PI2211 datasheet(19/26 Pages) VICOR

19 / 26 page





Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 19 of 26

PI2211



RDSON and RDSSCALAR

Using the VGS @ 4.5V trace, the RDSON can be taken from the

graph on the left in

Figure 14, using the desired operating current as an X

intercept. Once that value is chosen, the graph on the right

will determine the scalar of the RDSON value at the maximum

junction temperature. In this example it is about 1.58X, also

taken using the VGS @ 4.5V trace.

IDCMAX is the DC operating current that will heat the MOSFET

to its maximum junction temperature TJMAX 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 RDSON of the MOSFET, at its maximum

junction temperature, will yield the maximum operating

current squared. IDCMAX is calculated as 90% of this value.

√

Where; TJMAX = Maximum allowed junction temperature

TAMBMAX = Maximum ambient temperature

RDSON = The MOSFET "On" resistance @ room

temperature.

RDSSCALAR = 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 4.5VGS



a minimum VDS voltage rating of 20V



a RθJ-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

RDSon 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

RθJ-C thermal transfer properties and lower RDSon resistances.

3) PI2211 Resistor Calculations:

RACC 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.

RTIMER 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

default model parameters to accurately model a MOSFET in

the end user's application.

Figure 15 - MOSFET Power measurements.

RSOAS

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 RSOAS 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 RSOAS is:

[

] [

]

Where; 10Ω

2 is the SOAS current scaling factor,

the default case-to-ambient, RΘCA-DEFAULT, is 60°C/W,

the default temperature rise, ΔTDEFAULT, is 60°C,

PI2211 Datasheet(PDF) 19 Page - Vicor Corporation

PI2211 Datasheet(HTML) 19 Page - Vicor Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL