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 second 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 information.
Note: R
SET
values are in Kilo Ohms (kΩ)
Table 11. R
SET
vs. Segment Current and
LED Forward Voltage
Table 12. Package Thermal Resistance
Data
I
SEG
(mA)
V
LED
(V)
1.5 2.0 2.5 3.0 3.5
40 12.2 11.8 11.0 10.6 9.69
30 17.8 17.1 15.8 15.0 14.0
20 29.8 28.0 25.9 24.5 22.6
10 66.7 63.7 59.3 55.4 51.2
PACKAGE
THERMAL RESISTANCE
(θ
JA
)
24 Narrow DIP +75°C/W
24 Wide SO +85°C/W
24 CERDIP +80°C/W
Maximum Junction Temperature (T
J
) = +150°C
Maximum Ambient Temperature (T
A
) = +85°C
MAX7219/MAX7221 Serially Interfaced, 8-Digit LED Display Drivers
www.maximintegrated.com
Maxim Integrated │ 11