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Etmax
Etmax
Hmm I get 3 diodes at a cost of 4-5 cents. Compare that to 15 cents for an LED. ...
Green LED replaces LDO regulator
Kien Mach, Maxim Integrated Products Inc.
9/8/2010 7:21 AM EDT
Such systems often provide a low-dropout (LDO) regulator for each supply voltage, and each LDO also requires two bypass capacitors, one connected to the supply pin and one to the output pin. For portable devices constrained by low cost and limited space, however, we can eliminate the 1.8V LDO and its bypass capacitors by simply substituting a surface-mount green LED (case size 0603) across the level translator’s DVCC and VCC pins (Figure 1).
Figure 1: For low-power loads, a single green LED (D1) replaces a 1.8V LDO regulator in this level-translator circuit.
(Click on image to enlarge)
The VCC pin in this configuration assumes a voltage of ≈1.9V, which allows the translator to provide a 1.8V logic-high level to the slave devices. To ensure a stable VCC, the slave devices should operate on low power and draw reasonably constant supply currents. ♦
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Bob Lacovara
9/8/2010 10:53 AM EDT
It's nifty. It isn't the most general solution to the problem, but as stated by the author, and given the constraints he imposes, it's a nice trick to put in your toolbox. One can hardly fault either the complexity or the cost.
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Avatar Engineer
9/8/2010 10:16 PM EDT
Why not? it's cheaper than 3 diodes in series!
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Etmax
9/25/2011 9:47 PM EDT
Hmm I get 3 diodes at a cost of 4-5 cents. Compare that to 15 cents for an LED. What are you paying for LED's??
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t.alex
9/9/2010 8:25 PM EDT
Ah nice trick! The only factor is we need to select good-grade led as it is always ON. Do we need to have a current-limiting resistor?
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BicycleBill
9/12/2010 10:55 PM EDT
t.alex: I'll pass your query to the author. Of course, the circuit is so simple you could try it with and without the resitor, and see for for yourself! What have you got to lose, except an inexpensive LED?
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Jack Bauer
9/15/2010 9:12 AM EDT
This assumes the VCC current is relatively fixed to generate a nearly constant forward voltage in the green LED. It also assumes the larger supply is exactly a diode drop greater than the logic supply needs to be. That almost never happens. Interesting trick, but not exactly a universal solution.
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loggie
9/15/2010 12:03 PM EDT
I would be concerned about the variation in forward voltage drop across the LED over temperature range (0 to 80 C for a typical consumer device). Typically, core logic operating at 1.8V requires a +/-10% regulated supply voltage.
Given that the 3.6V supply has its own tempco and adding this to the LED tempco, it is possible that the 1.8V supply could have a variation over temperature exceeding 10%.
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NATRON
9/15/2010 2:51 PM EDT
You might consider leaving the led in the dark...they tend to generate a voltage when exposed to strong light...
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Etmax
9/25/2011 9:49 PM EDT
Given that it light up in ise, that may be difficuly :-)
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GREAT-Terry
9/16/2010 2:23 PM EDT
This is a trick that students usually come up with. I may say this is a simple trick but not always be useful. It may only work well if the load is really light and the tempco. (in other words, the output voltage accuracy) is not an issue. To nowadays demanding digital stuff, it surely is not a good idea. However, for many low voltage consumer stuff, it is still a viable solution but need to be used with great care.
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numberone2
9/28/2010 2:03 AM EDT
I disagree that the LED replaces 3 components compared with using an LDO. In this case the LDO would share bypassing with the other IC so this solution really only saves a few cents and marginal board space, ie an 0402 LED vs an SOT323 LDO. I would personally take the peace of mind and go with the LDO.
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