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AS1100

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

9.4 Calculating the Power Dissipation

The upper limit for power dissipation (PD) for the AS1100 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 8)

VLED = LED forward voltage

ISEG = segment current set by RSET

Dissipation Example:

ISEG = 40mA, N = 8, 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 8) = 1.07W

Thus, for a PDIP package

θJA = +75°C/W (from Table 13), the maximum allowed ambient temperature TA is given by:

TJ,MAX = TA + PD x θJA = 150°C = TA +1.07W x 75°C/W.

Where:

TA = +69.7°C.

The TA limit for SO Packages in the dissipation example above is +59.0°C.

Table 16. Package Thermal Resistance Data

Package

Thermal Resistance (

θJA)

24 Wide SO

+85ºC/w

Maximum Junction Temperature (TJ) = +150ºC

Maximum Ambient Temperature (TA) = +85ºC