RC1587 PRODUCT SPECIFICATION
6 REV. 1.5
Applications Information
General
The RC1587, RC1587-1.5, and RC1587-3.3 are three-terminal
regulators optimized for GTL+ V
TT
termination applications.
These devices are short-circuit protected, and offer thermal
shutdown to turn off the regulator when the junction temper-
ature exceeds about 150°C. The RC1587 series provides low
dropout voltage and fast transient response. Frequency com-
pensation uses capacitors with low ESR while still maintain-
ing stability. This is critical in addressing the needs of low
voltage high speed microprocessor buses like GTL+.
Stability
The RC1587 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. 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 RC1587 for low ripple and fast
transient response. When these bypassing capacitors are not
used at the adjust pin, larger values of output capacitors pro-
vide equally good results.
Protection Diodes
In normal operation, the RC1587 series does not require any
protection diodes. For the RC1587, 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 conditions.
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 RC1587 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 10. 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 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 10. Optional Protection
Ripple Rejection
In applications that require improved ripple rejection, a
bypass capacitor from the adjust pin of the RC1587 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 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 RC1587 regulator develops a 1.25V reference voltage
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 very precise output
voltage setting is required.
RC1587
ADJ
C
ADJ
R2
R1
65-1587-13
+
C2
22µF
V
OUT
+
C1
10µF
+
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
RC1587-1.5, -3.3
GND
C2
22µF
V
OUT
C1
10µF
+ +
IN OUT
D1
1N4002
(OPTIONAL)
V
IN
