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RDS ON and RDS SCALAR
Using the V GS @ 4.5V trace, the RDS ON can be taken from the
graph on the left in
intercept. Once that value is chosen, the graph on the right
will determine the scalar of the RDS ON value at the maximum
junction temperature. In this example it is about 1.58X, also
taken using the V GS @ 4.5V trace.
IDC MAX is the DC operating current that will heat the MOSFET
to its maximum junction temperature TJ MAX when the
surrounding environment is at some maximum ambient
temperature. It is found by taking the maximum allowed
temperature rise of the MOSFET, based on the difference
between the maximum MOSFET junction temperature and
the maximum ambient temperature, and dividing this by the
thermal resistance of the MOSFET, R ? J-A . The results will be
the maximum wattage (power) of the MOSFET. Dividing this
wattage by the RDS ON of the MOSFET, at its maximum
junction temperature, will yield the maximum operating
current squared. IDC MAX is calculated as 90% of this value.
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PI2211
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Rθ J-C thermal transfer properties and lower RDS on resistances.
3) PI2211 Resistor Calculations:
R ACC is the resistor used to program the internal current
source. Its value is always 20.0k and should be set using a 1%
tolerance resistor. The power through the resistor is about
30μW.
R TIMER is the resistor that programs the start-up delay timer
after board insertion; DELAY is in the range of 5ms to 100ms.
For a 25ms delay:
SOA Programming Components:
Every MOSFET has its own unique thermal characteristics,
due to die size, lead frame, packaging, etc., and these
characteristics need to be "programmed" into the PI2211
digital model for it to accurately emulate and predict the
junction temperature rise for any given MOSFET. The model
needs to know the MOSFET junction-to-case and junction-to-
ambient thermal impedances, as well as the power through
the MOSFET, in order to accurately predict the MOSFET
junction temperature
rise.
These unique
MOSFET
characteristics are used in conjunction with some built-in
Where; 10Ω is the SOAS current scaling factor,
Where; TJ MAX = Maximum allowed junction temperature
TAMB MAX = Maximum ambient temperature
RDS ON = The MOSFET "On" resistance @ room
temperature.
RDS SCALAR = The MOSFET "On" resistance multiplier @
the maximum junction temperature.
R ? JA = The MOSFET thermal resistance, junction to
ambient.
Recommended MOSFETs:
The ability to program the PI2211 to maintain a MOSFET in its
thermal SOA region allows for a wider selection of MOSFETs
to be used in a hot-swap application.
When selecting a MOSFET, the key features to look for are:
? low RDSON at 4.5V GS
? a minimum V DS voltage rating of 20V
? a Rθ J-C between 0.5°C/W and 7.5°C/W
? the peak pulsed current rating
? packages with metal thermal tabs
The PI2211 is designed to be used with a wide range of
MOSFETs in various packages, but to realize the greatest
efficiency Picor recommends the use of surface mount
MOSFETs with a metal drain tab for the best thermal and
RDS on performance. Packages such as Vishay's PowerPAK SO8
and 1212-8, Infineon's PG-TDSON-8 and PG-TSDSON-8, IR's 5
x 6 mm and 3 x 3 mm PQFN, and other similar tabbed
packages offer MOSFETs with high current ratings, better
default model parameters to accurately model a MOSFET in
the end user's application.
Figure 15 - MOSFET Power measurements.
R SOAS
The PI2211 determines the power across the MOSFET by
measuring the voltage drops across the MOSFET and the
sense resistor, as is shown in Figure 15. The R SOAS resistor is
used to program the PI2211 with the sense resistor's value.
The value range of this programming resistor is 1k minimum,
to 20k maximum.
The equation for calculating R SOAS is:
2
the default case-to- ambient, RΘ CA-DEFAULT , is 60°C/W,
the default temperature rise, ΔT DEFAULT , is 60°C,
Picor Corporation · picorpower.com
PI2211
Rev 1.0, Page 19 of 26
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