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MAX7219ENG Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX7219ENG Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 16 page If the scan-limit register is set for three digits or less, individual digit drivers will dissipate excessive amounts of power. Consequently, the value of the RSET resistor must be adjusted according to the number of digits dis- played, to limit individual digit driver power dissipation. Table 9 lists the number of digits displayed and the corresponding maximum recommended segment cur- rent when the digit drivers are used. Display-Test Register The display-test register operates in two modes: normal and display test. Display-test mode turns all LEDs on by overriding, but not altering, all controls and digit reg- isters (including the shutdown register). In display-test mode, 8 digits are scanned and the duty cycle is 31/32 (15/16 for MAX7221). Table 10 lists the display-test reg- ister format. No-Op Register The no-op register is used when cascading MAX7219s or MAX7221s. Connect all devices’ LOAD/CS inputs together and connect DOUT to DIN on adjacent devices. DOUT is a CMOS logic-level output that easily drives DIN of successively cascaded parts. (Refer to the Serial Addressing Modes section for detailed infor- mation on serial input/output timing.) For example, if four MAX7219s are cascaded, then to write to the fourth chip, sent the desired 16-bit word, followed by three no-op codes (hex XX0X, see Table 2). When LOAD/CS goes high, data is latched in all devices. The first three chips receive no-op commands, and the fourth receives the intended data. __________Applications Information Supply Bypassing and Wiring To minimize power-supply ripple due to the peak digit driver currents, connect a 10µF electrolytic and a 0.1µF ceramic capacitor between V+ and GND as close to the device as possible. The MAX7219/MAX7221 should be placed in close proximity to the LED display, and connections should be kept as short as possible to minimize the effects of wiring inductance and electro- magnetic interference. Also, both GND pins must be connected to ground. Selecting RSET Resistor and Using External Drivers The current per segment is approximately 100 times the current in ISET. To select RSET, see Table 11. The MAX7219/MAX7221’s maximum recommended seg- ment current is 40mA. For segment current levels above these levels, external digit drivers will be need- ed. In this application, the MAX7219/MAX7221 serve only as controllers for other high-current drivers or tran- sistors. Therefore, to conserve power, use RSET = 47k Ω when using external current sources as segment dri- vers. The example in Figure 2 uses the MAX7219/MAX7221’s segment drivers, a MAX394 single-pole double-throw analog switch, and external transistors to drive 2.3” AND2307SLC common-cathode displays. The 5.6V zener diode has been added in series with the decimal point LED because the decimal point LED forward volt- age is typically 4.2V. For all other segments the LED forward voltage is typically 8V. Since external transis- tors are used to sink current (DIG 0 and DIG 1 are used as logic switches), peak segment currents of 45mA are allowed even though only two digits are displayed. In applications where the MAX7219/MAX7221’s digit dri- vers are used to sink current and fewer than four digits are displayed, Table 9 specifies the maximum allow- able segment current. RSET must be selected accord- ingly (Table 11). Refer to the Power Dissipation section of the Absolute Maximum Ratings to calculate acceptable limits for ambient temperature, segment current, and the LED forward-voltage drop. Serially Interfaced, 8-Digit LED Display Drivers 10 ______________________________________________________________________________________ Table 9. Maximum Segment Current for 1-, 2-, or 3-Digit Displays Table 10. Display-Test Register Format (Address (Hex) = XF) 10 20 2 MAXIMUM SEGMENT CURRENT (mA) 1 30 3 NUMBER OF DIGITS DISPLAYED X X Normal Operation D6 X X Display Test Mode D4 X X D5 X X D2 X 0 REGISTER DATA D0 X X X D1 X D3 1 X D7 MODE Note: The MAX7219/MAX7221 remain in display-test mode (all LEDs on) until the display-test register is reconfigured for normal operation. |
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