Re: Pete Lefferts LED current source


On Tue, 21 Oct 2008 20:15:14 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:

Tim Wescott wrote:
Jim Thompson wrote:
On Tue, 21 Oct 2008 17:33:14 GMT, Walt Jung <tester3@xxxxxxxxxxxx>
wrote:

Jim Thompson wrote:
On Tue, 21 Oct 2008 16:32:42 GMT, Walt Jung <tester3@xxxxxxxxxxxx>
wrote:

John Larkin wrote:
On Tue, 21 Oct 2008 08:25:18 -0700 (PDT), miso@xxxxxxxxx wrote:

On Oct 21, 7:51 am, Walt Jung <test...@xxxxxxxxxxxx> wrote:
I am trying to establish who and when first used an LED (RED or
GRN) as
a voltage reference to bias the base of a bipolar xstr as a current
source/sink. To my best knowledge, this was Pete Lefferts of
NSC, back
in 1975.

1. Peter A. Lefferts, “LED Used as Voltage Reference Provides
Self-Compensating Temp Coefficient,” Electronic Design, Feb. 15,
1975,
pp. 92.

Comments welcome.

Walt Jung
LEDs were used in some audio power amps for biasing if you want a
commercial application.

An interesting side note, when the high efficiency LEDs hit the
market, some chips had difficultly driving them due to the higher
"on-
voltage" (forward bias). That is, you drove them with some chips,
and
the light was dimmer due to the current source running out of
compliance. When I did the MAX7219, I put in more headroom to handle
these higher voltage devices.
I had the opposite problem. We use blue led's on module front panels
as VMEbus activity indicators. The early Cree SiC parts neeeded 50
mA,
so I drove them with a couple of 74F38's, 5 volts, 27 ohms. As the
blue led's kept getting better, my customers started complaining
about
being blinded by them. We're down to about 4 mA now.

Which brings up another issue... for later.

Gotta go.

John

There is some misunderstanding here. I wasn't asking about an LED
*driver*, but using an LED as a Vref. Like D1 in this ckt:

+Vs >------+--------+
| |
D1 --- .-.
\ /~~> | | R1
----- | | 1K
| '-'
| |
| |
| |<' Q1
+------|
| |\
| |
| |
| |
.-. \ /
R2 | | | Iout
10K | | \ /
'-' |
| |
Gnd ------+--------+
So you're saying the LED has a -2mV/°C TC, but a larger forward?

...Jim Thompson
Yes, that is exactly what Lefferts said in the reference cited. So
the xstr Vbe then gets subtracted from the LED's Vf of 1.6-1.8V,
leaving a low TC drop of ~1V across the emitter R.

Walt Jung

Now wonder how I do that in an IC ?:-)

...Jim Thompson

Why, clearly you demand that the customer use a process that'll let you
build an LED into your otherwise all-silicon device.

What could be easier?


You could do it in SiGe...but you'd have about a 400 mV voltage
reference. 'Course you could put 3 in series.

Cheers,

Phil Hobbs

I'd just do a BandGap ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

"The American Republic will endure until the day Congress
discovers that it can bribe the public with the public's money."

- Alexis de Tocqueville
.

Relevant Pages

  • Re: Pete Lefferts LED current source
    ... a voltage reference to bias the base of a bipolar xstr as a current ... source/sink. ... ?LED Used as Voltage Reference Provides ... when the high efficiency LEDs hit the ...
    (sci.electronics.design)
  • Re: Pete Lefferts LED current source
    ... source/sink. ... ?LED Used as Voltage Reference Provides ... when the high efficiency LEDs hit the ...
    (sci.electronics.design)
  • Re: Pete Lefferts LED current source
    ... a voltage reference to bias the base of a bipolar xstr as a current ... source/sink. ... “LED Used as Voltage Reference Provides ... when the high efficiency LEDs hit the ...
    (sci.electronics.design)
  • Re: Pete Lefferts LED current source
    ... source/sink. ... voltage" (forward bias). ... There was an audio amp using LEDs ...
    (sci.electronics.design)
  • Re: Pete Lefferts LED current source
    ... source/sink. ... To my best knowledge, this was Pete Lefferts of NSC, back ... “LED Used as Voltage Reference Provides ... when the high efficiency LEDs hit the ...
    (sci.electronics.design)