FS8856
550mA LDO Linear Regulator
Fortune Semiconductor Co.
TEL : +886-2-2809-4742
9/13
TD-0405008
Rev. 1.3
http://www.fsc.com.tw
FAX: +886-2-2809-4874
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The FS8856 is a low-dropout linear regulator. The
device provides preset 2.5V and 3.3V output
voltages for output current up to 550mA. Other
mask options for special output voltages from 1.3V
to 5.0V with 100mV increment are also available.
As illustrated in function block diagram, it consists
of a 1.25V reference, error amplifier, 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 OUT pin. Additional
blocks include an output current limiter, thermal
sensor, and shutdown logic.
Internal P-channel Pass Transistor
The FS8856 features a P-channel MOSFET pass
transistor. Unlike similar designs using PNP pass
transistors, P-channel MOSFETs require no base
drive,
which
reduces
ground
pin
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 FS8856 does not suffer from these
problems and consumes only 65µA (Typ.) of
ground pin current under heavy loads as well as in
dropout conditions.
Output Voltage Selection
The FS8856 output voltage is preset at an
internally trimmed voltage 2.5V, 3.3V or can be
mask-optioned from 1.3V to 5.0V with 100mV
increment. The first two digits of part number suffix
identify
the
output
voltage
(see
Ordering
Information). For example, the FS8856-33CL has
a preset 3.3V output voltage.
Current Limit
The FS8856 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 0.8A.
Thermal Overload Protection
Thermal overload protection limits total power
dissipation in the FS8856. When the junction
temperature exceeds TJ = +155°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 FS8856 in the event of fault conditions. For
continuous operation, the maximum operating
junction temperature rating of TJ = +125°C should
not be exceeded.
Operating Region and Power Dissipation
Maximum power dissipation of the FS8856
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:
(
) ()
JA
A
J
CA
JC
A
J
MAX
T
T
T
T
P
θ
θ
θ
−
=
+
−
=
Where (TJ-TA) is the temperature difference
between the FS8856 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 IN, OUT, and GND pins.
If the FS8856 uses a SOT-89 package and this
package is mounted on a double sided printed