www.ams.com/LED-Driver-ICs/AS1105 Revision 1.36 12 - 15
AS1105
Datasheet - Application Information
9.4 Cascading Drivers
The AS1105 can be cascaded as well. The DOUT pin must be connected to the DIN pin of the following AS1105.
9.5 Computing Power Dissipation
The upper limit for power dissipation (PD) for the AS1105 is determined from the following equation:
PD = (VDD x 0.5mA) + (VDD - VLED)(DUTY x I
SEG
x N) (EQ 1)
Where:
VDD = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 4)
VLED = LED forward voltage
I
SEG
= segment current set by RSET
Dissipation Example:
I
SEG
= 40mA, N = 4, DUTY = 31/32, VLED = 1.8V at 40mA, VDD = 5.25V
PD = 5.25V(0.5mA) + (5.25V - 1.8V)(31/32 x 40mA x 4) = 0.54W
Thus, for a SO package θJA = +85ºC/W (see Table 13), the maximum allowed ambient temperature T
A
is given by:
T
J,MAX
= T
A
+ PD x
θ
JA = 150ºC = T
A
+0.54W x 85ºC/W (EQ 2)
Where: T
A
= +104ºC
Table 16. Package Thermal Resistance Data
Package
Thermal Resistance (θ
JA
)
20 Wide SO +85°C/W
Maximum Junction Temperature (TJ) = +150°C
Maximum Ambient Temperature (TA) = +85°C
