AS1108 datasheet(15/21 Pages) TAOS

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AS1108

Datasheet - Ty p i c a l A p p l i c a t i o n

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

ISEG (mA)

VLED(V)

1.5

2.0

Ω

4.4k

Ω

6.9k

Ω

5.9k

Ω

10.7k

Ω

9.6k

Ω

22.2k

Ω

20.7k

Ω

Table 20. RSET vs. Segment Current and LED Forward Voltage, VDD = 3.3V

ISEG (mA)

VLED(V)

1.5

2.0

2.5

6.7k

Ω

6.4k

Ω

5.7k

Ω

9.1k

Ω

8.8k

Ω

8.1k

Ω

13.9k

Ω

13.3k

Ω

12.6k

Ω

28.8k

Ω

27.7k

Ω

26k

Ω

Table 21. RSET vs. Segment Current and LED Forward Voltage, VDD = 3.6V

ISEG (mA)

VLED(V)

1.5

2.0

2.5

3.0

7.5k

Ω

7.2k

Ω

6.6k

Ω

5.5k

Ω

10.18k

Ω

9.8k

Ω

9.2k

Ω

7.5k

Ω

15.6k

Ω

15k

Ω

14.3k

Ω

13k

Ω

31.9k

Ω

31k

Ω

29.5k

Ω

27.3k

Ω

Table 22. RSET vs. Segment Current and LED Forward Voltage, VDD = 4.0V

ISEG (mA)

VLED(V)

1.5

2.0

2.5

3.0

3.5

8.6k

Ω

8.3k

Ω

7.9k

Ω

7.6k

Ω

5.2k

Ω

11.6k

Ω

11.2k

Ω

10.8k

Ω

9.9k

Ω

7.8k

Ω

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