PI2211 datasheet(21/26 Pages) VICOR

21 / 26 page

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Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 21 of 26

PI2211

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For example: A MOSFET with a RθJ-C of 2.5°C/W, a BUS

voltage of 12V, and a start-up current limit of 6.25A will yield

the following RθJ-C scaling factor:

Using the thermal impedance curves shown in Figure 16 and

a RθJ-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, TauP can be calculated using the

equation below:

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]

Where; PULSE is the pulse width taken from the RθJ-C thermal

impedance curves,

RθC-A is the default junction to case thermal

impedance of 60°C/W,

RθJ-C is the maximum junction-to-case value of the

MOSFET scaled by the ratio of the FET's RθC-A and the

default RθC-A .

Power is the nominal BUS voltage multiplied by the

current programmed via RSOAS.

Power = BUS * RSOAS * .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 RSOAS.

Knowing TauP, RSOAT can be calculated using the equation

below:

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 RθJ-C value at

about 50ms. MOSFETs with larger die sizes can take much

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 RθJ-A will become more dominate

and the RθJ-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 VBUSMAX , 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 (RHI, RMID, RLO); when VBUSMIN the

voltage on UV is 0.75V, when VBUSMAX the voltage on OV is

0.675V. The ratio of the RMID to RLOW can be determined once

the ratio of VBUSMAX to VBUSMIN is set.

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