Alt-BEAM Archive
Message #05417
To: beam@sgiblab.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Sun, 25 Jul 1999 13:33:45 -0700
Subject: [alt-beam] Re: h-bridge problems
The bottom line is: LED series current limiting resistors of 500 ohm to
1K are fine for visual indication of the output state of 74HC logic at
Vcc=5V. At lower Vcc, you can reduce the resistor values over a range
from 500 ohms at down to 0 ohms at about 2.0V (the latter strongly
dependend on the type of LED).
Want to know why? Read on!
For reasons of meeting requirements of logic levels the voltage drop on
the outputs of 74HC logic caused by output current must not exceed
certain limits. Based on Ohm's law, the maximum allowable output current
is therefore determined by the HC logic output resistance . An 74HC240
can sink 3 times as much current as a 74HC14 and so can have three times
smaller LED resistor and brighter LEDs. The lowest practical super
efficient LED current is about 2 ma. With a 74HC14 and a Vcc of 5V,
there is a typical 1.9V drop across the LED which leaves 3.1V across the
resistor and HC driver output resistance requiring about 1.5K to limit
the LED current to 2 ma. So at 5V use 1.5K max and 390 ohm minimum for
LED currents from 2 ma to 8ma. With a 74HC240 a minimum LED resistor
value can be reduced to 200 ohm which results in about 20ma of LED
current which may be required for very low efficiency LEDs. Without a
resistor at Vcc=5V, the LED current can be 70-150ma which is enough to
damage the LED and the HC logic in a very short time if not instantly.
Minimum resistor values at the other end of the Vcc spectrum are
(approximately): Vcc=4V - Rled = 330 and Vcc =3V - Rled = 150 ohm. At
Vcc voltages between 1.8 and 2.2V the value is a little uncertain since
it strongle depends on the forward voltage of the LED. Iit may even be
necesary to remove the LED resistor to get enough LED current since the
HC driver output resistance limits the current. Owing to the relatively
high LED Vf compared to Vcc= 1.8-2.2V, the effect of output loading on
logic voltage levels is minimized.
I found that the absolute minimum supply voltage that can be used with a
2ma LED is about 1.65V for some types of red LEDs. . More typical red and
green LED minimum voltages at 2 ma is around 1.9-2.0V.
I recently posted a 100ma Low Drop-Out voltage regulator design that
exploits this low Vcc concept to the max by using an actual LED as the
regulator voltage reference. That way the LDO regulator holds the 74HC
Vcc supply voltage at the optimum level for HC outputs to drive LEDs
directly. In addition, the LED Vf referenced regulator tracks Vcc with
temperature for constant LED brightness over a wide input voltage range
of 2.1V to 9V.
As Steven Bolt has pointed out many times, for low power applications
there is a big advantage to operating HCMOS at low voltage ie Vcc<2.5V
compared to Vcc=5.0V. For example, at Vcc=5.0V active oscillators (Nv)
and voltage detectors (Nu) can produce peak supply currents bumps (Ipeak)
of 20 ma per stage and average currents of many ma. But at Vcc<2.5V
these are typically reduced to Ipeak<1ma and I average <100ua. This
means that using a low power LDO voltage regulator can significantly
reduce overall power and give consistent performance for many BEAM
applications over a wide range of supply voltages.
enjoy
wilf
SG wrote:
> 10K ? wow! i was told just 1K at the highest, 500ohm at the lowest.
> -Sparky
>
> At 10:26 PM 7/23/99 +1000, you wrote:
> >done that, 10k, nothings caught on fire yet.
> >
> >
> >-----Original Message-----
> >From: SG
> >To: beam@sgiblab.sgi.com
> >Date: Saturday, 24 July 1999 12:48
> >Subject: Re: h-bridge problems
> >
> >
> >>>Please help, you wouldn't want all the
> >>>prizes to go to Darrel :-) David Perry
> >>
> >>be sure to put resistors on yur LEDs!!!
> >>heh heh
> >>-Sparky
> >>
> >
> >
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