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 VDD and GND to
avoid power supply ripple (see Figure 12 on page 16).
Note: Both GND pins must be connected to ground.
Selecting RSET Resistor Value and Using External Drivers
Brightness of the display segments is controlled via RSET. The current that flows between VDD and ISET defines the
current that flows through the LEDs.
Segment current is about 200 times the current in ISET. Typical values for RSET for different segment currents, operat-
ing voltages, and LED voltage drop (VLED) 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 RSET must be set
accordingly.
Note: The display brightness can also be logically controlled (see Selecting RSET Resistor Value and Using External
Drivers on page 14).
Table 19. RSET vs. Segment Current and LED Forward Voltage, VDD = 2.7V
VLED(V)
ISEG (mA)
1.5
2.0
40
30
20
10
5kΩ
4.4kΩ
5.9kΩ
9.6kΩ
20.7kΩ
6.9kΩ
10.7kΩ
22.2kΩ
Table 20. RSET vs. Segment Current and LED Forward Voltage, VDD = 3.3V
VLED(V)
2.0
ISEG (mA)
1.5
2.5
40
30
20
10
6.7kΩ
9.1kΩ
13.9kΩ
28.8kΩ
6.4kΩ
8.8kΩ
13.3kΩ
27.7kΩ
5.7kΩ
8.1kΩ
12.6kΩ
26kΩ
Table 21. RSET vs. Segment Current and LED Forward Voltage, VDD = 3.6V
VLED(V)
ISEG (mA)
1.5
2.0
2.5
3.0
40
30
20
10
7.5kΩ
7.2kΩ
9.8kΩ
15kΩ
31kΩ
6.6kΩ
9.2kΩ
14.3kΩ
29.5kΩ
5.5kΩ
7.5kΩ
13kΩ
10.18kΩ
15.6kΩ
31.9kΩ
27.3kΩ
Table 22. RSET vs. Segment Current and LED Forward Voltage, VDD = 4.0V
VLED(V)
2.5
ISEG (mA)
1.5
2.0
3.0
3.5
40
30
8.6kΩ
11.6kΩ
8.3kΩ
11.2kΩ
7.9kΩ
10.8kΩ
7.6kΩ
9.9kΩ
5.2kΩ
7.8kΩ
www.austriamicrosystems.com/LED-Driver-ICs/AS1108
Revision 2.13
14 - 20
AMSCO [ AMS(艾迈斯) ]
