www.austriamicrosystems.com/LED-Driver-ICs/AS1115

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AS1115

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 into 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, operating voltages, and LED volt-

age drop (VLED) are given in Table 22 & Table 23. The maximum current the AS1115 can drive is 47mA. If higher currents are needed, exter-

nal 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 16).

Calculating Power Dissipation

The upper limit for power dissipation (PD) for the AS1115 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 17).

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 TQFN(4x4)-24 package

ΘJA = +30.5°C/W, the maximum allowed TAMB is given by:

TJ,MAX = TAMB + PD x

ΘJA = 150°C = TAMB + 0.865W x 30.5°C/W

(EQ 4)

In this example the maximum ambient temperature must stay below 123.61°C.

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

ISEG (mA)

VLED

VLED

VLED

1.5V

2.0V

1.5V

2.0V

2.5V

1.5V

2.0V

2.5V

3.0V

40

5k

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

30

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

20

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

10

22.2k

Ω

20.7k

Ω

28.8k

Ω

27.7k

Ω

26k

Ω

31.9k

Ω

31k

Ω

29.5k

Ω

27.3k

Ω

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

ISEG

(mA)

VLED

VLED

1.5V

2.0V

2.5V

3.0V

3.5V

1.5V

2.0V

2.5V

3.0V

3.5V

4.0V

40

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

30

11.6k

Ω

11.2k

Ω

10.8k

Ω

9.9k

Ω

7.8k

Ω

15.4k

Ω

15.1k

Ω

14.7k

Ω

14.4k

Ω

13.6k

Ω 13.1kΩ

20

17.7k

Ω

17.3k

Ω

16.6k

Ω

15.6k

Ω 13.6kΩ

23.6k

Ω

23.1k

Ω

22.6k

Ω

22k

Ω

21.1k

Ω 20.2kΩ

10

36.89k

Ω

35.7k

Ω

34.5k

Ω

32.5k

Ω 29.1kΩ

48.9k

Ω

47.8k

Ω

46.9k

Ω

45.4k

Ω

43.8k

Ω

42k

Ω