NCP1117DTAT5G(2014) 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1117DTAT5G
(Rev.:2014)

1.0 A Low-Dropout Positive Fixed and Adjustable Voltage Regulators

ON Semiconductor 

NCP1117, NCV1117

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.

Frequency compensation for the regulator is provided by

capacitor Cout 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

33 mW (typ) to 2.2 W is required. See Figures 12 and 13. 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

The typical application circuits for the fixed and

adjustable output regulators are shown in Figures 23 and 24.

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, Iadj, 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 Ignd, ranges from 3.0 mA to 5.0 mA

depending upon the output voltage.

External Capacitors

Input bypass capacitor Cin 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.

Input

Cin

3 NCP1117 2 Output

XTXX

+

1

+

Cout

Ignd

Figure 23. Fixed Output Regulator

Input

Cin

3 NCP1117 2

XTA

+

1

Vref

Iadj

R2

Output

R1

+

Cout

+

Cadj

ǒ Ǔ Vout + Vref

1

)

R2

R1

) Iadj R2

Figure 24. Adjustable Output Regulator

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

Cadj, shown in Figure 24. The reactance of Cadj 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.

Cadj

u

2

p

1

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

transformer and a full wave bridge, the value for Cadj is:

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

The value for Cadj 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:

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.

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