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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 R SET Resistor Value and Using External Drivers
Brightness of the display segments is controlled via R SET . The current that flows into I SET defines the current that flows through the LEDs.
Segment current is about 200 times the current in I SET . Typical values for R SET for different segment currents, operating voltages, and LED volt-
age drop (V LED ) 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) .
Table 22. R SET vs. Segment Current and LED Forward Voltage, V DD = 2.7V & 3.3V & 3.6V
I SEG (mA)
1.5V
V LED
2.0V
1.5V
V LED
2.0V
2.5V
1.5V
2.0V
V LED
2.5V
3.0V
40
30
20
10
5k ?
6.9k ?
10.7k ?
22.2k ?
4.4k ?
5.9k ?
9.6k ?
20.7k ?
6.7k ?
9.1k ?
13.9k ?
28.8k ?
6.4k ?
8.8k ?
13.3k ?
27.7k ?
5.7k ?
8.1k ?
12.6k ?
26k ?
7.5k ?
10.18k ?
15.6k ?
31.9k ?
7.2k ?
9.8k ?
15k ?
31k ?
6.6k ?
9.2k ?
14.3k ?
29.5k ?
5.5k ?
7.5k ?
13k ?
27.3k ?
Table 23. R SET vs. Segment Current and LED Forward Voltage, V DD = 4.0V & 5.0V
I SEG
V LED
V LED
(mA)
40
30
20
10
1.5V
8.6k ?
11.6k ?
17.7k ?
36.89k ?
2.0V
8.3k ?
11.2k ?
17.3k ?
35.7k ?
2.5V
7.9k ?
10.8k ?
16.6k ?
34.5k ?
3.0V
7.6k ?
9.9k ?
15.6k ?
32.5k ?
3.5V
5.2k ?
7.8k ?
13.6k ?
29.1k ?
1.5V
11.35k ?
15.4k ?
23.6k ?
48.9k ?
2.0V
11.12k ?
15.1k ?
23.1k ?
47.8k ?
2.5V
10.84k ?
14.7k ?
22.6k ?
46.9k ?
3.0V
10.49k ?
14.4k ?
22k ?
45.4k ?
3.5V
10.2k ?
13.6k ?
21.1k ?
43.8k ?
4.0V
9.9k ?
13.1k ?
20.2k ?
42k ?
Calculating Power Dissipation
The upper limit for power dissipation (PD) for the AS1115 is determined from the following equation:
PD = (V DD x 5mA) + (V DD - V LED )(DUTY x I SEG x N)
Where:
V DD is the supply voltage.
N is the number of segments driven (worst case is 8)
V LED is the LED forward voltage
I SEG = segment current set by RSET
Dissipation Example:
I SEG = 40mA, N = 8, DUTY = 15/16, V LED = 2.2V at 40mA, V DD = 5V
PD = 5V(5mA) + (5V - 2.2V)(15/16 x 40mA x 8) = 0.865W
Thus, for a TQFN(4x4)-24 package Θ JA = +30.5°C/W, the maximum allowed T AMB is given by:
T J,MAX = T AMB + PD x Θ JA = 150°C = T AMB + 0.865W x 30.5°C/W
In this example the maximum ambient temperature must stay below 123.61°C.
(EQ 1)
(EQ 2)
(EQ 3)
(EQ 4)
Revision 1.08
19 - 25
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