FS8855
500mA LDO Linear Regulator
Fortune Semiconductor Co.
TEL : +886-2-2809-4742
9/13
TD-0405007
Rev. 1.4
http://www.fsc.com.tw
FAX: +886-2-2809-4874
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The FS8855 is a low-dropout linear regulator. The
device provides preset 2.5V and 3.3V output voltages
for output current up to 500mA. Other mask options
for special output voltages from 1.3V to 4.5V 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 FS8855 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 FS8855
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 FS8855 output voltage is preset at an
internally trimmed voltage 2.5V, 3.3V or can be
mask-optioned from 1.3V to 4.5V with 100mV
increment. The first two digits of part number suffix
identify
the
output
voltage
(see
Ordering
Information). For example, the FS8855-33CL has
a preset 3.3V output voltage.
Current Limit
The FS8855 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 FS8855. 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 FS8855 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 FS8855
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 FS8855 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 FS8855 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 θJA is