LX8384B-00CPT 데이터 시트보기 (PDF) - Microsemi Corporation

부품명

상세내역

일치하는 목록

LX8384B-00CPT

5A Low Dropout Positive Regulators

Microsemi Corporation 

LX8384x-xx

TM

®

5A Low Dropout Positive Regulators

PRODUCTION DATA SHEET

APPLICATION NOTES (CONTINUED)

OVERLOAD RECOVERY (continued)

voltage across the regulator at the time the short circuit is

removed from the output. If this limited current is not sufficient

to develop the designed voltage across the output resistor, the

voltage will stabilize at some lower value, and will never reach

the designed value. Under these circumstances, it may be

necessary to cycle the input voltage down to zero in order to

make the regulator output voltage return to regulation.

RIPPLE REJECTION

Ripple rejection can be improved by connecting a capacitor

between the ADJ pin and ground. The value of the capacitor

should be chosen so that the impedance of the capacitor is equal

in magnitude to the resistance of R1 at the ripple frequency. The

capacitor value can be determined by using this equation:

C

=

1

(6.28× FR

×

R1)

where:

C ≡ the value of the capacitor in Farads; select

an equal or larger standard value.

FR ≡ the ripple frequency in Hz

R1 ≡ the value of resistor R1 in ohms

At a Ripple frequency of 120Hz, with R1= 100Ω:

C

=

1

(6.28 ×120Hz

×100Ω)

=

13.3μF

The closest equal or larger standard value should be used, in

this case, 15µF. When an ADJ pin bypass capacitor is used,

output ripple amplitude will be essentially independent of the

output voltage. If an ADJ pin bypass capacitor is not used, output

ripple will be proportional to the ratio of the output voltage to the

reference voltage:

M = VOUT

VREF

where: M ≡ a multiplier for the ripple seen when the

ADJ pin is optimally bypassed.

VREF= 1.25V

For example, if VOUT = 2.5V the output ripple will be:

M = 2.5V = 2

1.25V

Output ripple will be twice as bad as it would be if the ADJ

pin were to be bypassed to ground with a properly selected

capacitor.

OUTPUT VOLTAGE

The LX8384/84A/84B ICs develop a 1.25V reference voltage

between the output and the adjust terminal (See Figure 2). By

placing a resistor, R1, between these two terminals, a constant

current is caused to flow through R1 and down through R2 to set

the overall output voltage. Normally this current is the specified

minimum load current of 10mA. Because IADJ is very small and

constant when compared with the current through R1, it

represents a small error and can usually be ignored.

VIN

IN

LX8384x

OUT

VOUT

ADJ

IADJ

50µA

VREF

R1

R2

VOUT

= VREF ⎜⎜⎝⎛1+

R2

R1

⎟⎞

⎠

+

I

ADJ

R

2

FIGURE 2 - BASIC ADJUSTABLE REGULATOR

LOAD REGULATION

Because the LX8384/84A/84B regulators are three-terminal

devices, it is not possible to provide true remote load sensing.

Load regulation will be limited by the resistance of the wire

connecting the regulator to the load. The data sheet specification

for load regulation is measured at the bottom of the package.

Negative side sensing is a true Kelvin connection, with the

bottom of the output divider returned to the negative side of the

load. Although it may not be immediately obvious, best load

regulation is obtained when the top of the resistor divider, (R1), is

connected directly to the case of the regulator, not to the load.

This is illustrated in Figure 3. If R1 were connected to the load,

the effective resistance between the regulator and the load would

be:

RPeff

=

RP

×

⎜⎛

⎝

R2 + R1

R1

⎟⎞

⎠

where: RP ≡ Actual parasitic line resistance.

When the circuit is connected as shown in Figure 3, the

parasitic resistance appears as its actual value, rather than the

higher RPeff.

IN

VIN

LX8384x

RP Parasitic Line

Resistance

OUT

ADJ

R1

Connect R1 to

Case of Regulator

RL

Connect R2 to

R2

Load

FIGURE 3 - CONNECTIONS FOR BEST LOAD REGULATION

Copyright © 2000

Rev. 2.1f, 2006-01-20

Microsemi Inc.

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 6

Share Link: