AN1196| Application Note
Maxim > App Notes > DISPLAY DRIVERS Keywords: LED display drivers, LED drivers, display drivers, SPI, serial, 7 segment, 7-segment
Sep 09, 2002
APPLICATION NOTE 1196
Using the MAX7219/7221 to Drive Higher Voltage or Current
Abstract: Techniques to amplify the current and voltage drive capabilities of the MAX7219 and MAX7221 serial interface LED drivers. The MAX7219/7221 is an 8-digit, 7-segment common-cathode multiplexed LED display driver which drives up to 40mA segment current while operating from a 4V to 5.5V supply. This application note discusses how the MAX7219/7221 can be used with external drive transistors for applications requiring higher peak segment currents and/or a higher drive voltage. LEDs using three or more LED dice per segment cannot be driven directly by the MAX7219/7221 from a 5V supply due to the multiple LED voltage drops. The drivers shown in Figures 1 thru 4 interface a MAX7219/7221 operating at 4V to 5V to common anode displays operating at a higher segment current and/or drive voltage than the MAX7219/7221 can deliver. Eight instances of the low current cathode column driver, and eight instances of the high current anode digit driver are required. The MAX7219/7221's PWM digital intensity control can s
till be used. The peak segment current, however, is now set by external current-limiting resistors in series with the LED cathodes instead of by resistor RSET. To use these drivers, choose R1 (see Figure 4) to set the desired peak LED segment current ISEG according to the driver supply voltage Vdriver and the LED forward voltage drop VLED: IPEAK = (Vdriver - VLED - VCE(sat)Q1) / (R1 + RDS(on)Q2) A Rate Q1 at segment current ISEG, and rate Q2 at digit current, which is 8 times ISEG. Note that the display must be the common anode (CA) type to interface the drivers described here; the MAX7219/7221 directly drive common cathode (CC) displays, but these external drivers invert the digit/segment drive polarity.
Digit Drivers
The digit drive outputs of the MAX7219/7221 are power switches which go active low one at a time to turn a digit on, and high impedance when off. Three alternative digit drive circuits are suggested here. The simplest circuit (Figure 1) suits an application where a higher than 40mA segment current is desired, and yet the 4V-5V MAX7219/7221 supply voltage can be used to drive the LEDs. The active low MAX7219/7221 digit drive outputs directly drive logic level PFET Q2. Resistor R3 turns Q2 off when the digit output is high impedance. The supply Vdriver is the same supply as used by the MAX7219/7221.
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