NCP1117, NCV1117
APPLICATIONS INFORMATION
Introduction
Frequency compensation for the regulator is provided by
The NCP1117 features a significant reduction in dropout
voltage along with enhanced output voltage accuracy and
temperature stability when compared to older industry
standard three−terminal adjustable regulators. These
devices contain output current limiting, safe operating area
compensation and thermal shutdown protection making
them designer friendly for powering numerous consumer
and industrial products. The NCP1117 series is pin
compatible with the older LM317 and its derivative device
types.
capacitor C and its use is mandatory to ensure output
out
stability. A minimum capacitance value of 4.7 mF with an
equivalent series resistance (ESR) that is within the limits of
0.25 W to 2.2 W is required. The capacitor type can be
ceramic, tantalum, or aluminum electrolytic as long as it
meets the minimum capacitance value and ESR limits over
the circuit’s entire operating temperature range. Higher
values of output capacitance can be used to enhance loop
stability and transient response with the additional benefit of
reducing output noise.
Output Voltage
Input
C
Output
3
2
NCP1117
XTA
The typical application circuits for the fixed and
adjustable output regulators are shown in Figures 20 and 21.
The adjustable devices are floating voltage regulators. They
develop and maintain the nominal 1.25 V reference voltage
between the output and adjust pins. The reference voltage is
programmed to a constant current source by resistor R1, and
this current flows through R2 to ground to set the output
voltage. The programmed current level is usually selected to
be greater than the specified 5.0 mA minimum that is
+
+
R1
C
C
V
in
out
ref
1
I
adj
+
adj
R2
R2
ref ǒ1 ) Ǔ) I
V
+ V
R2
out
adj
R1
Figure 21. Adjustable Output Regulator
required for regulation. Since the adjust pin current, I , is
adj
significantly lower and constant with respect to the
programmed load current, it generates a small output
voltage error that can usually be ignored. For the fixed
output devices R1 and R2 are included within the device and
The output ripple will increase linearly for fixed and
adjustable devices as the ratio of output voltage to the
reference voltage increases. For example, with a 12 V
regulator, the output ripple will increase by 12 V/1.25 V or
9.6 and the ripple rejection will decrease by 20 log of this
ratio or 19.6 dB. The loss of ripple rejection can be restored
to the values shown with the addition of bypass capacitor
the ground current I , ranges from 3.0 mA to 5.0 mA
gnd
depending upon the output voltage.
External Capacitors
Input bypass capacitor C may be required for regulator
in
C
adj
, shown in Figure 21. The reactance of C at the ripple
adj
stability if the device is located more than a few inches from
the power source. This capacitor will reduce the circuit’s
sensitivity when powered from a complex source impedance
and significantly enhance the output transient response. The
input bypass capacitor should be mounted with the shortest
possible track length directly across the regulator’s input
and ground terminals. A 10 mF ceramic or tantalum
capacitor should be adequate for most applications.
frequency must be less than the resistance of R1. The value
of R1 can be selected to provide the minimum required load
current to maintain regulation and is usually in the range of
100 W to 200 W.
1
C
u
adj
2 p f
R1
ripple
The minimum required capacitance can be calculated
from the above formula. When using the device in an
application that is powered from the AC line via a
transformer and a full wave bridge, the value for C is:
adj
Input
Output
3
2
f
120 Hz, R1 + 120 W, then C
u 11.1 mF
adj
NCP1117
XTXX
ripple +
+
+
The value for C is significantly reduced in applications
adj
C
C
out
in
1
where the input ripple frequency is high. If used as a post
regulator in a switching converter under the following
conditions:
I
gnd
f
+ 50 kHz, R1 + 120 W, then C
u 0.027 mF
adj
ripple
Figure 20. Fixed Output Regulator
Figures 10 and 11 shows the level of ripple rejection that
is obtainable with the adjust pin properly bypassed.
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