FAN1585A PRODUCT SPECIFICATION
6 REV. 1.1.6 7/8/05
Applications Information
General
The FAN1585A and FAN1585A-1.5 are three-terminal regu-
lators optimized for GTL+ V
TT
termination and logic appli-
cations. These devices are short-circuit protected and offer
thermal shutdown to turn off the regulator when the junction
temperature exceeds about 150°C. The FAN1585A 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 microprocessor buses
like GTL+.
Stability
The FAN1585A series requires an output capacitor as a part
of the frequency compensation. It is recommended to use a
22µF solid tantalum or a 100 µF 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 <300mΩ. It is also recommended to use
bypass capacitors such as a 22µF tantalum or a 100µF alumi-
num on the adjust pin of the FAN1585A for low ripple and
fast transient response. When these bypassing capacitors are
not used at the adjust pin, larger values of output capacitors
provide equally good results. Refer to Typical Performance
Characteristics for graph of stability of output capacitance
ESR vs load current.
Protection Diodes
In normal operation, the FAN1585A series does not require
any protection diodes. For the FAN1585A, 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 FAN1585A 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 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 cur-
rent; a diode from output to input is then recommended, as
shown in Figure 10. Usually, normal power supply cycling
or system “hot plugging and unplugging” will not generate
current large enough to do any damage.
The adjust pin can be driven on a transient basis ±7V with
respect to the output, 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 func-
tional.
Figure 10. Optional Protection
Ripple Rejection
In applications that require improved ripple rejection, a
bypass capacitor from the adjust pin of the FAN1585A 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 100Ω to 120Ω) in the feedback divider network
in Figure 10. Therefore, the value of the required adjust pin
capacitor is a function of the input ripple frequency. For
example, if R1 equals 100Ω and the ripple frequency equals
120Hz, the adjust pin capacitor should be 22µF. At 10kHz,
only 0.22µF is needed.
Output Voltage
The FAN1585A regulator develops a 1.25V reference volt-
age between the output pin and the adjust pin (see Figure
11). 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.
The current out of the adjust pin adds to the current from R1
and is typically 35µA. Its output voltage contribution is
small and only needs consideration when a very precise out-
put voltage setting is required.
FAN1585A
ADJ
C
ADJ
R2
R1
+
C2
22µF
V
OUT
+
C1
22µF
+
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
FAN1585A-1.5
GND
C2
22µF
V
OUT
C1
22µF
+ +
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
