AD9662_15 Datasheet, PDF (11/16 Pages) Analog Devices – 3-Channel Laser Diode Driver with Oscillator

The change in junction temperature can then be determined.
TJ = T1 + ΔVBE/(1.9 mV/°C)
AD9662
I_CC
V_CC
R_S
5V
–
V1, V2
+
I_BE
1MΩ
ENR
10V
Figure 15. Junction Temperature Measurement Circuit
This 2-point measurement allows the rise in die temperature to
be calculated for any given power dissipation. The θJA of the
system can be calculated using the power dissipation of the LDD.
PD = VCC × ICC – VDIODE × IDIODE
θJA = (TJ – T1)/PD
Figure 16 shows a graph of the measured voltage between ENR
and VCC (VENR − VCC) vs. the die temperature. This graph was
constructed using a 2-layer evaluation board for the AD9662
(see Figure 17).
AD9662
Using the preceding method, actual data was taken to
determine the θJA of the AD9662 in the evaluation board.
Immediately after power-up, V1 was measured to be 593 mV.
The supply current was 27 mA. The AD9662 was adjusted to
deliver 200 mA into a 10 Ω load. This resulted in a total supply
current of 244 mA. After allowing the part to reach thermal
equilibrium, V2 measured 412 mV. The voltage drop across the
120 mΩ internal resistor due to the change in supply current
was then calculated.
(244 mA – 27 mA) × 120 mΩ = 26 mV
This 26 mV internal voltage drop was then added to the
measured voltage reduction to determine the actual ΔVBE.
ΔVBE = (593 mV – 412 mV + 26 mV) = 207 mV
The die temperature change measured 82.4°C. The output of
the AD9662 was at a voltage of 2 V. The part dissipated an
additional 600 mW of power (3 V × 200 mA). The θJA for the
AD9962 mounted on its 2-layer board was calculated to be:
600 mW/82.4°C = 137°C/W.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
20
40
60
80
100
120
140
160
TEMPERATURE (°C)
Figure 16. VENR − VCC vs. Internal Temperature
Rev. C | Page 11 of 16