AS1118 datasheet(16/21 Pages) AMSCO

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AS1118

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

9 Typical Application

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 Table 22 & Table 23. The maximum current the AS1118 can

drive is 47mA. If higher currents are needed, external drivers must be used, in which case it is no longer necessary

that the devices drive high currents.

Note: The display brightness can also be logically controlled (see Intensity Control Register (0x0A) on page 12).

Calculating Power Dissipation

The upper limit for power dissipation (PD) for the AS1118 is determined from the following equation:

PD = (VDD x 5mA) + (VDD - VLED)(DUTY x ISEG x N)

(EQ 1)

Where:

VDD is the supply voltage.

DUTY is the duty cycle set by intensity register (page 13).

N is the number of segments driven (worst case is 8)

VLED is the LED forward voltage

ISEG = segment current set by RSET

Dissipation Example:

ISEG = 40mA, N = 8, DUTY = 15/16, VLED = 2.2V at 40mA, VDD = 5V

(EQ 2)

PD = 5V(5mA) + (5V - 2.2V)(15/16 x 40mA x 8) = 0.865W

(EQ 3)

Thus, for a QSOP-24 package

ΘJA = +88°C/W, the maximum allowed T AMB is given by:

TJ,MAX = TAMB + PD x

ΘJA = 150°C = T AMB + 0.865W x 88°C/W

(EQ 4)

In this example the maximum ambient temperature must stay below 73.88°C.

Table 22. RSET vs. Segment Current and LED Forward Voltage, VDD = 2.7V & 3.3V & 3.6V

ISEG (mA)

VLED

1.5V

2.0V

1.5V

2.0V

2.5V

1.5V

2.0V

2.5V

3.0V

Ω

4.4k

Ω

6.7k

Ω

6.4k

Ω

5.7k

Ω

7.5k

Ω

7.2k

Ω

6.6k

Ω

5.5k

Ω

6.9k

Ω

5.9k

Ω

9.1k

Ω

8.8k

Ω

8.1k

Ω

10.18k

Ω

9.8k

Ω

9.2k

Ω

7.5k

Ω

10.7k

Ω

9.6k

Ω

13.9k

Ω

13.3k

Ω

12.6k

Ω

15.6k

Ω

15k

Ω

14.3k

Ω

13k

Ω

22.2k

Ω

20.7k

Ω

28.8k

Ω

27.7k

Ω

26k

Ω

31.9k

Ω

31k

Ω

29.5k

Ω 27.3kΩ

Table 23. RSET vs. Segment Current and LED Forward Voltage, VDD = 4.0V & 5.0V

ISEG

(mA)

VLED

1.5V

2.0V

2.5V

3.0V

3.5V

1.5V

2.0V

2.5V

3.0V

3.5V

4.0V

8.6k

Ω

8.3k

Ω

7.9k

Ω

7.6k

Ω

5.2k

Ω

11.35k

Ω 11.12kΩ 10.84kΩ 10.49kΩ 10.2kΩ 9.9kΩ

11.6k

Ω 11.2kΩ 10.8kΩ 9.9kΩ

7.8k

Ω

15.4k

Ω 15.1kΩ 14.7kΩ 14.4kΩ 13.6kΩ 13.1kΩ

17.7k

Ω 17.3kΩ 16.6kΩ 15.6kΩ 13.6kΩ

23.6k

Ω 23.1kΩ 22.6kΩ

22k

Ω 21.1kΩ 20.2kΩ

36.89k

Ω 35.7kΩ 34.5kΩ 32.5kΩ 29.1kΩ

48.9k

Ω 47.8kΩ 46.9kΩ 45.4kΩ 43.8kΩ 42kΩ

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