rc1585t-1.5 Fairchild Semiconductor, rc1585t-1.5 Datasheet - Page 6

• Manufacturer Part Number: rc1585t-1.5
• Description: 5a Adjustable/fixed Low Dropout Linear Regulator
• Manufacturer: Fairchild Semiconductor

RC1585
Applications Information
General
The RC1585 and RC1585-1.5 are three-terminal regulators
optimized for GTL+ V
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 RC1585 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 RC1585 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 RC1585 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 RC1585 series does not require any
protection diodes. For the RC1585, 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 RC1585 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 current; a diode from output to input is then recom-
mended, as shown in Figure 10. Usually, normal power sup-
ply 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.
6
TT
termination and logic applica-
Ripple Rejection
In applications that require improved ripple rejection, a
bypass capacitor from the adjust pin of the RC1585 to
ground reduces the output ripple by the ratio of V
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 RC1585 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 a very precise out-
put voltage setting is required.
V
V
IN
IN
10µF
10µF
C1
C1
+
Figure 10. Optional Protection
+
IN
IN
(OPTIONAL)
(OPTIONAL)
RC1585-1.5
RC1585
1N4002
1N4002
GND
+
ADJ
D1
D1
C
OUT
OUT
ADJ
PRODUCT SPECIFICATION
+
R1
R2
C2
22µF
REV. 1.1.4 10/8/02
+
V
OUT
OUT
65-1585-12
C2
22µF
V
/1.25V.
OUT