LED driving questions.

An led requires a least 5mA to make it lite. so """"The LED circuit should draw

Naa. A Led is clearly visible with just 1mA. If switched on at 1mA, but 10uA average, this would mean a 100:1 deadtime.

Further. The VCE of an NPN is as low as

50mV saturated. Better than a resistor in terms of power is the use of PWM, a coil and a cap, in case you have a spare pin on your controller.

Rene

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To me that means that the LED driver should consume as little power as possible, so the LED has to be off in this condition and leakage currents should not exceed 20uA.

You should not be using a bipolar transistor to drive the LED.

--DF

Got you.

but I saw a lot of design use bipolar transistor...

do you mean use MOSFET to drive is better?

Best regards, Boki.

currents

Well, "should not" is a bit strong, but you are already in a "battery bad" condition when you have to activate the LED so you have to limit the power consumption as much as possible. If you can avoid a base current by using a FET, that's good. BTW there exist low current LEDs and personally I almost never need more than 2mA through a LED to get good visibility.

--DF

I interpret it to mean that

Fine.

You mean "forward" voltage of the LED.

For a 10mA LED current and a 20uA maximum driving circuit current, you will have to use a MOSFET with a maximum threshold Vgs less than your worst case (minimum) available gate drive voltage, or a very high beta transistor with gain > 10/0.02=500. A small MOSFET will just keep worst case leakage under 20uA. The simplest circuit is the LED in series with a current limiting resistor and MOSFET. At 2.8V you then have

2.8V-1.9V=10mA*(R+RDS,ON)=10mA*R most of the time, making R=90 ohms or so.

Hi, Boki. There are dozens of battery monitor ICs that will do the job. Why not just go with a one IC solution?

Many have MOSFET outputs that can sink 10mA with an Rds(on) of less than 20 ohms(if that's what the customer expects -- I'd go with a couple of mA strobed at 1Hz if all you want is a visible LED indication, and you want to conserve a low battery rather than drive it into the ground).

If you need help in specifying the IC, possibly you could describe what kind of battery you're trying to monitor? Also you might want to tell a little more about your project. And since we're at it, how about a wish list of any other features you could use? This wheel has been reinvented so many times, manufacturers have added all kinds of bells and whistles to differentiate their products. You can easily take advantage of that, or just go with a bare-bones minimum if cost is your only consideration.

Good luck Chris

If by this cryptic language you mean the LED is a LO battery indicator, then the requirements are more challenging than first impression. The

20uA refers to the battery current draw by the *all* of the LO battery indicator circuit in the inactive state. These circuits will have some kind of threshold comparison for the LO battery condition and the challenge here is that most comparator circuits exhibit a large increase of bias current draw from the battery as the battery voltage slowly drifts close to that threshold. There are several varieties of specialized LO battery indicator ICs that overcome this drawback. Go to and check them out.

Why not just ask your customer what was meant?

It is hard for me to imagine that you are supposed to keep the quiescent current down below 20uA, if the LED is going to draw at least 10mA when the battery is good. For one thing, 10mA is a lot to waste on an LED if a circuit is running off of some batteries (and I'm imagining a Lithium button, right now, which pushes the total circuit draw to a few mA at most, anyway.) For another, 20uA is such a small price to pay if you are already paying the price of 10mA for the LED that I can't easily see why that would be such a concern as to get spec'd like this.

If this is a circuit which must somehow indicate battery good by lighting the LED, but can only draw 20uA to do that, you will have to accept pulsing the LED. A 10mA guaranteed pulse current with an average draw of 20uA implies a 500:1 period, in ideal conditions. And if this is the desire, then that still leaves out what should be done when the battery is determined to not be good. Would the circuit simply be OFF, in this case?

If this is really a circuit driven off of a lithium button and that is the reason that the battery good LED circuit must draw |--/\\/\\---------+-------------, | | | --- | | --- 47uF ,---------, ,--------, | | | | | | | voltage |->-| SCR | gnd ,--->| trigger | | switch |->--, | | | | | | | '---------' '--------' | 4.7nF | | | --- | gnd gnd --- | | '--------------------------------' positive feedback

You can get this to run at under the 20uA and off of 2.8V, generating pulses, and use normal, cheap BJTs throughout (such as 2N3906 and

2N3904) -- including the "SCR" which can be fabricated from the same. In fact, I set down a possible circuit on the above model that in LTSpice draws about 20uA, pulsing the LED at 2Hz or so with RC decay shaped pulses having half-power widths of 1ms or so, with peak pulses of 12mA. It uses (2) 2N3906 and (1) 2N3904 and a couple of 1N4148s.

I'm baffled why you can't ask the customer for more details, though. That seems to be the place to go for clarification.

Jon

Sorry, that should be large C and modest R.

Jon

As has been mentioned, 20 uA will not light an LED. This will work if the LED off means "battery good" and the LED on means "battery low".

You must have missed the part where he says Vf is 1.9V and Vs is 2.8V. He also seemed to indicate that he needs 10 mA ILED, so R = (2.8 - 1.9) / .01. And, Vsat (of the driver transistor) can go as high as .3V at 15 mA for the PN2222, so at 10 mA it will be less and that's the max so it could be way less. So it should be R = ((Vs - Vf) - Vcesat)/If, or 60 ohms if your Vcesat is .3, or 80 if it's .1. So, use a 75 ohm resistor.

Cheers! Rich

Google for a LED switching supply.Those can convert up from the powersupply, and sense the resulting current at ground connection. If a Led fails "shorted",the others just keep on working. They work by switching a coil current into the stack of LEDS,and adjust switching freq. to get the wanted current. Note that it works without series resistor, so you use your energy very efficiently.

--
This is perfect for your application:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1463,P1172,D2764

Agree.

Best regards, Boki.

Typical indicator LEDs won't light in any way usefully if at all at 20uA, but the technology has been improving.

I've got some white 5mm "26,000mcd" 20 degree viewing angle clear LEDs from here:

There is some noticeable part to part variation at extremely low currents, but for the most part they seem to light quite visibly in subdued lighting conditions all the way down to less than 1uA. They glow very nicely at

20uA, and in fact I use them as indicators at that current level, even for quite bright office fluorescent standard lighting conditions. Higher current than a few tens of microamps isn't really desirable for indicator use since they quickly start to become dazzling to look at straight on.

There is one drawback however... Typical transistors/MOSFETs and whatnot can often have fully off state leakage currents exceeding 1uA. This means you can't always guarantee the LEDs to be fully extinguished when switched with typical transistors/MOSFETs, unless you place a parallel resistance next to the LED to soak up the leakage without producing visible light.

Hmm... Redoing the math, I take part of that back. I use them at 200uA as indicators in office standard lighting conditions. At 20uA they still glow very much visibly, but only if you look at them within the 20 degree viewing angle. As indicators I prefer to easily be able to establish if they are lit or not when viewed on or off angle.

Or drive with a CMOS output. Not like you'll be needing a big transistor for 20uA!

Best regards, Spehro Pefhany

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I know of LEDs that light damn well and bright at half a milliamp! Please mention upper limits and lower limits of voltage available to the LED or upper end of the LED's dropping resistor! Also color requirements

- deep pure red can require a good couple milliamps but with lower voltage drop in the LED! Worst is where the color is required to be yellow or yellowish.

- Don Klipstein ( snipped-for-privacy@misty.com)