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PI2211 Application/Theory of Operation
Maintaining a MOSFET within its SOA boundary:
The PI2211 has a programmable digital model of a MOSFET
thermal response to transient and static loads that it uses to
predict a junction temperature rise, as a function of power,
for a given MOSFET. The equivalent analog model is shown in
thermal equivalent of the junction-to-case and case-to-
ambient characteristics of the MOSFET and its package. In the
model, the case-to-ambient characteristics are fixed while the
junction-to-case can be tuned to match the published data
for a specific MOSFET by two programming resistors; R SOAT
and R SOAR .
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PI2211
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The R SOAT resistor controls the time constant (Tau P ) of the SOA
junction-to-case model. This resistor programs the model to
adhere to the manufacturer’s transient thermal impedance
graph of the junction-to-case response to "single pulse"
power changes, as well as the extended SOA curves beyond
the DC area limit. This instantaneous power calculation
refreshes in less than 50μs and predicts junction temperature
rise within the 1ms extended SOA MOSFET curves so the
PI2211 will protect the MOSFET from prolonged heating with
excessive static loads and hot-spotting from transient loads.
The R SOAR resistor programs the model with the R ? J-C of the
MOSFET. Scaled by the ratio of the junction-to-case/case-to-
ambient thermal impedances (R ? J-C /R ? C-A ), referenced to the
fixed internal 60°C/W R ? C-A of the PI2211.
The R SOAS resistor programs the magnitude of the calculated
current through the MOSFET and the power it is dissipating.
All three of these resistors have a maximum value 20 kΩ and a
1.30 kΩ minimum value. Values outside of these ranges will
not stop the PI2211 from working, but will force the internal
references to either their minimum or maximum values. See
the Recommended Design Steps section for more details on
calculating the required SOA programming resistors.
Figure 2 - Simplified representation of the PI2211 thermal
model.
10
R θ J-C =
Tau= 1ms
7.5 ° C/W
Tau= 15ms
1
R θ J-C =
0.5 ° C/W
Tau= 1ms
0.1
0.01
Tau= 15ms
1E-05
0.0001
0.001
0.01
0.1
Time
.5°C/W, Tau = 1ms
.5°C/W, Tau = 15ms
7.5°C/W, Tau = 1ms
7.5°C/W, Tau = 15ms
Figure 3 - The range of PI2211 R θ J-C thermal impedance adjustment at 1W.
Picor Corporation · picorpower.com
PI2211
Rev 1.0, Page 9 of 26
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