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For example: A MOSFET with a R θ J-C of 2.5°C/W, a BUS
voltage of 12V, and a start-up current limit of 6.25A will yield
the following R θ J-C scaling factor:
Using the thermal impedance curves shown in Figure 16 and
a R θ J-C scalar of 0.32, the intersection with the single pulse
curve produces a pulse width of 1.4ms. To summarize, a
1.4ms pulse of 75W across this particular MOSFET should
raise the junction to case temperature by 60°C.
Knowing the pulse width, Tau P can be calculated using the
equation below:
Where; PULSE is the pulse width taken from the R θ J-C thermal
impedance curves,
R θ C-A is the default junction to case thermal
impedance of 60°C/W,
R θ J-C is the maximum junction-to-case value of the
MOSFET scaled by the ratio of the FET's R θ C-A and the
default R θ C-A .
Power is the nominal BUS voltage multiplied by the
current programmed via R SOAS .
Power = BUS * R SOAS * .0025
The power across the MOSFET is calculated by the PI2211 as
the product of the BUS voltage and the programmed current
as determined by R SOAS .
Knowing Tau P , R SOAT can be calculated using the equation
below:
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PI2211
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longer to reach their normalized value, even though they
might be in the same package. As this single pulse time
increases, then effect of the R θ J-A will become more dominate
and the R θ J-A single-pulse thermal impedance curve should be
used to determine the pulse width for a 60°C junction rise.
4) Under and Over-Voltage Programming:
As is shown in Figure 7, there is a programmable window of
BUS voltage range where the PI2211 is guaranteed to be
operational. This window is defined by the OV pin voltage
being ≤ 0.675Vdc and the UV/EN pin voltage being ≥
0.750Vdc. The user should be aware that VBUS MAX , when
calculated using the following equations, is the maximum
voltage where the controller is guaranteed to be operational,
not the maximum voltage where the controller faults.
Figure 17 - Resistor divider options for UV and OV threshold
programming.
To calculate the OV and UV resistor divider ratios for separate
networks:
Using a three resistor string (R HI , R MID , R LO ); when VBUS MIN the
voltage on UV is 0.75V, when VBUS MAX the voltage on OV is
0.675V. The ratio of the R MID to R LOW can be determined once
the ratio of VBUS MAX to VBUS MIN is set.
Using junction-to-case thermal impedance curves is a good
method to determine the duration of the power pulse since
the junction-to-case heats up much faster than the case-to-
ambient and they are typically the most accurate thermal
curves published.
The MOSFET in Figure 16 reaches its normalized R θ J-C value at
about 50ms. MOSFETs with larger die sizes can take much
Picor Corporation · picorpower.com
PI2211
Rev 1.0, Page 21 of 26
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