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AS1105

Datasheet - Application In form atio n

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 ISEG 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

ISEG = segment current set by RSET

Dissipation Example:

ISEG = 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 TA is given by:

TJ,MAX = TA + PD x

θJA = 150ºC = TA +0.54W x 85ºC/W

(EQ 2)

Where: TA = +104ºC

Table 16. Package Thermal Resistance Data

Package

Thermal Resistance (

θJA)

20 Wide SO

+85°C/W

Maximum Junction Temperature (TJ) = +150°C

Maximum Ambient Temperature (TA) = +85°C