S e ria lly In t e rfa c e d , 8 -Dig it LED Dis p la y Drive rs
If the scan-limit register is set for three digits or less,
individual digit drivers will dissipate excessive amounts
fourth chip, sent the desired 16-bit word, followed by
thre e no-op c ode s (he x XX0X, se e Ta b le 2). Whe n
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.
of power. Consequently, the value of the R
resistor
SET
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.
__________Ap p lic a t io n s In fo rm a t io n
S u p p ly Byp a s s in g a n d Wirin g
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.
Dis p la y-Te s t Re g is t e r
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.
Table 9. Maximum Segment Current for
1-, 2-, or 3-Digit Displays
S e le c t in g R
Us in g Ex t e rn a l Drive rs
The current per segment is approximately 100 times
Re s is t o r a n d
S ET
9/MAX721
MAXIMUM SEGMENT
the current in ISET. To select R
, see Table 11. The
SET
NUMBER OF DIGITS
CURRENT
MAX7219/MAX7221’s maximum recommended seg-
me nt c urre nt is 40mA. For s e g me nt c urre nt le ve ls
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-
DISPLAYED
(mA)
1
2
3
10
20
30
sistors. Therefore, to conserve power, use R
= 47kΩ
SET
when using external current sources as segment dri-
vers.
Table 10. Display-Test Register Format
(Address (Hex) = XF)
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 c ommon-c a thode disp la ys. 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-
REGISTER DATA
MODE
D7 D6 D5 D4 D3 D2 D1 D0
Normal
Operation
X
X
X
X
X
X
X
X
X
X
X
X
X
X
0
1
Display Test
Mode
Note: The MAX7219/MAX7221 remain in display-test mode
(all LEDs on) until the display-test register is reconfigured
for normal operation.
No -Op Re g is t e r
The no-op register is used when cascading MAX7219s
or MAX7221s. Connect all devices’ LOAD/CS inputs
tog e the r a nd c onne c t DOUT to DIN on a d ja c e nt
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 a re c a s c a d e d , the n to write to the
able segment current. R
ingly (Table 11).
must be selected accord-
SET
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.
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
