calculate the RSet resistor value for MAX7221 IC
The current per segment is approximately 100 timesthe current in ISET. To select RSET, see Table 11. TheMAX7219/MAX7221's maximum recommended segmentcurrent is 40mA. For segment current levelsabove these levels, external digit drivers will be needed.In this application, the MAX7219/MAX7221 serveonly as controllers for other high-current drivers or transistors.Therefore, to conserve power, use RSET = 47k[ch937]when using external current sources as segment drivers.
how should I drive 7-segment displays with that same IC with 9 volts
One thing that I found out is that I could use MAX394 with MAX7221
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