, the data sheets have many recipes laid out.

Graham

Alsowolfman
12-11-2009, 06:50 AM
the propeller robot controller from parallax uses a dual switching power supply for 3.3 and 5 volts, and there is a schematic with all of the component values available for download. it states the voltage range of 6-15 volts, just a little outside your range. it uses an LT3501.

http://www.parallax.com/Store/Microcontrollers/PropellerDevelopmentBoards/tabid/514/CategoryID/73/List/0/Level/a/ProductID/584/Default.aspx?SortField=ProductName%2cProductName

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My head is pasming!

Bobb Fwed
12-11-2009, 07:08 AM
22 external parts!? Goodness! But the LT3501 supports up to 25V input (unlike the LTC3412). The efficiency and frequency looks like the right direction too. I like the idea of both the 3.3V and 5V on a single chip.

Is that many external parts normal? I am looking at some regulators (LM2576T-3.3) that have 4 external parts, but they are 70-80% efficient, and only 52kHz, they are also higher voltage (up to 60V input). I was hoping to find some parts that would have a small footprint on the board.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver (http://obex.parallax.com/objects/488/) - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database (http://obex.parallax.com/objects/493/) - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library (http://obex.parallax.com/objects/543/) - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm (http://obex.parallax.com/objects/546/) - Fast communication between two propellers (1.37MB/s @100MHz)!

Toby Seckshund
12-11-2009, 07:18 AM
Dr_Acula uses two on his Drac_blade, one for 5Volts and another for the 3.3Volts

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Style and grace : Nil point

Leon
12-11-2009, 07:36 AM
Fairchild makes some nice ones, as well. They tend to be simpler than the LT ones.

Leon

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Amateur radio callsign: G1HSM

Bobb Fwed
12-11-2009, 07:57 AM
Leon said...
Fairchild makes some nice ones, as well. They tend to be simpler than the LT ones.

Leon

Any specific suggestions?

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver (http://obex.parallax.com/objects/488/) - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database (http://obex.parallax.com/objects/493/) - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library (http://obex.parallax.com/objects/543/) - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm (http://obex.parallax.com/objects/546/) - Fast communication between two propellers (1.37MB/s @100MHz)!

Leon
12-11-2009, 08:20 AM
FAN8301? I've got some but haven't tried them. I started designing a PCB so that I could test one; I ought to finish it off, make a PCB, and try it. I want 1.0 V at 2A from a 5V input.

Leon

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Amateur radio callsign: G1HSM

dMajo
12-11-2009, 08:36 AM
Have a look at Maxim also. They have many partr above 90% efficiency

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Propeller Object Exchange (last Publications (http://obex.parallax.com/objects/?o=0&ot=dsc&n=20)/ Updates (http://obex.parallax.com/objects/?o=8&ot=dsc&n=50)); Vaati's custom search (http://www.google.com/cse/home?cx=014602381951657504386:0v5gwyvny3m&hl=en)

Dr_Acula
12-11-2009, 11:44 AM
There are lots of options. I can confirm the LM25xx series of switchers work fine with the propeller. See www.smarthome.viviti.com/propeller (http://www.smarthome.viviti.com/propeller) for a photo and near the bottom of the text is a link to the schematic. Given all regulators need capacitors on the inputs and outputs, the extra components you need for a LM25xx simple switcher are the switcher, the inductor and the diode. The diode is 10c. The inductors vary a lot and can be up to $2, and you need to get one that is rated for 1amp or more. But you save money by not needing a heatsink so the total cost ends up similar.

I bought mine from futurlec and they happened to have a LM2575 in 5V for a good price and a LM2574 in 3V for a good price (the latter is a DIP8). But any of the simple switchers will work and it would be a matter of where you can get them at the right price ($2 to $3 each).

The only word of caution is to keep the caps, inductor and diode all as close to each other as possible and to make those tracks or wires reasonably thick.

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www.smarthome.viviti.com/propeller (http://www.smarthome.viviti.com/propeller)

Ale
12-11-2009, 02:42 PM
If you use the LM25XX try to use the ones that work at ~150kHz. The 50 kHz parts need huge inductors! (and I bought many of them :-(). If saving space is what you want use either one of the higher frequencies LTs or Fairchild, Maxim or Texas. The LT parts are expensive compared to Texas parts.

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Visit some of my articles at Propeller Wiki:
MATH on the propeller propeller.wikispaces.com/MATH (http://propeller.wikispaces.com/MATH)
pPropQL: propeller.wikispaces.com/pPropQL (http://propeller.wikispaces.com/pPropQL)
pPropQL020: propeller.wikispaces.com/pPropQL020 (http://propeller.wikispaces.com/pPropQL020)
OMU for the pPropQL/020 propeller.wikispaces.com/OMU (http://propeller.wikispaces.com/OMU)

Bobb Fwed
12-15-2009, 08:58 AM
Here is what I think I have settled on: LM2674. 260kHz, 500mA, SOIC package, 86/90% efficiency at 3.3/5V.

Thanks for your help.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver (http://obex.parallax.com/objects/488/) - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database (http://obex.parallax.com/objects/493/) - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library (http://obex.parallax.com/objects/543/) - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm (http://obex.parallax.com/objects/546/) - Fast communication between two propellers (1.37MB/s @100MHz)!