Philips Semiconductors Linear Products

Product specification

NE587

LED decoder/driver

August 31, 1994

536

because the current source output is operating with 3.25V across it.

In practice, the outputs operate satisfactorily down to 0.5V, and so

the extra voltage may be dropped external to the integrated circuit.

required to keep the display active, and hence 2.0V may be dropped

calculated by:

R

2.0

7 x I

SEG

[

10

W (

1

2

W rating)

Hence now

= 5.25

× 50 + 1.25 × 7 × 30mW

= 525mW

and

If a diode (or 2) is used to reduce voltage to the display, then the

voltage appearing across the display driver will be independent of

the number of “ON” segments and will be equal to

Where n is the number of diodes used, power dissipation can be

calculated in a similar manner.

In a multiplexed display system, the voltage drop across the digit

driver must also be considered in computing device power

dissipation. It may even be an advantage to use a digit driver which

drops an appreciable voltage, rather than the saturating PNP

transistors shown in Figure 9. For example a Darlington PNP or

NPN emitter-follower may be preferable. Figure 8 shows the NE591

as the digit driver in a multiplexed display system. The NE591 output

drops about 1.8V which means that the power dissipation is evenly

distributed between the two integrated circuits.

optimized for minimum system power dissipation and/or cost.

so this supply can be made much cheaper than the regulated 5V

supply used in the rest of the system. In fact, a simple unsmoothed

full-wave rectified sine wave works extremely well if a slight loss in

brightness can be tolerated. A transformer voltage of about

shown below:

VS

ISEG

characteristics of the display driver.

With

The duty cycle is approximately 60%.