If the scan-limit register is set for three digits or less,

individual digit drivers will dissipate excessive amounts

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.

Using External Drivers

The current per segment is approximately 100 times

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-

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-

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.