Re: Thanks to all - well most...



Paul E. Schoen wrote:
"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message news:47F39C88.6030404@xxxxxxxxxxxxx


Paul E. Schoen wrote:

"legg" <legg@xxxxxxxxxxxxxxx> wrote in message news:dcc3v3198qtdint7ag1d8jap33pa2ii2pj@xxxxxxxxxx


On Mon, 31 Mar 2008 20:31:08 +0100, "Dave" <daveh@xxxxxxxxxxxx> wrote:


What kind of LED are you working with and would an LM10BL with one
sensing resistor work for you? The reference is 200mV and supply
current ~300uA.

Others might have ideas for low current, low voltage references that
might complement the tlv2760's 20uA supply current.

RL


I found an LT6650 that combines a 400 mV reference and an op-amp with a supply current of 5.6 uA typical, in a 5 lead SOT23 package. I made a SwitcherCad simulation, with one 2.7 kOhm resistor, that holds LED current to 148.524 uA (exactly), from 2.2 volts to 5 volts. Even better, it draws only 17.75 uA from the battery, so you get an efficiency of 770%. Yes, 39 uW in and 300 uW out. There must be an error in the model, but I think it will run on 170 uA. Of course the LT6650 costs $1.88, but *what a deal* for an "over unity" part!

This has been an interesting thread, whether or not it has helped the OP. The designs offered are useful in general, and I've learned a few things.

Paul

=====================================================================================

That's okay but you would want to buy yourself a bit more headroom with something like this:
View in a fixed-width font such as
Courier.

.
.
.
.
.
. LTC6650
.
. |\
. -----|+\ ~~
. | | >-+----+--|>|--.
. 400mV .-|-/ | | |
. | | |/ === [110k] |
. | | 1u| | |
. --- | | | |
. '------|----+-[22k]-+
. | |
. | [300]
. | |
. --- ---


Yes, that could be done if you really needed it to work below 2.2 volts supply. But comes at the expense of more parts, less efficiency, and not quite as good current regulation if the diode Vf varies due to temperature, production lot, aging, etc. The battery is probably just about shot at 2.2 volts, and a gradual dimming might be a "good thing".

I don't know that a simple LED indicator drive requires all that expense and precision. You're the one who went with a $2 versus a $0.45 part. I don't see that it's justified unless something else in the product requires a reference. I did jump the gun on the first circuit, absolutely terrible dI/dV, unusable actually. Taking the self-referential opamp circuit as a minimum acceptable performance, with dI/dV=-I/V, the reference circuit meets it with overheads, VOH, in excess of Vref/2. This gets you good-enough regulation down to 2V, and you have a reference you can tap into if necessary.
View in a fixed-width font such as Courier.

..
..
..
..
..
..
..
.. |\ V
.. .-----|+\ + - Vref-VOH
.. | | >---+---|>|----. k= --------
.. Vref .-|-/ | ~ | V
.. | | |/ [R1] || I
.. | | | |v
.. | | | | dI I Vref-VOH
.. | '--------+--[R2]----+ + -- = - - * --------
.. | + - | dV V VOH
.. | kV [Rcs] VOH
.. | | -
.. '-----------------------' > Vref
.. => VOH = ----
.. 2
.. LT6650
..
.. |\ V
.. .-----|+\ + -
.. | | >---+---|>|----.
.. 400mV .-|-/ | ~ |
.. | | |/ [120k] || I k=0.1
.. | | | |v
.. | | | |
.. | '--------+--[13k]---+ +
.. | + - |
.. | kV [1.2k] 200mV
.. | | -
.. '-----------------------'
..
..



I didn't notice the requirement for the 1 uF capacitor. Thanks for pointing that out. Now the simulation shows a little start-up oscillation, but the model is still flawed, since I am still drawing only 18 uA from the supply for 150 uA output. And it's not April 1 anymore.

I don't know that the 1u is necessary unless there is capacitive loading somewhere, the amp is unity gain stable, and they seem to go to great lengths to filter the power supply because the PSRR is *real* bad on that part...


Paul

================================================================================

Version 4
*** 1 880 680
WIRE 112 144 -80 144
WIRE 112 208 112 144
WIRE -80 224 -80 144
WIRE 256 256 224 256
WIRE 320 256 256 256
WIRE 384 320 384 256
WIRE 384 320 224 320
WIRE 384 336 384 320
WIRE 256 352 256 256
WIRE -80 416 -80 304
WIRE 112 416 112 368
WIRE 112 416 -80 416
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TEXT -112 504 Left 0 !.tran .05 startup



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