At 01:38 PM 3/17/2006 -0700, you wrote:
>Good day to all.
>
>I'm starting a new project that uses 3 digits of LED seven-segment
>display.  My usual techniques for driving these is either direct
>drive with shift registers or multiplexed.  Current limit is always
>with the appropriate series resistors.
>
>I was playing around with the shift registers that I normally use
>(74hc595) and see that the RDSon of the output FETs is fairly
>predictable.  That got me to wondering if I could carefully control
>the supply voltage to the LEDs and make use of the RDSon to eliminate
>the current limit resistors.
>
>First glance: seems possible.  I'm using a LED of similar
>characteristics to the display LEDs within the feedback loop of the
>LED power supply for temperature tracking.
>
>I'm looking at a couple of slightly different approaches:
>
>1) keep the LED supply voltage just a bit higher than the Vf of the
>displays.  Amount of "higher" determines brightness.  Relies upon
>RDSon for current limit.
>
>2) Operate the LED supply as a constant current supply.  Monitor
>total LED current through a current sense resistor and adjust the
>total current in accordance with the number of segments turned on at
>that instant.  Relies upon RDSon for ballasting.
>
>I should mention that there is already a simple-switcher supply
>dedicated to driving the displays so the cost of the supply is not a
>factor (its already in the budget).  Only the cost of controlling that supply.
>
>Has anyone done anything like this before?
>
>Any thoughts?
>
>dwayne

Yes, it's been done. I don't think it's a very good idea in most cases-
it greatly increases the tolerance on the LED current and sensitivity to
the HC595 batch/mfr and LED batch/technology. Note that the
LEDs have an equivalent series resistance as well (the slope of the
Vf vs. If curve near the operating point). This might be 10 ohms or
something like that, depending on current and LED technology.  Chances are
you are close to the ABS. MAX. rating of the HC595 if your display is
multiplexed.

Granted, the visual brightness difference for a +/-50% variation in current is
not all that huge, but I'm not sure it's worth it to save two penny-ish
SMD components per HC595. You'll not be able to guarantee the current will be
within reasonable limits using only the data sheet numbers. Matching of
adjacent displays will probably be worse as well.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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