Re: Driving a transistor array

From: Chris W (1qazse4_at_cox.net)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 16:12:38 -0600

Larry Brasfield wrote:

>"James Beck" <jim@reallykillersystems.com> wrote in message
> news:MPG.1c9246f892570c65989ae7@news.west.earthlink.net...
>
>
>>In article <ZUTVd.108$j8.1145@news.uswest.net>,
>>donotspam_larry_brasfield@hotmail.com says...
>>
>>
>>>"Chris W" <1qazse4@cox.net> wrote in message
>>> news:rvTVd.7469$2s.3903@lakeread06...
>>>
>>>
>>>>Larry Brasfield wrote:
>>>>
>>>>
>>>>
>>>>>"Chris W" <1qazse4@cox.net> wrote in message
>>>>>news:%iRVd.7251$2s.3412@lakeread06...
>>>>>
>>>>>
>>>>>
>>>>>>Will there be any problem driving one of these transistor arrays
>>>>>>http://focus.ti.com/lit/ds/symlink/sn75468.pdf
>>>>>>With these D-Type Flip Flops
>>>>>>http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/SN74HCT74.pdf
>>>>>>
>>>>>>
>>>>>>
>>>>>If you are demanding less than 500 mA sunk into each
>>>>>Darlington collector, plan to drive their bases with an
>>>>>HCT device, and don't expect saturation on the output
>>>>>(or what might be called saturation for a Darlington), you
>>>>>should be OK. Do you plan to use the logic output for
>>>>>anything else, at the same time?
>>>>>
>>>>>
>>>Upon closer inspection of the data***, I would revise
>>>than collector current down to 350 mA.
>>>
>>>
>>>
>>>>I plan on driving either 1 to 4 LEDs in parallel, or a 12v around 2 or so amp relay, on each Darlington collector. The flip
>>>>flops
>>>>will be driving nothing else, just the Darlington array. I need 16 Darlingtons so with 7 in a chip that will leave 5 unused
>>>>Darlingtons on the one of the 3 arrays or 1 on one chip and 2 on the others. I am still new to all this.
>>>>
>>>>
>>>You had better be careful about burning up your
>>>array(s) if you put more than about 150 mA into each
>>>of the 7 devices in a package. The 2 Amps (or so) you
>>>mention cannot be switched by just one of those devices.
>>>
>>>Maybe you should be looking a power MOSFET rather
>>>than a Darlington transistor array.
>>>
>>>
>>>
>>>>What exactly does it mean to have saturation on the output?
>>>>
>>>>
>>>It means something a bit too involved to explain in all
>>>its glory. For your purposes, the issue is: What will
>>>be the voltage drop between the collector-emitter
>>>terminals when the device is supposed to be switched
>>>on? I urge you to consult the data*** about that.
>>>
>>>
>>>
>>>
>>I think he means that the relay is reated for a 2A load NOT that it
>>requires 2A to operate.
>>
>>
>
>I imagine you are right. But I've seen and
>wired some pretty big relays.
>
>

To clarify, yes I did mean a relay that is rated for a 2A load.

>
>
>>Also, what in the data*** leads you to
>>believe that the HCT part can't drive the darlingtons to saturation?
>>
>>
>
>Well, saturation is kind of a funny thing for the
>Darlington transistor pair. The input transistor
>can be readily saturated but the output transistor
>is generally not saturated, having its base drive
>modified such that its base and collector voltages
>are about equal. So, using the term "saturation"
>loosely to mean "as on as can be", there is no
>reason the HCT device cannot cause that to
>occur in the Darlington. But for the more precise
>definition, requiring an excess of carriers in the
>base region and a forward biased collector-base
>junction, the output transistor certainly cannot be
>brought to that state.
>
>The reason I thought this might matter to the OP
>is that Darlingtons often present a dissipation issue
>in switching application due to their drop while in
>the on state and that drop can be significant with
>respect to the load voltage as well.
>
>

I've been looking at that data *** and I don't really understand a lot
of what it is saying. On page 4 it has the chart with all the values
and refers you to diagrams on page 5. Most of the diagrams say "Open"
by the output of the clamping diode. Doesn't that mean the Darlington
is off?

One line says Vce and has 1V typical for a 200ma load. Does that mean
that 1 volt will be dropped by the Darlington? If so I will need to
make a significant change to my voltage to drive my high intensity
LED's. For the 12V relay, that will probably have something more like
13.8 volts, it probably won't make any difference though.

I don't understand what the Vi(on) numbers are trying to tell me. For
the SN75469 in the MAX column they vary from 5V to 8V depending on the
collector current. What exactly does that mean? Is that the maximum
voltage the input line can handle? What about the minimum voltage
needed to turn the Darlington on?

Then the Vf for the clamp diode is 1.7V .... does that mean there will
be 1.7V drop from my supply?

Ii(off) seems obvious and Ii seems to be well with in what my flip flop
can provide. I assume Ir is the amount of revers current that will flow
for inductive kick back from a relay?

Finally what about Icex? Is that the minimum amount of current the
collector needs to stay on?

Thanks for everyones help. I really need to get a good book. The
problem is most books about things I am interested in are either way too
slow paced, or over my head. I learn this kind of thing very quickly
and often find beginner type books very slow paced. 

-- 
Chris W
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