PRODUCT SPECIFICATION FAN1587A
REV. 1.0.5 11/10/03 7
Applications Information
General
The FAN1587A, FAN1587A-1.5, and FAN1587A-3.3 are
three-terminal regulators optimized for GTL+ V
TT
termina-
tion 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 FAN1587A
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+.
Stability
The FAN1587A series require 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. It is recommended to use bypass capacitors
such as a 22µF tantalum or a 100µF aluminum on the adjust
pin of the FAN1587A 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 Charac-
teristics for graph of stability of output capacitance ESR vs
load current.
Protection Diodes
In normal operation, the FAN1587A series does not require
any protection diodes. For the FAN1587A, 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 shortcircuit condi-
tions.
A protection diode between the input and output pins is usu-
ally not needed. An internal diode between the input and out-
put pins on the FAN1587A 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 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 11. 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
functional.
Figure 11. Optional Protection
Ripple Rejection
In applications that require improved ripple rejection, a
bypass capacitor from the adjust pin of the FAN1587A to
ground reduces the output ripple by the ratio of V
OUT
/1.25V.
The impedance of the adjust pin capacitor at the ripple fre-
quency should be less than the value of R1 (typically in the
range of 100Ω to 120Ω) in the feedback divider network in
Figure 11. 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 FAN1587A regulator develops a 1.25V reference volt-
age between the output pin and the adjust pin (see Figure
12). 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 very precise output
voltage setting is required.
FAN1587A
ADJ
C
ADJ
R2
R1
65-1587-13
+
C2
22µF
V
OUT
+
C1
22µF
+
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
FAN1587A-1.5, -3.3
GND
C2
22µF
V
OUT
C1
22µF
+
+
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
