19 - 24
AS1115
Datasheet - Typical Application
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, operat-
ing voltages, and LED voltage drop (V
LED
) are given in
Table 22
&
Table 23
. The maximum current the AS1115 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 16)
.
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)
(EQ 1)
Where:
V
DD
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)
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
(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
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
(EQ 4)
In this example the maximum ambient temperature must stay below 123.61°C.
Table 22. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 2.7V & 3.3V & 3.6V
V
LED
1.5V
2.0V
1.5V
40
V
D
10
22.2k
Ω
20.7k
Ω
28.8k
Ω
I
SEG
(mA)
V
LED
2.0V
6.4k
Ω
8.8k
Ω
13.3k
Ω
27.7k
Ω
V
LED
2.5V
5.7k
Ω
8.1k
Ω
12.6k
Ω
26k
Ω
1.5V
7.5k
Ω
10.18k
Ω
15.6k
Ω
31.9k
Ω
2.0V
7.2k
Ω
9.8k
Ω
15k
Ω
31k
Ω
2.5V
6.6k
Ω
9.2k
Ω
14.3k
Ω
29.5k
Ω
3.0V
5.5k
Ω
7.5k
Ω
13k
Ω
27.3k
Ω
5k
Ω
6.9k
Ω
10.7k
Ω
4.4k
Ω
5.9k
Ω
9.6k
Ω
V
D
6.7k
Ω
9.1k
Ω
13.9k
Ω
V
D
30
20
Table 23. R
SET
vs. Segment Current and LED Forward Voltage, V
DD
= 4.0V & 5.0V
V
LED
1.5V
2.0V
2.5V
40
V
D
10
36.89k
Ω
35.7k
Ω
34.5k
Ω
32.5k
Ω
29.1k
Ω
I
SEG
(mA)
V
LED
3.0V
7.6k
Ω
9.9k
Ω
15.6k
Ω
13.6k
Ω
3.5V
5.2k
Ω
7.8k
Ω
1.5V
2.0V
2.5V
3.0V
3.5V
4.0V
8.6k
Ω
11.6k
Ω
17.7k
Ω
8.3k
Ω
11.2k
Ω
17.3k
Ω
7.9k
Ω
10.8k
Ω
16.6k
Ω
V
D
11.35k
Ω
11.12k
Ω
10.84k
Ω
10.49k
Ω
10.2k
Ω
9.9k
Ω
15.4k
Ω
15.1k
Ω
14.7k
Ω
23.6k
Ω
23.1k
Ω
22.6k
Ω
48.9k
Ω
47.8k
Ω
46.9k
Ω
30
20
14.4k
Ω
13.6k
Ω
13.1k
Ω
22k
Ω
21.1k
Ω
20.2k
Ω
45.4k
Ω
43.8k
Ω
42k
Ω