Philips Semiconductors Linear Products

Product specification

NE587

LED decoder/driver

August 31, 1994

534

Figure 1.

I

P +

V

rP

R

P

+

1.3V

R

P

RP

IP

IO

VCC

1:20

PIN 8

BAND GAP

REFERENCE

TIMING DIAGRAMS

LE = L

D0–D3

OUTPUT

tPLH

tPHL

tPLH

tPHL

LE

D0–D3

OUTPUT

Segment Identification

a

b

c

d

e

f

g

An output current of 10 to 50mA was chosen so that it would be

suitable for multiplexed operation of large-size LED digits. When

designing a display system, particular care must be taken to

minimize power dissipation within the IC display driver. Since the

output is a constant-current source, all the remaining supply voltage,

which is not dropped across the LED (and the digit driver, if used),

will appear across the output. Thus, the power dissipation will go up

sharply if the display power supply voltage rises. Clearly, then, it is

good design practice to keep the display supply voltage as low as

possible, consistent with proper operation of the supply output

current sources. Inserting a resistor or diode in series with the

display supply is a good way of reducing the power dissipation

within the integrated circuit segment driver, although, of course, total

system power remains the same.

Power dissipation may be calculated as follows. Referring to Figure

these will be the same voltage. Necessary parameters are:

Supply voltage to driver

Supply voltage to display

Quiescent supply current of driver

LED segment current

% Duty cycle

(hence the color) and the forward current. The actual forward

voltage drops should be obtained from the LED display

manufacturer’s literature for the peak segment current selected;

however, approximate voltages at nominal rated currents are:

Red

1.6 to 2.0V

Orange

2.0 to 2.5V

Yellow

2.2 to 3.5V

Green

2.5 to 3.5V

Figure 4. Setup and Hold Times

LE

D0–D3

OUTPUT

tW

tS

tH