FAN1084 Fairchild Semiconductor, FAN1084 Datasheet - Page 4

• Manufacturer Part Number: FAN1084
• Description: 4.5A Adjustable/Fixed Low Dropout Linear Regulator
• Manufacturer: Fairchild Semiconductor

FAN1084
Applications Information
General
The FAN1084, FAN1084-1.5, and FAN1084-3.3 are three-
terminal regulators optimized for GTL+ V
and logic applications. These devices are short-circuit pro-
tected, and offer thermal shutdown to turn off the regulator
when the junction temperature exceeds about 150°C. The
FAN1084 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 micro-
processor buses like GTL+.
Stability
The FAN1084 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 capacitors.
In general, it is suggested to use capacitors with an ESR of
<0.2 . It is also recommended to use bypass capacitors such
as a 22µF tantalum or a 100µF aluminum on the adjust pin of
the FAN1084 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.
Protection Diodes
In normal operation, the FAN1084 series does not require
any protection diodes. For the FAN1084, 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 condi-
tions.
A protection diode between the input and output pins is
usually not needed. An internal diode between the input and
the output pins on the FAN1084 series can handle micro-
second 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
output capacitance, such as 1000 F to 5000 F, 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 recom-
mended, as shown in Figure 2. Usually, normal power supply
cycling or system “hot plugging and unplugging” will not
generate current large enough to do any damage.
4
TT
termination
The adjust pin can be driven on a transient basis ±7V with
respect to the ouput, without any device degradation. As with
any IC regulator, exceeding the maximum input-to-output
voltage differential causes the internal transistors to break
down and none of the protection circuitry is then functional.
Ripple Rejection
In applications that require improved ripple rejection, a bypass
capacitor from the adjust pin of the FAN1084 to ground
reduces the output ripple by the ratio of V
impedance of the adjust pin capacitor at the ripple frequency
should be less than the value of R1 (typically in the range of
100 to 120 ) in the feedback divider network in Figure 2.
Therefore, the value of the required adjust pin capacitor is a
function of the input ripple frequency. For example, if R1
equals 100 and the ripple frequency equals 120Hz, the
adjust pin capacitor should be 22µF. At 10kHz, only 0.22µF is
needed.
Output Voltage
The FAN1084 regulator develops a 1.25V reference voltage
between the ouput pin and the adjust pin (see Figure 3). 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.
V
V
IN
IN
+
+
C1
10 F
C1
10 F
Figure 2. Optional Protection
IN
IN
FAN1084-1.5
(OPTIONAL)
(OPTIONAL)
FAN1084
1N4002
1N4002
+
GND
ADJ
D1
D1
C
ADJ
OUT
OUT
PRODUCT SPECIFICATION
+
REV. 1.0.6 11/18/02
OUT
R1
R2
C2
22 F
/1.25V. The
+
C2
22 F
V
V
OUT
OUT