PI2125 datasheet(15/20 Pages) VICOR

15 / 20 page

The PI2125 internal MOSFET power dissipation can

be calculated with the following equation:

)

(

Rds

MOSFET

∗

Where:

: Source Current

Rds(on) : MOSFET on-state resistance

Note:

Calculate with Rds(on) at maximum MOSFET

temperature because Rds(on) is

temperature

dependent, Refer to figure 11 for normalized

Rds(on) values over temperature. PI2125 nominal

Rds(on) at 25°C is 3mΩ and will increase by 35% at

125°C junction temperature.

The junction temperature rise is a function of power

dissipation and thermal resistance.

)

(

Rds

Rth

Trise

MOSFET

∗

Where:

: Junction-to-Ambient thermal

resistance (54°C/Watt)

Rth

(3)

This may require iteration to get to the final junction

temperature.

Figures 13, 14, and 17 show the

PI2125 internal MOSFET final junction temperature

curves versus conducted current at given ambient

temperatures and air flow.

OV/UV resistor selection:

The UV and OV comparators inputs are used to

monitor the input voltage and will indicate a fault

condition when this voltage is out of range. It can be

used to monitor the input voltage (Vin) level if the

PI2125 is referenced to the same ground as Vin. If

the PI2125 is floating on Vin then OV and UV can

not monitor Vin and should be disabled by

connecting UV to the VC pin and OV to the GND pin

of the same PI2125.

The Fault pin ( FT ) will indicate a fault (active low)

when the UV pin is below the threshold or when the

OV pin is above the threshold. The threshold is

0.50V typical with 25mV hysteresis and the input

current is less than ±1µA. It is important to consider

the maximum current that will flow in the resistor

divider and maximum error due to UV and OV input

current. Set the resistor current to 100µA or higher

to maintain better than 1% accuracy for UV and OV

due to the bias current.

The three-resistor voltage divider configuration for

both UV and OV to monitor the same voltage node

is shown in Figure 20:

)

(

Set

value based on system allowable current

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

(

)

(

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

(

)

(

Where:

)

(

: UV threshold voltage at Vin.

)

(

: OV threshold voltage at Vin.

)

(UV

: UV voltage set

current.

Figure 20:

UV & OV three-resistor divider

configuration

Alternatively,

two-resistor

voltage

divider

configuration can be used and is shown in Figure

21.

Figure 21

: UV & OV two-resistor divider

configuration

UV resistor voltage divider from the following

equations:

RUV

)

(

Set

value based on system allowable current

RUV

100

≥

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

)

(

)

(

Where:

V(UVTH) : UV Threshold Voltage

RUV

current

Picor Corporation • picorpower.com

PI2125

Rev 1.0

Page 15 of 20

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