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MAX7221 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX7221 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page 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+ - VLED)(DUTY x ISEG x N) where: V+ = supply voltage DUTY = duty cycle set by intensity register N = number of segments driven (worst case is 8) VLED = LED forward voltage ISEG = segment current set by RSET Dissipation Example: ISEG = 40mA, N = 8, DUTY = 31/32, VLED = 1.8V at 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 ( θJA = +60°C/W from Table 12), the maximum allowed ambient temperature TA is given by: TJ(MAX) = TA + PD x θJA + 150°C = TA +1.11W x 60°C/W where TA = +83.4°C. 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 Table 11. RSET 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 VLED (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 ISEG (mA) 24.5 63.7 51.2 55.4 24 Narrow DIP 24 Wide SO PACKAGE +85°C/W 24 CERDIP Maximum Ambient Temperature (TA) = +85°C Maximum Junction Temperature (TJ) = +150°C +60°C/W THERMAL RESISTANCE ( θJA) +75°C/W |
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