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'[EE:] Lowest power 12V to 3.3V SMPS?'
2007\06\01@130951 by enkitec

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       Hi,

       I need to power a uP circuit with 3.3V @ 6mA from a 4700uF capacitor
charged to 12V. The circuit has to run until the capacitor discharges.
       Have tried the Roman Black's SMPS circuits, but could'n go over 50%
efficiency at this 6mA current output. Even the Zener diode current is
excessive...

       Any ideas? Low cost and part number is a constrain.

       Thanks.
       Mark Jordan

2007\06\01@134718 by Mike Harrison

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face
On Fri, 01 Jun 2007 14:09:35 -0300, you wrote:

>
>        Hi,
>
>        I need to power a uP circuit with 3.3V @ 6mA from a 4700uF capacitor
>charged to 12V. The circuit has to run until the capacitor discharges.
>        Have tried the Roman Black's SMPS circuits, but could'n go over 50%
>efficiency at this 6mA current output. Even the Zener diode current is
>excessive...
>
>        Any ideas? Low cost and part number is a constrain.
>
>        Thanks.
>        Mark Jordan

For maximum efficiency at low voltages you should look at SMPS chips with synchronous rectification.
These can be up to 95% efficient.
Unfortunately some of the niceer ones won't go up to 10v input ( e.g. TI TPS62050 series), however
at this low output current you may have a reasonable selection of parts to choose from.



2007\06\03@091813 by Russell McMahon

face
flavicon
face
> I need to power a uP circuit with 3.3V @ 6mA from a 4700uF capacitor
> charged to 12V. The circuit has to run until the capacitor
> discharges.
> Have tried the Roman Black's SMPS circuits, but could'n go over 50%
> efficiency at this 6mA current output. Even the Zener diode current
> is
> excessive...
>
> Any ideas? Low cost and part number is a constrain.

Depending on how important those two things are relatively 'my' GSR
circuit may be useful.
I presently have a version giving over 90% efficient driving a Luxeon
1 Watt LED from 10 volts.
Getting this efficiency takes 'a little care' (tm)  :-)

Parts count is not as low as using any number of off the shelf
controller ICs.

At 3.3V out a Schottky flyback diode alone at 0.2V will account for
about 0.2/(3.3+0.2) =~6% efficiency loss.
A FET could be used as a synchronous flyback diode in the GSR if
desired at the cost of added driver complexity.

Can you give us more details about how low the cost needs to be, what
"low part number means" and approximate production volumes.

I may be interested in developing you a GSR (or some other solution)
to a predefined spec as a paid job (payment only on meeting
specification) if other alternatives didn't meet your need. This would
only make economic sense if volumes were large. Advice on doing it
yourself with a GSR is (up to a point :-) ) free.

The main advantage of the GSR is liable to be low component cost as it
uses 3 or 4 low cost transistors and no controller IC. The
disadvantage is higher parts count than an integrated IC controller.
In an SMD implementation the board area may not be significantly
larger than for an IC solution. .

Another highly successful alternative is the use of an eg 74C14 hex
Schmidt inverter as a controller. I have a design which draws under uA
providing voltage controlled no load output. This is liable to be
physically larger than a "proper" IC smps based solution but with far
lower component cost.

BOTE calculation indicates that the 4700 uF / 12V max storage
capacitor is liable to provide about 6 seconds of storage with a
linear regulator and approaching 15 with a very good smps. Tripling
the storage capacity would give you the same result with a compact
linear solution and MAY be a superior solution depending on other
factors that you haven't discussed. (Cap volume, cap already exists,
...)



       Russell


Linear time:

8 volts drop, 6 mA load.
T = CV/I = 0.0047*8/.006
~=6

SMPS

Energy available = 0.5 x C x (Vmax^2-Vmin^2)
Vmax = 12V
Vmin = 5V say
0.5 x 0.0047 x (12^2-5^2) ~= 280 mJ
Power out = 3.3V x 6mA = 19.8 mW
Time = 280/19.9 ~= 14.






2007\06\03@110351 by Alan B. Pearce

face picon face
>For maximum efficiency at low voltages you should look at SMPS chips
>with synchronous rectification. These can be up to 95% efficient.

Agreed.

>Unfortunately some of the niceer ones won't go up to 10v input
>( e.g. TI TPS62050 series), however at this low output current
>you may have a reasonable selection of parts to choose from.

That would be my comment as well. As another source look at Linear
Technology, they also have some nice devices, and another couple of features
is that at the end of their datasheets they list some other devices that
could be suitable (leads you on a data sheet chase a bit like some of
Russells OT posts), and they have excellent sampling policy, just like
Microchips, 3 off of each of 3 devices no questions asked.

2007\06\03@122210 by Vasile Surducan

face picon face
On 6/1/07, spam_OUTenkitecTakeThisOuTspamgmail.com <.....enkitecKILLspamspam@spam@gmail.com> wrote:
>
>        Hi,
>
>        I need to power a uP circuit with 3.3V @ 6mA from a 4700uF capacitor
> charged to 12V. The circuit has to run until the capacitor discharges.
>        Have tried the Roman Black's SMPS circuits, but could'n go over 50%
> efficiency at this 6mA current output. Even the Zener diode current is
> excessive...
>
>        Any ideas? Low cost and part number is a constrain.

1 resistor and one zenner (maybe a TL431) if you can loose around 60mW.

2007\06\04@020905 by Russell McMahon

face
flavicon
face
>> I need to power a uP circuit with 3.3V @ 6mA from a 4700uF
>> capacitor
>> charged to 12V. The circuit has to run until the capacitor
>> discharges.
>>        Have tried the Roman Black's SMPS circuits, but could'n go
>> over 50%
>> efficiency at this 6mA current output. Even the Zener diode current
>> is
>> excessive...

>>        Any ideas? Low cost and part number is a constrain.

> 1 resistor and one zenner (maybe a TL431) if you can loose around
> 60mW.

Any linear regulator solution can achieve, at best, about 6.7 seconds
of operation with the stated parameters.

A smps solution which uses 90% of the energy in the capacitor when
discharged from 12V to 3V3 will give about 14 seconds.

That's only 7.3 seconds longer, or, a whole 108% more than the best
linear solution, depending on whether the cup is half full or half
empty*.



       Russell

* Hey - I ordered fries.

2007\06\04@064216 by Alan B. Pearce

face picon face
Recently received a new product announcement from Analog Devices, describing
the ADP1864 constant frequency current mode converter with 14V input rating.
Described as having 40% cost saving over competing ICs.

Just had a quick look at the web page for it, and it really looks like it is
designed for CPU power blocks, Vin 3.15 to 14V, output illustrated as 2.5V
2A. Uses a PMOS transistor and high side current sense. Vout 0.8V to Vin
possible. Package TSOT-6 (yuk).

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