Re: Thanks to all - well most...


"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 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.

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
WIRE 256 416 112 416
WIRE 384 416 256 416
WIRE -80 464 -80 416
FLAG -80 464 0
SYMBOL SpecialFunctions\\LT6650 112 288 R0
SYMATTR InstName U1
SYMBOL voltage -80 208 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 132 Left 0
SYMATTR SpiceLine Rser=.1
SYMATTR InstName V1
SYMATTR Value 3
SYMBOL res 368 320 R0
SYMATTR InstName R1
SYMATTR Value 2.7k
SYMBOL LED 320 272 R270
WINDOW 0 72 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D1
SYMATTR Value QTLP690C
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 240 352 R0
SYMATTR InstName C1
SYMATTR Value 1µ
SYMATTR SpiceLine V=16 Irms=1.816 Rser=0.031 MTBF=0 Lser=0 mfg="KEMET"
pn="C0805C105K4RAC" type="X7R" ppPkg=1
TEXT -112 504 Left 0 !.tran .05 startup


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