Discrete Schottky?

How fast? How much current?

John

(1) How "quick" does ANY circuit need to be when flashing an LED?

(2) A 2N2369A is gold-doped, thus does not need to be Schottky-clamped to prevent charge storage.

(3) The 1N5711 data sheet shows only 2pF of capacitance, so it should be OK.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.      Sometimes I even put it in the food.

Maybe it is signaling to something not a person?

Gold doped BJTs still exhibit charge storage. That doping reduces carrier lifetime, so such transistors can switch off in a few 10's of nS, but preventing saturation will still speed up the turn-off.

If speed mattered, I would use a small MOSFET. They are inexpensive and good for interface to logic.

....

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--Larry Brasfield
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Above views may belong only to me.

Have a look the Hewlett-Packard Application Note 1066: "Fiber-Optic Solutions for 125 MBd Data Communication Applications at Copper Wire Prices". They suggest direct drive by three 74HCTQ00 gates in parallel. They easily achive rise/fall times below 10ns; the problem then is the LED, not the driver.

Martin.

In discrete land, the voltage swing is often too large for just a schottky. In those cases a "Baker clamp"[1] is used. I have used the schottky clamp method at 10V and had it work.

[1] Baker clamp is like this

Simple version:

D1 -------+------->!--------+---- ! ! ! D2 !/ ------->!------! ! e !

It takes away base drive when the collector gets down to one diode drop. D2 is often the E-B junction of another transistor. This circuit works ok but the speed and capacitance of D1 is an issue during turn off.

All that said, you want to use a MOSFET to drive the IR LED. They switch quite fast with no extra work. The Supertex TN0604 driven by HC logic like this:

+12V +-------+ ! ! /R1 /R2 ! V ! --- ! ! ! !!----+------- -----!! !!-- ! GND

R1 sets the diodes current.

R2 speeds up the discharging of capacitances during turn off. It needs to be about 100-1K depending on how fast you need.

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kensmith@rahul.net   forging knowledge

The 2N2369 at 10mA load turns off in 30ns. With the 1N5711's CJO=2pF I doubt that you will get much improvement... might even be worse.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

What speed did you have in mind ? Perhaps with a FET ?

Rene

Wats> IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode

Hi Larry,

That depends. If the control signal originates from 3.3V logic or even lower levels a FET becomes rather sluggish without some major level shifting or toroid transformers. I have been in that pickle more than once and reverted to bipolar. Besides the good old 2N2369 there are some other fine RF transistors such as the BFS17A.

Regards, Joerg

I usually put a diode parallel with the base resistor to discharge the base capacitance more quickly. But like the others already asked, what's the required speed? And why aren't you connecting the LED directly to the open collector output?

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 Thanks,
    - Win

One can actually do far better than that with a 2n2369.

I assume you get the 30ns number from the datasheet's MAX specs, which are 13ns storage time and 18ns turn-off switching time at Ic=10mA and Ib_on = 3mA (wow, way to high) and Ib_off = -1.5mA (wow, way too low). Even under these less-than ideal conditions a real transistor would be faster than the MAX specs. Moreover, we'd be making a discrete circuit with our 2n2369, so we would have capacitors available, which we'd use to increase the base drive during transitions. This allows us to reduce steady ON base drive, reducing OFF storage time, and to increase turn-off base drive, speeding up switch-off time. In this fashion I have been able to push 2n2369 style parts down to the 5ns territory.

2pF is the MAX value, for most of the swing the capacitance is closer to 0.8pF (and smaller Schottky diodes are available for super-fast switching). But even a 2pF 1n5711 would not be much of a problem, 10mA applied to 2pF will slew 5 volts in just 1ns. A Schottky diode is often worth its small extra capacitance.

--
 Thanks,
    - Win

Right, but provided by the speed-up cap or whatever, for a few ns.

--
 Thanks,
    - Win

the

drop.

works ok

switch

logic

needs to

Thank you to all for all the advice. In the schem directly above, if the Vdd is the same as V+, which is usually the case with only the single supply, the conditions couldn't be met. So using the circuit in the first schem above would be the wiser choice.

Here's how a guy in Prague, Czech Republic does it. He uses massively paralleled 74HC04s to drive the HP spider LED.

(scroll down to near bottom for the schem)And for the main project page:

But that's a requirement of 10mA of DRIVE current.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I thought that the 2N2369A was fast because it was bribed with gold...

There are plenty of drivers that avoid saturation and charge recovery delays. View in a fixed-width font such as Courier.

Second the 2n2369a idea, which can be darn quick when driven by a modern CMOS gate.

If you're set on push-pull drive as in the cool 'HC04 driver you cited above, other CMOS families, such as 74AC, would be much faster & stronger with fewer driver gates needed.

Need any free parts? -- they're yours for the asking.

Cheers, James Arthur

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Yup.

John