
FAN2558/FAN2559
PRODUCT SPECIFICATIONS 
6
REV. 1.0.4 3/15/04
Functional Description
Utilizing BiCMOS technology, the FAN2525/FAN2559 
product family is optimized for use in compact battery pow-
ered systems. These LDOs offer a unique combination of 
high ripple rejection, low noise, low power consumption, 
high tolerance for a variety of output capacitors, and less 
than 1μA “OFF” current. In the circuit, a differential current 
sense ampli
fi
er controls a series-pass P-Channel MOSFET to 
achieve high ripple rejection. A separate error ampli
fi
er com-
pares the load voltage at the output with an onboard trimmed 
low voltage bandgap reference for output regulation. 
Thermal shutdown and current limit circuits protect the 
device under extreme conditions. When the device tempera-
ture reaches 150
°
C, the output is disabled. When the device 
cools down by 10
°
C, it is re-enabled. The user can shut down 
the device using the Enable control pin at any time. The cur-
rent limit circuit is trimmed, which leads to consistent power 
on /enable delays, and provides safe short circuit current 
densities even in narrow traces of the PCB.
A carefully optimized control loop accommodates a wide 
range of ESR values in the output bypass capacitor, allowing 
the user to optimize space, cost, and performance require-
ments.
An Enable pin shuts down the regulator output to conserve 
power, reducing supply current to less than 1μA. 
The 
fi
xed-voltage FAN2559 has a noise bypass pin. Power 
Good is available as a diagnostic function to indicate that the 
output voltage has reduced within 5% of the nominal value.
The six pin adjustable-voltage version utilizes pin 5 to con-
nect to an external voltage divider which feeds back to the 
regulator error ampli
fi
er, thus setting the output voltage to 
the desired value. 
Applications Information
External Capacitors – Selection
The FAN2558/FAN2559 gives the user the 
fl
exibility to uti-
lize a wide variety of capacitors compared to other LDOs. 
An innovative design approach offers signi
fi
cantly reduced 
sensitivity to ESR, which degrades regulator loop stability in 
older designs. While the improvements featured in the 
FAN2558/FAN2559 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, and a smaller 
case size than tantalum capacitors.
Input Capacitor
An input capacitor of 2.2μF (nominal value) or greater, 
connected between the Input pin and Ground, placed in close 
proximity to the device, will improve transient response and 
ripple rejection. Higher values will further improve ripple 
rejection and transient response. An input capacitor is rec-
ommended when the input source, either a battery or a regu-
lated AC voltage, is located far from the device. Any good 
quality ceramic, tantalum, or metal 
fi
lm capacitor will give 
acceptable performance; however, in extreme cases capacitor 
surge current ratings may have to be considered. 
Output Capacitor
An output capacitor is required to maintain regulator loop 
stability. 
Stable operation will be achieved with a wide vari-
ety of capacitors with ESR values ranging from 0m
 up to 
400m
. Multilayer ceramic, tantalum or aluminum electro-
lytic capacitors may be used. A nominal value of at least 1μF 
is recommended. Note that the choice of output capacitor 
affects load transient response, ripple rejection, and it has a 
slight effect on noise performance as well.
An internal resistor of approximately 100
 is connected 
between V
OUT
 and GND in shutdown mode, to discharge the 
output capacitor at a faster rate.
Bypass Capacitor (FAN2559 Only)
In the 
fi
xed-voltage con
fi
guration, connecting a capacitor 
between the bypass pin and ground can signi
fi
cantly reduce 
output noise. Values ranging from 0pF to 47nF can be used, 
depending on the sensitivity to output noise in the applica-
tion. 
At the high-impedance Bypass pin, care must be taken in the 
PCB 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 
fi
lm types are recommended for their 
low leakage characteristics. Cost-sensitive applications not 
concerned with noise can omit this capacitor.