Computing Power Dissipation

The upper limit for power dissipation (PD) for the

MAX7219/MAX7221 is determined from the following

equation:

where:

V+ = supply voltage

DUTY = duty cycle set by intensity register

N = number of segments driven (worst case is 8)

Dissipation Example:

40mA, V+ = 5.25V

PD = 5.25V(8mA) + (5.25V - 1.8V)(31/32 x

40mA x 8) = 1.11W

Thus, for a CERDIP package (

Table 12), the maximum allowed ambient temperature

80°C/W

example above are +66.7°C and +55.6°C, respectively.

Cascading Drivers

The example in Figure 3 drives 16 digits using a 3-wire

µP interface. If the number of digits is not a multiple of

8, set both drivers’ scan limits registers to the same

number so one display will not appear brighter than the

other. For example, if 12 digits are need, use 6 digits

per display with both scan-limit registers set for 6 digits

so that both displays have a 1/6 duty cycle per digit. If

11 digits are needed, set both scan-limit registers for 6

digits and leave one digit driver unconnected. If one

display for 6 digits and the other for 5 digits, the sec-

ond display will appear brighter because its duty cycle

per digit will be 1/5 while the first display’s will be 1/6.

Refer to the No-Op Register section for additional infor-

mation.

Serially Interfaced, 8-Digit LED Display Drivers

______________________________________________________________________________________

11

LED Forward Voltage

Table 12. Package Thermal Resistance

Data

12.2

17.8

1.5

40

11.0

2.5

11.8

30

9.69

15.8

10.6

29.8

2.0

66.7

20

17.1

14.0

25.9

15.0

28.0

10

3.5

22.6

59.3

3.0

24.5

63.7

51.2

55.4

24 Narrow DIP

24 Wide SO

PACKAGE

+85°C/W

24 CERDIP

+80°C/W

THERMAL RESISTANCE

(

+75°C/W