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Detail Description
The RS7100 is a low‐dropout linear regulator. The device provides preset 2.5V and 3.3V output voltages for output current up
to 300mA. Other mask options for special output voltages from 1.5V to 5.0V with 100mV increment are also available. As
illustrated in function block diagram, it consists of a 1.25V reference, error amplifier, a P‐channel pass transistor, and an
internal feedback voltage divider.
The 1.25V 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 with
output current limiter and shutdown logic.
Internal P‐channel Pass Transistor
The RS7100 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 conditions when the pass transistor saturates, and use high base‐drive currents under large loads. The RS7100 does
not suffer from these problems and consumes only 65μA (Typical) of ground pin current under heavy loads as well as in
dropout conditions.
Output Voltage Selection
The RS7100 output voltage is preset at an internally trimmed voltage 2.5V or 3.3V or can be mask optioned from 1.5V to 5.0V
with 100mV increment The first two digits of part number suffix identify the output voltage (see Ordering Information). For
example, RS7100‐33 has a preset 3.3V output voltage.
Current Limit
The RS7100 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 600mA.
Thermal Overload Protection
Thermal overload protection limits total power dissipation in the RS7100. When the junction temperature exceeds T
J
=+155°C,
a thermal sensor turns off the pass transistor, allowing the IC to cool down. The thermal sensor turns the pass transistor
active 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 RS7100 in the event of fault conditions. For continuous operation, the
maximum operating junction temperature rating of T
J
=+125°C should not be exceeded.
Operating Region and Power Dissipation
Maximum power dissipation of the RS7100 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:
(T
J
−
T
A
) (T
J
−
T
A
)
=
P
MAX
=
θ
JC
+ θ
CA
θ
JA
Where (T
J
‐T
A
) is the temperature difference between the RS7100 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.
If the RS7100 uses a SOT‐89 package and this package is mounted on a double sided printed circuit board with two square
inches of copper allocated for “heat spreading”, the resulting θ
JA
is 180°C/W.
DS‐RS7100‐14
April, 2010
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
