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'[EE]: Darlington Speed-trap'
2001\05\04@173800 by Dan Michaels

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I was testing the ULN2803 mentioned in another thread, and
was amazed at how fast this chip is. I drove a 130 mA load
30 ohm resistor [5v - Vcesat(=1v)] at 2 mhz. Nice and clean.

For those not familiar, the 2803 chip is an 8-channel darlington
driver, rated at nominal 0.25usec turn-on/turn-off delays [ala
50-50 points] --> and it actually does this under load.

Anyone else have experience using these chips at high-frequencies
under load? I thought darlingtons were supposed to be slow.

thanks,
- dan michaels
http://www.oricomtech.com
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2001\05\04@201959 by rottosen

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Dan:
Yes, I have seen similar times. I once tried using them to pulse a 5MHz
(yes, MegaHertz) transducer with the '2803. Seemed to work. Never
pursued it doe to other factors.

As you may know, Sprauge (Allegro) spec's the turn-on time at 250ns typ
and 1us max. They "forgot" to give a test circuit. They don't spec a
rise or fall time at all.

One observation I have is that the smaller the load resistance, the
better the rise and fall times should be. This is because of lower gain
and therefor less Miller effect capacitance.

Another feature of these parts is that under low loading, they have low
saturation voltages. This means that they can be used as "protected"
buffers to "fragile" PIC input pins.


-- Rich


Dan Michaels wrote:
{Quote hidden}

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2001\05\05@050535 by Peter L. Peres

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Several Allegro chips probably drive half the bubble printer heads in the
civilized world right now. That's pulses down to 1usec. Several older
switcher designs used Darlingtons.

Darlingtons are not slow when designed not to be. In theory the Ton/Toff
of a Darlington is the sum of the TonToff of the component transistors. In
practice it is slower because of capacitance (esp. Toff). This is why most
Darlingtons have a built-in resistor network near the base of the second
transistor (including in the ULN2803).

The worst problem with Darlingtons is Vcesat. Try to run a ULN2803 at max
spec and you will exceed SOA. I have a small stepper driver (experimental)
where I had to glue a small fin on the ULN2803 to cool it while driving a
motor with Iin (total) 450ma at 12V. With three outputs on at max spec you
exceed SOA (power dissipation). As long as you keep this in mind, you will
be fine.

Peter

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2001\05\05@220949 by Russell McMahon

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> I was testing the ULN2803 mentioned in another thread, and
> was amazed at how fast this chip is. I drove a 130 mA load
> 30 ohm resistor [5v - Vcesat(=1v)] at 2 mhz. Nice and clean.
>
> For those not familiar, the 2803 chip is an 8-channel darlington
> driver, rated at nominal 0.25usec turn-on/turn-off delays [ala
> 50-50 points] --> and it actually does this under load.

I am using a ULN2004 (7 channel relative to ULN280x) with pulse widths down
to around 1 microsecond using the active pullup mentioned below.

Word of warning for those reading this and thinking they can use it as a
"fast" driver with a resistive load (my active pullup cures this).
The ULN2803 is of course an "open collector" driver. There is an active pull
down (darlington pair) but NO pullup whatsoever. The turn off time will be
no faster than the turnoff of the driver (which Dan is commenting on) BUT
the actual time for the output voltage to rise will then depend entirely on
the characteristics of the driven circuit. If you use a highish value load
resistor and drive a capacitive load you MAY get a time constant of far
longer than the driver turn off time. This is just basic electronics but is
an easy trap to fall into if you are used to using a standard push-pull
output in which there is a low and a high output driver thus providing a low
impedance on and off drive circuit.

A ULNxxxx driver (or any open collector driver) can be converted to an
active on and off driver but the addition of a transistor, a diode and one
resistor.
While this reduces the advantage of the single chop drive there are
circumstances where this is a very useful approach. In one design I use this
on one channel of a 7 channel driver to drive a power FET at high speed.

Parts:
   NPN transistor for hi side driver (eg BC337 TO92 package)
   Diode (say 1N4003)
   Resistor (1k to 10k range)

Transistor collector to positive supply
Transistor base to ULN2803 output
Transistor emitter to output.
Diode cathode to base and anode to emitter (ie diode conducts to pull output
LOW when ULN28-3 output goes LOW).
Resistor from transistor base to positive supply.

While this circuit features inside many ICs I was first introduced to it as
a cheap and simple FET driver using discrete transistors by AFAIR Lance
Allen.
I have never seen it used with a ULNxxxx input but it is an obvious
extension of the basic principle.

Turnoff time is now mainly controlled by the ULNxxxx to transistor drive
time constants and can be designed to be as fats as required essentially
regardless of load.

It's main shortcoming (especially here) is that the off/low output voltage
is 1 diode drop plus ULNxxxx saturation voltage above ground. This would be
a drawback in some circuits but is unimportant when driving MOSFETS. Apart
from power wasting and loss of available voltage this is not usually a
problem when driving eg motors, relays etc.




Russell McMahon

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