Transistor Help.

A

Andy Ladouceur

Guest
Hi everyone,

I took a basic electronics course a few years back, otherwise only know
what I've found online, and am having one hell of a time trying to work
out the right voltages / resistances for a project I'm working on.

I have 2.4V @ 24mA coming off an output from an IC, that I'd like to
use to power an LED. The LED needs 3.3V, 20mA.

I'm able to vary the 24mA output from the IC in 16 1.5mA steps. (From
1.5mA -> 24mA)

I could be mistaken, but I'm quite sure what I'm looking for here is a
NPN transistor. I just don't know what values of.. -anything-, I'd
need. I had it calculated as:

NPN Transistor, Beta 50.

Power to collector: 12V, with a 420 Ohm resistor.
Power to base: 2.4V
Power to emitter: 0V (Ground), with an 80 Ohm resistor.

This would give me 3.25V at 20mA coming out of the collector. (Er..
right?) Which would work perfectly, if I didn't plan on varying the
current. But, I'm not sure how to calculate what the current going to
the LED would be when the base current is dropped to 1.5V. What I'd
ultimately like is to be able to vary the current and have 1.5mA
pretty much be 'off' for the LED, and 24mA at its brightest.

With my current setup, is this how it would work, or am I right in
assuming most of its current comes from the 12V collector?

Could someone please advise me of a setup that'd let me do the above,
if my current(sorry) one doesn't allow for it? Thanks in advance for
any help you might provide. I'd like to be able to understand the
workings of a transistor enough to be able to figure this out myself,
but it's been two hours, and it seems that's not happening, right now.
:)

-Andy
 
Hi Rich,

Sorry for not including that in my original post. :) The IC is a
MAX6956.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3503/ln/

Direct link to PDF:
http://pdfserv.maxim-ic.com/en/ds/MAX6956.pdf

Thanks!

-Andy
 
Andy Ladouceur wrote:
Hi everyone,

I took a basic electronics course a few years back, otherwise only
know
what I've found online, and am having one hell of a time trying to
work
out the right voltages / resistances for a project I'm working on.

I have 2.4V @ 24mA coming off an output from an IC, that I'd like to
use to power an LED. The LED needs 3.3V, 20mA.

I'm able to vary the 24mA output from the IC in 16 1.5mA steps. (From
1.5mA -> 24mA)

I could be mistaken, but I'm quite sure what I'm looking for here is
a
NPN transistor. I just don't know what values of.. -anything-, I'd
need. I had it calculated as:

NPN Transistor, Beta 50.

Power to collector: 12V, with a 420 Ohm resistor.
Power to base: 2.4V
Power to emitter: 0V (Ground), with an 80 Ohm resistor.

This would give me 3.25V at 20mA coming out of the collector. (Er..
right?) Which would work perfectly, if I didn't plan on varying the
current. But, I'm not sure how to calculate what the current going to
the LED would be when the base current is dropped to 1.5V. What I'd
ultimately like is to be able to vary the current and have 1.5mA
pretty much be 'off' for the LED, and 24mA at its brightest.

With my current setup, is this how it would work, or am I right in
assuming most of its current comes from the 12V collector?

Could someone please advise me of a setup that'd let me do the above,
if my current(sorry) one doesn't allow for it? Thanks in advance for
any help you might provide. I'd like to be able to understand the
workings of a transistor enough to be able to figure this out myself,
but it's been two hours, and it seems that's not happening, right
now.
:)

-Andy
Click to expand...
<plus>

Hi Rich,


Sorry for not including that in my original post. :) The IC is a
MAX6956.


http://www.maxim-ic.com/quick_­view2.cfm/qv_pk/3503/ln/


Direct link to PDF:
http://pdfserv.maxim-ic.com/en­/ds/MAX6956.pdf


Thanks!


-Andy
Click to expand...
Hi, Andy. I'm not entirely sure you're interpreting the data sheet
properly. But the data sheet isn't too clear, either.

The data sheet for the MAX6956 isn't very clear on output pin voltage
while sinking LED current, if you set it for current drive rather than
voltage drive. It does mention on page 21 of the data sheet that the
expected minimum voltage at an output pin set to sink current as an LED
driver will be about 0.6V. If you've got 3.3V V(f) white LEDs, you
might want to start thinking about using a 5V supply, and using series
33 ohm resistors to limit total package power dissipation (if you're
lighting up most or all 28 outputs with LEDs). A 33 ohm series
resistor will drop .66V, which will add to the 3.3V with the LED to
leave 1V remaining (20mW) dropped across the IC output. That would be
560mW due to voltage drop if you're using all 28 port pins as LED
drivers, which keeps you in safe limits for all packages at reasonable
temperatures.

+5V
o--------o-------------.
| |
| White LED
| |
.----o-----. V~
| | -~
| | |
| | o1.7V
| | |
| | .-.
| |33 ohm| | |
| | | | |
| | '-' |
| MAX6956 | | V
| | | 20mA
| | |
| o-------o 1.0V
| |
| |
'----o-----'
|
|
o--------'
GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

This is a good thing, because you can't get 66mW (3.3V * 20mA) out of
57mW (2.4V * 24mA) anyway. Just won't work. TAANSTAAFL and all that.

If this is confusing, or you're stuck with a supply less than 4V, look
at Maxim app note 2197 for more information and how to interface the IC
to transistors:

http://www.maxim-ic.com/appnotes.cfm/appnote_number/2197

If this doesn't clear things up, please provide further information
with your response:

* What's your power supply for the MAX6956? If it's less than 4V,
would you be willing to look at a DC-DC converter to increase the power
supply voltage to the IC?

* How many LEDs are you driving? Anything else besides LEDs? The
trick to using this chip successfully is total power dissipation.

* Which IC do you have? You have to be more careful with the SSOP
packages than the PDIP on power dissipation. The complete IC part
number would have been helpful.

I hope this has been of help. Again, feel free to respond, but with
more information, please.

Good luck with the programming
Chris
 
I had a chance to look at the data sheet for the MAX6956 again. It
notes on page three that

Port Drive Logic Sink Current, Port Configured as LED Driver
V+ = 5.5V, VOUT = 0.6V at maximum sink current = 24mA typ.

So the data sheet does guarantee a saturation voltage for the pin when
configured as a current sink of 0.6V (in a backwards kind of way).

Let's try putting 3.3V white LEDs as a load on all the pins of your
MAX6956 (5VDC power supply) without series resistors. By the
definition of a current sink, once the voltages across the load are
taken care of, the rest of the voltage will be across the driver. That
means each pin will have about 1.7V across it. Accounting for the
power, that means 1.7V * .02A = .034 watts will be dissipated by the
driver at each pin. If you've got 28 pins, that would mean 28 * .034
Watts = .952 Watts will be generated by the outputs alone (this doesn't
include power dissipation from the rest of the IC). The graph on page
4 suggests the IC itself will take up about 10mA by itself with all
outputs set as current sinks, all outputs on, and no load on any of the
pins. That would mean another 5V * .01A = .05 Watts. So, worst case
(all LEDs on), that would mean 1 watt of power would be dissipated by
the chip.

We're getting pretty close to the dreaded abs-max (absolute maximum
ratings, usually listed on the top of the data sheet, do not exceed or
you'll be sorry). It depends on your IC package. The DIP package is
supposed to be rated for 1.667 watts, and I'm assuming that's the IC
you've got here. But if you have an SSOP IC, you may already be over
abs-max. Either way, that's probably an uncomfortable amount of power
dissipation.

But all of this depends on the details. You could be using some of the
pins as inputs. You could be using some as outputs, but not to drive
LEDs. You might say that, except for a few moments for a reset, all of
the LEDs will never be on. Any of these would change what's going on
here, and affect the basic question of whether you need series
resistors. The thing is, much of the advantage of the chip is based on
not needing them.

If you're driving this with a PIC, you might want to look into using an
adjustable regulator to set the power supply voltage to around 4.2V or
so. That's an oddball but perfectly permissible solution that will
reduce the power dissipation of the chip by almost half.

All of this is fairly simple hardware stuff. Your main issues will be
the programming for that chip, which is not trivial, and the cost of
the IC itself.

Good luck
Chris
 
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