VITESSE
SEMICONDUCTOR CORPORATION
Advance Product Information
SDH/SONET 2.5Gb/s
Laser Diode Driver
VSC7927
Calculation of the Maximum Case Temperature
The VSC7927 is designed to operate with a maximum junction temperature of 125°C. The rise from the
case to junction is determined by the power dissipation of the device. The power dissipation is determined by
the V current plus the operating I
and I
currents.
BIAS
SS
MOD
The power of the chip is determined by the following formula:
= (-V * I ) + ((V – V ) * I
P
) + ((V
– V ) * I
)
BIAS
D
SS
SS
IOUT
SS
MOD
IBIAS
SS
For example with:
V
=
=
=
=
=
-5.2V
SS
I
I
40mA
20mA
-2.0V
-2.0V
MOD
BIAS
V
V
IBIAS
IOUT
P
=
=
(-5.2 * 150mA) + ((5.2 - 2.0) * 40mA) + ((5.2 - 2.0) * 20mA)
D
-
P
780mW + 128mW + 64mW = 972mW
D
The thermal rise from junction to case is θ * P . For the metal glass package, θ = 32 °C/W. Thus the
JC
D
JC
thermal rise is:
32°C/W * 1.336W = 31.1°C
The maximum case temperature is:
125°C – 31.1°C = 93.9°C
The absolute maximum power dissipation of the device is at:
V
=
=
=
=
=
-5.5V
60mA
50mA
0V
SS
I
I
MOD
BIAS
V
V
IBIAS
IOUT
0V
P
=
(5.5 * 150mA) + (5.5 * 60mA) + (5.5mA * 50mA)
P
= 1.43W
D
D
This will net a maximum junction to case thermal rise of: 1.43W * 32°C/W = 45.8°C
This situation will allow maximum case temperature of: 125°C – 58°C = 79.2°C
G52201-0, Rev 2.2
10/13/99
VITESSE SEMICONDUCTOR CORPORATION
741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896
Page 5
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