S e ria lly In t e rfa c e d , 8 -Dig it LED Dis p la y Drive rs
9/MAX721
Table 11. R
LED Forward Voltage
vs. Segment Current and
Table 12. Package Thermal Resistance
Data
SET
THERMAL RESISTANCE
PACKAGE
V
(V)
LED
I
(mA)
SEG
(θ
)
JA
1.5
2.0
2.5
3.0
3.5
24 Narrow DIP
24 Wide SO
24 CERDIP
+75°C/W
+85°C/W
+60°C/W
40
12.2
17.8
29.8
66.7
11.8
17.1
28.0
63.7
11.0
15.8
25.9
59.3
10.6
15.0
24.5
55.4
9.69
14.0
22.6
51.2
30
20
10
Maximum Junction Temperature (T ) = +150°C
J
Maximum Ambient Temperature (T ) = +85°C
A
Co m p u t in g P o w e r Dis s ip a t io n
Ca s c a d in g Drive rs
The up p e r limit for p owe r d is s ip a tion (PD) for the
MAX7219/MAX7221 is determined from the following
equation:
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.
PD = (V + x 8mA) + (V+ - V )(DUTY x I
x N)
LED
SEG
where:
V+ = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 8)
V
= LED forward voltage
LED
I
= segment current set by R
SEG
SET
Dissipation Example:
= 40mA, N = 8, DUTY = 31/32, V
I
= 1.8V at
LED
SEG
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 (θ
= +60°C/W from
JA
Table 12), the maximum allowed ambient temperature
T is given by:
A
T
= T + PD x θ + 150°C = T +1.11W x
J(MAX)
A
JA
A
60°C/W
where T = +83.4°C.
A
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
