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Detail Description
The RS7225 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. Other mask options for special output voltages are also available. As illustrated in function block diagram,
it consists of a 0.95V bandgap reference, an error amplifier, a P-channel pass transistor and an internal feedback voltage
divider.
The 0.95V bandgap reference 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 feedback through an internal resistive divider connected to V
OUT
pin. Additional blocks include an output
current limiter, thermal sensor, and shutdown logic.
Internal P-channel Pass Transistor
The RS7225 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 RS7225 does not suffer
from these problems and consumes only 60μA (Typ.) of current consumption under heavy loads as well as in dropout
conditions.
Enable Function
EN pin starts and stops the regulator. When the EN pin is switched to the power off level, the operation of all internal circuit
stops, the build-in P-channel MOSFET output transistor between pins V
IN
and V
OUT
is switched off, allowing current consumption
to be drastically reduced. The V
OUT
pin enters the GND level through the internal discharge path between V
OUT
and GND pins.
Soft Start Function
The capacitor can be connected between the BYPASS pin and GND. This capacitor can be a low cost Polyester Film variety. A
larger capacitor improves the AC ripple rejection, but also makes the output come up slowly. This "Soft" turn-on is desirable in
some applications to limit turn-on surges.
Fast Discharge Function
The RS7225 fixed type has fast discharge Function on EN pin disable. When user turns off the device, its internal pull-low
resistor will discharge output capacitor charge. It’ll avoid other device to arise wrong motions.
Output Voltage Selection
For fixed voltage type of RS7225, 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 RS7225-33 has a preset 3.3V output
voltage.
Current Limit
The RS7225 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 1.3A (Typ.)
Thermal Overload Protection
Thermal overload protection limits total power dissipation in the RS7225. When the junction temperature exceeds T
J
=+165°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 RS7225 in the event of fault conditions. For continuous operation, the
absolute maximum operating junction temperature rating of T
J
=+125°C should not be exceeded.
DS-RS7225-02
September, 2009
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ORISTER [ ORISTER CORPORATION ]
