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| 型号： | FAN8800 |
| 文件页数： | 5/191页 |
| 文件大小： | 2018K |
| 代理商： | FAN8800 |
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FAN1084
PRODUCT SPECIFICATION
4
P
Applications Information
General
The FAN1084 and FAN1084-1.5 are three-terminal regula-
tors optimized for GTL+ V
TT
termination and logic applica-
tions. These devices are short-circuit protected, and offer
thermal shutdown to turn off the regulator when the junction
temperature exceeds about 150°C. The FAN1084 series pro-
vides low dropout voltage and fast transient response. Fre-
quency compensation uses capacitors with low ESR while
still maintaining stability. This is critical in addressing the
needs of low voltage high speed microprocessor 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
<1
. 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 usu-
ally not needed. An internal diode between the input and the
output pins on the FAN1084 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 cur-
rents in normal operation. Only with large values of output
capacitance, such as 1000
μ
F to 5000
μ
F, and with the input
pin instantaneously shorted to ground can damage occur. A
crowbar circuit at the input can generate those levels of cur-
rent; a diode from output to input is then recommended, as
shown in Figure 2. Usually, normal power supply cycling or
system “hot plugging and unplugging” will not generate cur-
rent large enough to do any damage.
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.
Figure 2. Optional Protection
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
OUT
/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
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.
FAN1084
ADJ
C2
22
μ
F
V
OUT
+
C1
10
μ
F
+
+
IN
OUT
D1
1N4002
(OPTIONAL)
V
IN
FAN1084-1.5
IN
GND
C2
22
μ
F
V
OUT
+
C1
10
μ
F
+
OUT
D1
1N4002
(OPTIONAL)
V
IN
R1
R2
C
ADJ
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