Functional Description

The MM5480 is specifically designed to operate 3

-digit

alphanumeric displays with minimal interface with the dis-

play and the data source Serial data transfer from the data

source to the display driver is accomplished with 2 signals

serial data and clock Using a format of a leading ‘‘1’’ fol-

lowed by the 35 data bits allows data transfer without an

additional load signal The 35 data bits are latched after the

36th bit is complete thus providing non-multiplexed direct

drive to the display Outputs change only if the serial data

bits differ from the previous time Display brightness is de-

termined by control of the output current for LED displays A

0001 mF ceramic or mica disc capacitor should be connect-

ed to brightness control pin 13 to prevent possible oscilla-

tions

A block diagram is shown in

Figure 1 The output current is

typically 20 times greater than the current into pin 13 which

is set by an external variable resistor There is an internal

limiting resistor of 400X nominal value

Figure 4 shows the input data format A start bit of logical

‘‘1’’ precedes the 35 bits of data At the 36th clock a LOAD

signal is generated synchronously with the high state of the

clock which loads the 35 bits of the shift registers into the

latches At the low state of the clock a RESET signal is

generated which clears all the shift registers for the next set

of data The shift registers are static master-slave configura-

tion There is no clear for the master portion of the first shift

register thus allowing continuous operation

There must be a complete set of 36 clocks or the shift regis-

ters will not clear

When the chip first powers ON an internal power ON reset

signal is generated which resets all registers and all latches

The START bit and the first clock return the chip to its nor-

mal operation

Figure 5 shows the Output Data Format for the 5480 Be-

cause it uses only 23 of the possible 35 outputs 12 of the

bits are ‘Don’t Cares’

Figure 3 shows the timing relationships between data and

clock A maximum clock frequency of 05 MHz is assumed

For applications where a lesser number of outputs are used

it is possible to either increase the current per output or

equation can be used for calculations

where

junction temperature 150 C max

the voltage at the LED driver outputs

the LED current

thermal coefficient of the package

ambient temperature

58 CW

52 CW

TLF6138 3

FIGURE 3

TLF6138 4

FIGURE 4 Input Data Format

START 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 5451

START X 1 2 3 4 5 6 7 X X X 8 9 10 11 X X X X 12 13 14 15 16 17 X 18 X X 19 20 21 22 23 X 5480

FIGURE 5 Output Data Format

3