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Detail Description
The RS7213 is a low‐dropout linear regulator. The device provides preset 1.8V, 2.5V and 3.3V output voltages for output
current up to 300mA. Other mask options for special output voltages are also available. As illustrated in function block
diagram, it consists of a 0.87V bandgap reference, an error amplifier, a P‐channel pass transistor and an internal feedback
voltage divider.
The bandgap reference for 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’s 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’s gate is pulled up to decrease the output voltage.
The output voltage is feed back through an internal resistor divider connected to V
OUT
pin. Additional blocks include an
output current limiter, thermal sensor, and shutdown logic.
Internal P‐channel Pass Transistor
The RS7213 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 RS7213 does not suffer
from these problems and consumes only 30μA (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.
Operating Region and Power Dissipation
Maximum power dissipation of the RS7213 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 =
I
OUT
x (V
IN
‐V
OUT
). The resulting maximum power dissipation is:
P
MAX
=
( T
J
−
T
A
) ( T
J
−
T
A
)
=
θ
JC
+ θ
CA
θ
JA
Where (T
J
‐T
A
) is the temperature difference between the RS7213 die junction and the surrounding air,
θ
JC
is the thermal
resistance of the package chosen, and
θ
CA
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 V
IN
,
V
OUT
, and GND pins.
The thermal resistance θ
JA
of SOT‐25 package of RS7213 is 250°C/W. Based on a maximum operating junction temperature
125°C with an ambient of 25°C, the maximum power dissipation will be:
P
MAX
=
( T
J
−
T
A
) (125
−
25)
=
=
0.40W
θ
JC
+ θ
CA
250
Thermal characteristics were measured using a double sided board with 1”x2” square inches of copper area connected to the
GND pin for “heat spreading”.
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 RS7213 use a P‐ channel MOSFET
pass transistor, its dropout voltage is a function of drain‐to‐source on‐resistance R
DS(ON)
multiplied by the load current.
V
DROPOUT
=
V
IN
−
V
OUT
=
R
DS ( ON)
×
I
OUT
DS‐RS7213‐02
September, 2009
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
