13 - 19
AS1108
Datasheet - Typical Application
9 Typical Application
Supply Bypassing and Wiring
In order to achieve optimal performance the AS1108 should be placed very close to the LED display to minimize
effects of electromagnetic interference and wiring inductance.
Furthermore, a 10μF electrolytic and a 0.1μF ceramic capacitor should be connected between pins V
DD
and GND to
avoid power supply ripple (see Figure 12 on page 15).
Note:
Both GND pins must be connected to ground.
Selecting R
SET
Resistor Value and Using External Drivers
Brightness of the display segments is controlled via R
SET
. The current that flows between V
DD
and I
SET
defines the
current that flows through the LEDs.
Segment current is about 200 times the current in I
SET
. Typical values for R
SET
for different segment currents, operat-
ing voltages, and LED voltage drop (V
LED
) are given in Tables 19 - 23. The maximum current the AS1108 can drive is
40mA. If higher currents are needed, external drivers must be used, in which case it is no longer necessary that the
device drive high currents.
In cases where the device drives only a few digits, Table 16 specifies the maximum currents, and R
SET
must be set
accordingly.
Note:
The display brightness can also be logically controlled (see Selecting RSET Resistor Value and Using External
Drivers on page 13).
Table 19. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 2.7V
V
LED
(V)
1.5
40
5k
Ω
30
6.9k
Ω
20
10.7k
Ω
10
22.2k
Ω
I
SEG
(mA)
2.0
4.4k
Ω
5.9k
Ω
9.6k
Ω
20.7k
Ω
Table 20. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 3.3V
I
SEG
(mA)
V
LED
(V)
2.0
6.4k
Ω
8.8k
Ω
13.3k
Ω
27.7k
Ω
1.5
6.7k
Ω
9.1k
Ω
13.9k
Ω
28.8k
Ω
2.5
5.7k
Ω
8.1k
Ω
12.6k
Ω
26k
Ω
40
30
20
10
Table 21. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 3.6V
I
SEG
(mA)
V
LED
(V)
1.5
7.5k
Ω
10.18k
Ω
15.6k
Ω
31.9k
Ω
2.0
7.2k
Ω
9.8k
Ω
15k
Ω
31k
Ω
2.5
6.6k
Ω
9.2k
Ω
14.3k
Ω
29.5k
Ω
3.0
5.5k
Ω
7.5k
Ω
13k
Ω
27.3k
Ω
40
30
20
10
Table 22. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 4.0V
I
SEG
(mA)
V
LED
(V)
2.5
7.9k
Ω
10.8k
Ω
1.5
8.6k
Ω
11.6k
Ω
2.0
8.3k
Ω
11.2k
Ω
3.0
7.6k
Ω
9.9k
Ω
3.5
5.2k
Ω
7.8k
Ω
40
30