www.austriamicrosystems.com/LED-Driver-ICs/AS1106_07

Revision 2.29

15 - 20

AS1106, AS1107

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

9.3 Calculating the Power Dissipation

The upper limit for power dissipation (PD) for the AS1106/AS1107 is determined from the following equation:

PD = (VDD x 1mA) + (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 12).

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 = 31/32, VLED = 1.8V at 40mA, VDD = 5.25V

(EQ 2)

PD = 5.25V(1mA) + (5.25V - 1.8V)(31/32 x 40mA x 8) = 1.075W

(EQ 3)

Thus, for a PDIP package

ΘJA = +75°C/W (from Table 24), the maximum allowed TAMB is given by:

TJ,MAX = TAMB + PD x

ΘJA = 150°C = TAMB +1.07W x 75°C/W

(EQ 4)

Where:

TAMB = +69.4°C.

The TAMB limit for SO Packages in the dissipation example above is +58.6°C.

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

ISEG (mA)

VLED (V)

1.5

2.0

2.5

3.0

3.5

4.0

40

11.35k

Ω

11.12k

Ω

10.84k

Ω

10.49k

Ω

10.2k

Ω

9.9k

Ω

30

15.4k

Ω

15.1k

Ω

14.7k

Ω

14.4k

Ω

13.6k

Ω

13.1k

Ω

20

23.6k

Ω

23.1k

Ω

22.6k

Ω

22k

Ω

21.1k

Ω

20.2k

Ω

10

48.9k

Ω

47.8k

Ω

46.9k

Ω

45.4k

Ω

43.8k

Ω

42k

Ω

Table 24. Package Thermal Data

Package

Thermal Resistance (

ΘJA)

24 Wide SOIC

+85°C/W