ISL58831 Datasheet, PDF (10/12 Pages) Intersil Corporation – Dual Laser Driver with APC Amplifier and Spread Spectrum Oscillator

ISL58831
Furthermore, the case-to-ambient thermal coefficient may not be
known precisely.
To assist in worst case conditions, it is possible to monitor the
silicon temperature of the ISL58831 by forcing current into the
ENABLE pin, which will then be at a voltage of V_DD + V_PN, where
V_PN is the forward biassed voltage of the ESD protection diode.
Since ENABLE = HI is necessary for normal operation, the device
can be operated as it would be in the real-life applications, while
the temperature is monitored. The ISL58831 has been calibrated
with a 1MΩ resistor to +10V connected in series with the ENABLE
pin, which results in an input current of approximately 4.5µA.
Figure 7 allows the silicon temperature to be determined directly.
The graph shows the measured ENABLE pin to VDD pin
differential voltage, which shows a linear voltage sensitivity of
-2.26mV/°C. Users may wish to measure their specific part at
+20°C (no warm-up) to allow for any statistical/process
distribution, but the method is reliable and accurate.
By applying this method to the ISL58831 in an actual
application, users can measure the silicon temperature under all
operating conditions to determine whether their thermal
engineering is sufficient. The thermal resistance of the QFN24 is
+140°C/W when tested on a standard JEDEC JESD51-3 (single
layer) test board. When using a standard JEDEC JESD51-7 (four
layer) test board, the thermal resistance is +112°C/W. Actual
thermal resistance is highly dependent on circuit board layout
considerations.
Temperature Measurement Set-Up and
Results
Example: Measure ENABLE - V_DD under coolest condition of
V_DD = 0V and V_ENABLE = 5V through 1MΩ. Suppose the result
was 580mV at T_AMBIENT = +20°C.
Now measure ENABLE - V_DD under the actual operating
conditions. Suppose result (must be after thermal equilibrium
has been reached) is 450mV, and the new I_CC value is 100mA.
Now one can calculate the temperature rise of (450 to 580)/
-2.26 = +57°C. Using the power dissipation of
PW = (V_DD * I_CC) - (I_CC * V_DD), the _JA of the application can be
calculated.
600
550
500
450
400
350
300
ENA WITH 1MΩ TO +10V
250
0
25
50
75 100 125 150
SILICON TEMPERATURE (°C)
1M
+10V
V
+5V
ENABLE
ISL58831
V_DD
FIGURE 7. ISL58831 ON-CHIP THERMOMETER
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FN7440.1
January 28, 2016