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FAN1582D25X View Datasheet(PDF) - Fairchild Semiconductor

Part NameDescriptionManufacturer
FAN1582D25X 3A Adjustable/Fixed Ultra Low Dropout Linear Regulator Fairchild
Fairchild Semiconductor Fairchild
FAN1582D25X Datasheet PDF : 11 Pages
1 2 3 4 5 6 7 8 9 10
FAN1582
PRODUCT SPECIFICATION
General
The FAN1582, FAN1582-1.5, and FAN1582-2.5 are three-
terminal regulators optimized for GTL+ VTT termination and
logic applications. These devices are short-circuit protected,
and offer thermal shutdown to turn off the regulator when the
junction temperature exceeds about 150°C. The FAN1582
series provides low dropout voltage and fast transient
response. Frequency compensation uses capacitors with low
ESR while still maintaining stability. This is critical in
addressing the needs of low voltage high speed microproces-
sor buses like GTL+.
VIN and VCNTL Functions
The FAN1582 utilizes a dual supply approach to maximize
efficiency. The collector of the power device is brought out
to the VIN pin to minimize internal power dissipation under
high current loads. VCNTL provides power for the control
circuitry and the drive for the output NPN transistor. VCNTL
should be at least 1.2 V higher than the output voltage.
Special care was taken to ensure there are no supply-
sequencing problems. The output voltage will not turn on
until both supplies are operating. If the control voltage
comes up first, the output current will be typically limited to
about 3.0 mA until the power input voltage comes up. If the
power input voltage comes up first the output will not turn on
at all until the control voltage comes up. The output can
never come up unregulated.
output pins on the FAN1582 series can handle microsecond
surge currents of 50A to 100A. Even with large value output
capacitors it is difficult to obtain those values of surge
currents in normal operation. Only with large values of out-
put capacitance, such as 1000µF to 5000µF, and with the
input pin instantaneously shorted to ground can damage
occur. A crowbar circuit at the input can generate those levels of
current; a diode from output to input is then recommended,
as shown in Figure 1. Usually, normal power supply cycling
or system “hot plugging and unplugging” will not generate
current large enough to do any damage.
D1
1N4002
(OPTIONAL)
VOUT
VIN +
C1
10µF
Vcntl Vsense
FAN1582
Vin Adj Vout
+
CADJ
+
R1
VOUT
C2
22µF
R2
D1
1N4002
(OPTIONAL)
The FAN1582 can also be used as a single supply device
with the control and power inputs tied together. In this mode,
the dropout is determined by the minimum control voltage.
Stability
The FAN1582 series requires an output capacitor as a part of
the frequency compensation. It is recommended to use a 22µF
solid tantalum or a 100µF aluminum electrolytic on the output
to ensure stability. The frequency compensation of these
devices optimizes the frequency response with low ESR capaci-
tors. In general, it is suggested to use capacitors with an ESR
of <0.3. It is also recommended to use bypass capacitors
such as a 22µF tantalum or a 100µF aluminum on the
adjust pin of the FAN1582 for low ripple and fast transient
response. When these bypassing capacitors are not used at the
adjust pin, smaller values of output capacitors provide equally
good results. A graph showing stability of output capacitance
ESR vs load current can be found under Typical Performance
Characteristics.
Protection Diodes
In normal operation, the FAN1582 series does not require any
protection diodes. For the FAN1582, internal resistors limit
internal current paths on the adjust pin. Therefore, even with
bypass capacitors on the adjust pin, no protection diode is
needed to ensure device safety under short-circuit conditions.
A protection diode between the input and output pins is usu-
ally not needed. An internal diode between the input and the
VCNTL
VIN +
Vcntl Vsense
FAN15821.5, 2.5 +
C1 Vin Gnd Vout
10µF
C2
22µF
VOUT
Figure 1. Optional Protection Diode
Ripple Rejection
In applications that require improved ripple rejection, a bypass
capacitor from the adjust pin of the FAN1582 to ground
reduces the output ripple by the ratio of VOUT/1.25V. The
impedance of the adjust pin capacitor at the ripple frequency
should be less than the value of R1 (typically in the range of
100to 120) in the feedback divider network in Figure 1.
Therefore, the value of the required adjust pin capacitor is a
function of the input ripple frequency. For example, if R1 equals
100and the ripple frequency equals 120Hz, the adjust pin
capacitor should be 22µF. At 10kHz, only 0.22µF is needed.
Output Voltage
The FAN1582 regulator develops a 1.25V reference voltage
between the output pin and the adjust pin (see Figure 2).
Placing a resistor R1 between these two terminals causes a
constant current to flow through R1 and down through R2 to
set the overall output voltage. Normally, this current is the
specified minimum load current of 10mA.
6
REV. 1.1.3 11/9/03
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