PI2211 datasheet(20/26 Pages) VICOR

20 / 26 page

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Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 20 of 26

PI2211

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the difference between the max junction and the max

ambient temperatures is ΔTUSER .

The additional terms for temperature rise and case-to-

ambient ratios will scale the RSOAS resistor value, allowing

for the fine tuning of the PI2211 digital model to the MOSFET.

For example, if the steady-state RθC-A of the MOSFET is less

than the default 60°C/W of the model, the value of RSOAS

would be scaled down by this ratio. This would make the

value of the sense resistor appear to be larger to the PI2211.

For a given voltage drop across the sense resistor, a larger

sense resistor value would mean less apparent current

flowing through it and therefore the calculated power across

the MOSFET is less. By scaling the apparent power a wide

range of MOSFETs can be accurately emulated by the digital

thermal model.

Similarly, if the user has a lower ΔT than the default, the net

result is that the value of the sense resistor appears to be

lower, therefore the current flowing through it appears to be

greater to the SOA model and it will calculate more power

across the MOSFET.

The scaled value of current used to determine RSOAS is used to

calculate the power across the MOSFET when calculating TauP

and in determining the value of RSOAT.

RSOAT , RSOAR

The PI2211 thermal model has a default case-to-ambient

thermal impedance of 60°C/W, and a Tau of 2 seconds. The

junction-to-case

thermal

impedance

and

TauP

are

programmed by the RSOAT and RSOAR resistors. These can have

a maximum value of 20.0k and a minimum value of 1.30k.

RSOAR programs the PI2211 with the junction-to-case (RθJ-C)

thermal impedance of the selected MOSFET. The ratio of the

internal RθC-A (60°C/W) and the user's RθC-A is used to scale

the MOSFET RθJ-C in order to maintain the correct RθJ-C to RθC-

A ratio.

[

]

Where; 0.38 °C/(W∙k) is the SOAR to RθJ-C scaling factor,

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

RθJA-USER is the steady-state junction-to-ambient value

of the MOSFET, taken from the manufacturer's

datasheet,

RθJC-USER is the maximum junction-to-case value of the

MOSFET, taken from the manufacturer's datasheet.

RSOAT programs the PI2211 with the TauP (time constant) of

the MOSFET RθJ-C and its value can be calculated after first

determining the pulse width that will cause a 60°C

temperature rise of the MOSFET junction. This can typically

be done using RθJ-C transient thermal impedance curves

provided by the FET manufacturer.

The power used for determining the pulse width is the

product of the BUS voltage and the start-up current.

Multiplying this power by RθJ-C results in a temperature rise.

Setting the maximum temperature rise to 60°C for the same

power means that the RθJ-C will have to be scaled down. Using

this scaling factor and the RθJ-C thermal impedance curves for

a single pulse, the pulse width can determined. An example

of normalized RθJ-C thermal impedance curves is shown in

Figure 16.

Figure 16 - Determining the pulse width.

PI2211 Datasheet(PDF) 20 Page - Vicor Corporation

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