Datasheet

Computing Power Dissipation
The upper limit for power dissipation (PD) for the
MAX7219/MAX7221 is determined from the following
equation:
PD = (V+ x 8mA) + (V+ - V
LED
)(DUTY x I
SEG
x N)
where:
V+ = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 8)
V
LED
= LED forward voltage
I
SEG
= segment current set by R
SET
Dissipation example:
I
SEG
= 40mA, N = 8, DUTY = 31/32, V
LED
= 1.8V at
40mA, V+ = 5.25V
PD = (5.25V x 8mA) + (5.25V - 1.8V)(31/32 x
40mA x 8) = 1.11W
Thus, for a CERDIP package (θ
JA
= +80°C/W from
Table 12), the maximum allowed ambient temperature
T
A
is given by:
T
J(MAX)
= T
A
+ PD x θ
JA
150°C = T
A
+1.11W x 80°C/W
where T
A
= +61.2°C.
The T
A
limits for PDIP and SO packages in the dissipation
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.
Table 11. R
SET
vs. Segment Current and
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
V
LED
(V)
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
I
SEG
(mA)
24.5
63.7 51.255.4
24 Narrow DIP
24 Wide SO
PACKAGE
+85°C/W
24 CERDIP
Maximum Ambient Temperature (T
A
) = +85°C
Maximum Junction Temperature (T
J
) = +150°C
+80°C/W
THERMAL RESISTANCE
(
θθ
JA
)
+75°C/W
Maxim Integrated
11
Serially Interfaced, 8-Digit LED Display Drivers
MAX7219/MAX7221