NCP1117LP
www.onsemi.com
10
APPLICATIONS INFORMATION
Introduction
The NCP1117LP is a low dropout positive fixed or
adjustable mode regulator with 1 A output capability. This
LDO is guaranteed to have 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 NCP1117LP series is
pin compatible with the older NCP1117.
Output Voltage
The typical application circuits for the fixed and
adjustable output regulators are shown in Figures 33 and 34.
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
required for regulation. Since the adjust pin current, I
adj
, is
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 ground current I
gnd
is 550 mA (typ).
External Capacitors
Input bypass capacitor C
in
may be required for regulator
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.
Figure 33. Fixed Output Regulator
1
2
Output
3
Input
NCP1117LP
++
C
out
C
in
I
gnd
Frequency compensation for the regulator is provided by
capacitor C
out
and its use is mandatory to ensure output
stability. A minimum capacitance value of 4.7 mF with an
equivalent series resistance (ESR) that is within the limits of
20 mW to 20 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.
Figure 34. Adjustable Output Regulator
1
2
Output
3
Input
NCP1117LP
++
C
out
C
in
I
adj
R2
+
C
adj
V
ref
R1
V
out
+ V
ref
ǒ
1 )
R2
R1
Ǔ
) R2 @ I
adj
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 5 V
regulator, the output ripple will increase by 5 V/1.25 V or 4
and the ripple rejection will decrease by 20 log of this ratio
or 12 dB. The loss of ripple rejection can be restored to the
values shown with the addition of bypass capacitor C
adj
,
shown in Figure 34. The reactance of C
adj
at the ripple
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.
C
adj
u
1
2p @ f
ripple
@ R1
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
adj
is:
f
ripple +
120 Hz, R1 + 120 W, then C
adj
u 11.1 mF
The value for C
adj
is significantly reduced in applications
where the input ripple frequency is high. If used as a post
regulator in a switching converter under the following
conditions:
f
ripple
+ 50 kHz, R1 + 120 W, then C
adj
u 0.027 mF
