Data Sheet

www.austriamicrosystems.com/LED-Driver-ICs/AS1115 Revision 1.08 19 - 25
AS1115
Datasheet - Typic a l A ppli c a t ion
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 Θ JA = 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
VDD = 2.7V
5kΩ 4.4kΩ
VDD = 3.3V
6.7kΩ 6.4kΩ 5.7kΩ
VDD = 3.6V
7.5kΩ 7.2kΩ 6.6kΩ 5.5kΩ
30 6.9kΩ 5.9kΩ 9.1kΩ 8.8kΩ 8.1kΩ 10.18kΩ 9.8kΩ 9.2kΩ 7.5kΩ
20 10.7kΩ 9.6kΩ 13.9kΩ 13.3kΩ 12.6kΩ 15.6kΩ 15kΩ 14.3kΩ 13kΩ
10 22.2kΩ 20.7kΩ 28.8kΩ 27.7kΩ 26kΩ 31.9kΩ 31kΩ 29.5kΩ 27.3kΩ
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
VDD = 4.0V
8.6kΩ 8.3kΩ 7.9kΩ 7.6kΩ 5.2kΩ
VDD = 5.0V
11.35kΩ 11.12kΩ 10.84kΩ 10.49kΩ 10.2kΩ 9.9kΩ
30 11.6kΩ 11.2kΩ 10.8kΩ 9.9kΩ 7.8kΩ 15.4kΩ 15.1kΩ 14.7kΩ 14.4kΩ 13.6kΩ 13.1kΩ
20 17.7kΩ 17.3kΩ 16.6kΩ 15.6kΩ 13.6kΩ 23.6kΩ 23.1kΩ 22.6kΩ 22kΩ 21.1kΩ 20.2kΩ
10 36.89kΩ 35.7kΩ 34.5kΩ 32.5kΩ 29.1kΩ 48.9kΩ 47.8kΩ 46.9kΩ 45.4kΩ 43.8kΩ 42kΩ
ams AG
Technical content still valid