The scheme in Figure 10 is useful when the reference and

LED intensity must be adjusted independently over a wide

range. The R

with no effect on LED current. Since the internal divider here

does not load down the reference, minimum LED current is

much lower. At the minimum recommended reference load

of 80 µA, LED current is about 0.8 mA. The resistor values

shown give a LED current range from 1.5 mA to 20 mA.

At the low end of the intensity adjustment, the voltage drop

across the 510

Ω current-sharing resistors is so small that

chip to chip variation in reference voltage may yield a visible

variation in LED intensity. The optional approach shown of

connecting the bottom end of the intensity control pot to a

negative supply overcomes this problem by allowing a larger

voltage drop across the (larger) current-sharing resistors.

Other Applications

For increased resolution, it’s possible to obtain a display with

a smooth transition between LEDs. This is accomplished by

varying the reference level at pin 6 by 3 dBp-p as shown in

Figure 11. The signal can be a triangle, sawtooth or sine

wave from 60 Hz to 1 kHz. The display can be run in either

dot or bar mode.

When an exponentially decaying RC discharge waveform is

applied to pin 5, the LM3915’s outputs will switch at equal

intervals. This makes a simple timer or sequencer. Each time

interval is equal to RC/3. The output may be used to drive

logic, opto-couplers, relays or PNP transistors, for example.

00510416

FIGURE 8. Varying LED Intensity

00510417

FIGURE 9. Independent Adjustment of Reference Voltage and LED Intensity for Multiple LM3915s

www.national.com

14