NCP1117, NCV1117

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8

APPLICATIONS INFORMATION

Introduction

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.

Output Voltage

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

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

depending upon the output voltage.

External Capacitors

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 20. Fixed Output Regulator

1

2

Output

3

Input

NCP1117

XTXX

++

Frequency compensation for the regulator is provided by

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.

Figure 21. Adjustable Output Regulator

1

2

Output

3

Input

NCP1117

XTA

++

R2

+

R1

Vout + Vref 1 )

R2

R1

) Iadj R2

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

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.

Cadj u

1

2

p fripple 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

fripple

+ 120 Hz, R1 + 120 W, then Cadj u 11.1 mF

where the input ripple frequency is high. If used as a post

regulator in a switching converter under the following

conditions:

fripple + 50 kHz, R1 + 120 W, then Cadj u 0.027 mF

Figures 10 and 11 shows the level of ripple rejection that

is obtainable with the adjust pin properly bypassed.