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FAN2508SX View Datasheet(PDF) - Fairchild Semiconductor

Part NameDescriptionManufacturer
FAN2508SX 50 mA CMOS LDO Regulators Fairchild
Fairchild Semiconductor Fairchild
FAN2508SX Datasheet PDF : 10 Pages
1 2 3 4 5 6 7 8 9 10
Depending on the model selected, a number of control and
status functions are available to enhance the operation of the
LDO regulator. An Enable pin, available on all devices,
allows the user to shut down the regulator output to conserve
power, reducing supply current to less than 1µA. The
adjustable-voltage versions of the device utilize pin 4 to
connect to an external voltage divider which feeds back to
the regulator error amplifier, thereby setting the voltage as
desired. Two other functions are available at pin 4 in the
fixed-voltage versions: in noise-sensitive applications, an
external Bypass capacitor connection is provided that allows
the user to achieve optimal noise performance at the output,
while the Error output functions as a diagnostic flag to
indicate that the output voltage has dropped more than 5%
below the nominal fixed voltage.
Applications Information
External Capacitors – Selection
The FAN2508/09 allows the user to utilize a wide variety of
capacitors compared to other LDO products. An innovative
design approach offers significantly reduced sensitivity to
ESR (Effective Series Resistance), which degrades regulator
loop stability in older designs. While the improvements fea-
tured in the FAN2508/09 family greatly simplify the design
task, capacitor quality still must be considered if the designer
is to achieve optimal circuit performance. In general,
ceramic capacitors offer superior ESR performance, at a
lower cost and a smaller case size than tantalums. Those with
X7R or Y5Vdielectric offer the best temperature coefficient
characteristics. The combination of tolerance and variation
over temperature in some capacitor types can result in signif-
icant variations, resulting in unstable performance over rated
Input Capacitor
An input capacitor of 2.2µF (nominal value) or greater,
connected between the Input pin and Ground, located in
close proximity to the device, will improve transient
response and noise rejection. Higher values will offer supe-
rior input ripple rejection and transient response. An input
capacitor is recommended when the input source, either a
battery or a regulated AC voltage, is located far from the
device. Any good quality ceramic, tantalum, or metal film
capacitor will give acceptable performance, however tanta-
lum capacitors with a surge current rating appropriate to the
application must be selected to avoid catastrophic failure.
Output Capacitor
An output capacitor is required to maintain regulator loop
stability. Unlike many other LDO regulators, the
FAN2508/09 family of products are nearly insensitve to
output capacitor ESR. Stable operation will be achieved with
a wide variety of capacitors with ESR values ranging from
10mto 10or more. Tantalum or aluminum electrolytic,
or multilayer ceramic types can all be used. A nominal value
of at least 1µF is recommended.
Bypass Capacitor (FAN2508 Only)
In the fixed-voltage configuration, connecting a capacitor
between the bypass pin and ground can significantly reduce
noise on the output. Values ranging from 470pF to 10nF can
be used, depending on the sensitivity to output noise in the
At the high-impedance Bypass pin, care must be taken in the
circuit layout to minimize noise pickup, and capacitors must
be selected to minimize current loading (leakage). Noise
pickup from external sources can be considerable. Leakage
currents into the Bypass pin will directly affect regulator
accuracy and should be kept as low as possible; thus, high-
quality ceramic and film types are recommended for their
low leakage characteristics. Cost-sensitive applications not
concerned with noise can omit this capacitor.
Control Functions
Enable Pin
Applying a voltage of 0.4V or less at the Enable pin will
disable the output, reducing the quiescent output current to
less than 1µA, while a voltage of 2.0V or greater will enable
the device. If this shutdown function is not needed, the pin
can simply be connected to the VIN pin. Allowing this pin to
float will cause erratic operation.
Error Flag (FAN2509 only)
To indicate conditions such as input voltage dropout
(low VIN), overheating, or overloading (excessive output cur-
rent), the ERR pin indicates a fault condition. It is an open-
drain output which is HIGH when the voltage at VOUT is
greater than 95% of the nominal rated output voltage and
LOW when VOUT is less than 95% or the rated output volt-
age, as specified in the error trip level characteristics.
A logic pullup resistor of 100Kis recommended at this
output. The pin can be left disconnected if unused.
Thermal Protection
The FAN2508/09 is designed to supply high peak output cur-
rents of up to 1A for brief periods, however this output load
will cause the device temperature to increase and exceed
maximum ratings due to power dissipation. During output
overload conditions, when the die temperature exceeds the
shutdown limit temperature of 150°C, onboard thermal
protection will disable the output until the temperature drops
below this limit, at which point the output is then re-enabled.
During a thermal shutdown situation the user may assert the
power-down function at the Enable pin, reducing power
consumption to the minimum level IGND · VIN.
REV. 1.0.6 2/14/03
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