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Operating Region and Power Dissipation
Maximum power dissipation of the RS7225 depends on the thermal resistance of the case and circuit board, the temperature
difference between the die junction and ambient air, and the rate of airflow. The power dissipation across the devices is P=IOUT
x (VIN-VOUT). The resulting maximum power dissipation is:
(TJ −TA) (TJ −TA)
PMAX =
=
θJC + θCA
θJA
Where (TJ-TA) is the temperature difference between the RS7301 die junction and the surrounding air, θJC is the thermal
resistance of the package chosen, and θCA is the thermal resistance through the printed circuit board, copper traces and other
materials to the surrounding air. For better heat-sinking, the copper area should be equally shared between the VIN, VOUT, and
GND pins.
Dropout Voltage
A regulator’s minimum input-output voltage differential, or dropout voltage, determines the lowest usable supply voltage. In
battery-powered systems, this will determine the useful end-of-life battery voltage. The RS7225 use a P-channel MOSFET pass
transistor, its dropout voltage is a function of drain-to-source on-resistance RDS(ON) multiplied by the load current.
VDROPOUT =VIN −VOUT = RDS (ON ) × IOUT
DS-RS7225-02
September, 2009
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
