LM1117MPX-ADJ datasheet(9/21 Pages) NSC

9 / 21 page

Typical Performance Characteristics (Continued)

LM1117-5.0 Line Transient Response

10091911

Application Note

1.0 EXTERNAL CAPACITORS/STABILITY

1.1 Input Bypass Capacitor

An input capacitor is recommended. A 10µF tantalum on the

input is a suitable input bypassing for almost all applications.

1.2 Adjust Terminal Bypass Capacitor

The adjust terminal can be bypassed to ground with a by-

pass capacitor (C

ADJ) to improve ripple rejection. This by-

pass capacitor prevents ripple from being amplified as the

output voltage is increased. At any ripple frequency, the

impedance of the C

ADJ should be less than R1 to prevent the

ripple from being amplified:

1/(2

π*f

RIPPLE*CADJ) < R1

The R1 is the resistor between the output and the adjust pin.

Its value is normally in the range of 100-200

Ω. For example,

with R1 = 124

Ω and f

RIPPLE = 120Hz, the CADJ should be >

11µF.

1.3 Output Capacitor

The output capacitor is critical in maintaining regulator sta-

bility, and must meet the required conditions for both mini-

mum amount of capacitance and ESR (Equivalent Series

Resistance). The minimum output capacitance required by

the LM1117 is 10µF, if a tantalum capacitor is used. Any

increase of the output capacitance will merely improve the

loop stability and transient response. The ESR of the output

capacitor should range between 0.3

Ω -22Ω. In the case of

the adjustable regulator, when the C

ADJ is used, a larger

output capacitance (22µf tantalum) is required.

2.0 OUTPUT VOLTAGE

The LM1117 adjustable version develops a 1.25V reference

voltage, V

REF, between the output and the adjust terminal.

As shown in Figure 1, this voltage is applied across resistor

R1 to generate a constant current I1. The current I

ADJ from

the adjust terminal could introduce error to the output. But

since it is very small (60µA) compared with the I1 and very

constant with line and load changes, the error can be ig-

nored. The constant current I1 then flows through the output

set resistor R2 and sets the output voltage to the desired

level.

For fixed voltage devices, R1 and R2 are integrated inside

the devices.

3.0 LOAD REGULATION

The LM1117 regulates the voltage that appears between its

output and ground pins, or between its output and adjust

pins. In some cases, line resistances can introduce errors to

the voltage across the load. To obtain the best load regula-

tion, a few precautions are needed.

Figure 2, shows a typical application using a fixed output

regulator. The Rt1 and Rt2 are the line resistances. It is

obvious that the V

LOAD is less than the VOUT by the sum of

the voltage drops along the line resistances. In this case, the

load regulation seen at the R

LOAD would be degraded from

the data sheet specification. To improve this, the load should

be tied directly to the output terminal on the positive side and

directly tied to the ground terminal on the negative side.

10091917

FIGURE 1. Basic Adjustable Regulator

LM1117MPX-ADJ Datasheet(PDF) 9 Page - National Semiconductor (TI)

LM1117MPX-ADJ Datasheet(HTML) 9 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL