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Detail Description
The RS7300 is a low‐dropout linear regulator. The device provides preset 1.8V, 2.5V and 3.3V output voltages for output
current up to 600mA. Adjustable output voltage and other mask options for special output voltages are also available. As
illustrated in function block diagram, it consists of a 1.25V bandgap (Fixed voltage type is 0.95V) reference, an error amplifier,
a P‐channel pass transistor and an internal feedback voltage divider.
The bandgap reference for adjustable voltage type is connected to the error amplifier, which compares this reference with
the feedback voltage and amplifies the voltage difference. If the feedback voltage is lower than the reference voltage, the
pass transistor gate is pulled lower, which allows more current to pass to the output pin and increases the output voltage. If
the feedback voltage is too high, the pass transistor gate is pulled up to decrease the output voltage.
The output voltage is feed back through an internal resistive divider (or external resistive divider for adjustable output
voltage type) connected to VOUT pin. Additional blocks include an output current limiter, thermal sensor, and shutdown logic.
Internal P‐channel Pass Transistor
The RS7300 features a P‐channel MOSFET pass transistor. Unlike similar designs using PNP pass transistors, P‐channel
MOSFETs require no base drive, which reduces quiescent current. PNP‐based regulators also waste considerable current in
dropout when the pass transistor saturates, and use high base‐drive currents under large loads.
The RS7300 does not suffer from these problems and consumes only 60μA (Typ.) of current consumption under heavy loads
as well as in dropout conditions.
Output Voltage Selection
For fixed voltage type of RS7300, the output voltage is preset at an internally trimmed voltage. The first two digits of part
number suffix identify the output voltage (see Ordering Information). For example, the RS7300‐33 has a preset 3.3V output
voltage.
For adjustable voltage type of RS7300, the output voltage is set by comparing the feedback voltage at adjust terminal to the
internal bandgap reference voltage. The reference voltage VREF is 1.25V. The output voltage is given by the equation:
VOUT=VREF x (1+R2/R1)+IADJ x R2
Current Limit
The RS7300 also includes a fold back current limiter. It monitors and controls the pass transistor’s gate voltage, estimates the
output current, and limits the output current within 1.0A.
Thermal Overload Protection
Thermal overload protection limits total power dissipation in the RS7300. When the junction temperature exceeds TJ=+150°C,
a thermal sensor turns off the pass transistor, allowing the IC to cool down. The thermal sensor turns the pass transistor on
again after the junction temperature cools down by 20°C, resulting in a pulsed output during continuous thermal overload
conditions.
Thermal overload protection is designed to protect the RS7300 in the event of fault conditions. For continuous operation, the
absolute maximum operating junction temperature rating of TJ=+125°C should not be exceeded.
Operating Region and Power Dissipation
Maximum power dissipation of the RS7300 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:
(T
J
−T
A
)
(TJ
−T )
A
PMAX
=
=
θ
JC + θCA
θ
JA
Where (TJ‐TA) is the temperature difference between the RS7300 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.
DS‐RS7300‐14 September, 2009
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