LX8819 データシートの表示（PDF） - Microsemi Corporation

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LX8819

Dual Channel 1A Low Dropout Regulator

Microsemi Corporation 

LX8819

TM

®

Dual Channel 1A Low Dropout Regulator

DATA SHEET

APPLICATION INFORMATION

Using the calculated total power dissipation, the

maximum junction temperature can be calculated using the

given temperature coefficients (See Thermal Data) for the

selected package.

THERMAL CALCULATION EXAMPLE

Given the following application parameters:

TA = 50°C

VIN(MAX) = 5V + 5%

VOUT1 = 1.5V

VOUT2 = 2.5V

IOUT1(MAX) = IOUT2(MAX) = 500mA

The maximum junction temperature of the LX8819 can be

calculated using the following equation:

( ) TJ = TA + PD(REG1) + PREG2 ⋅ θJA

with:

( ) PD(REG1) = VIN(MAX) − VOUT1 ⋅IOUT1

and

( ) PD(REG2) = VIN(MAX) − VOUT2 ⋅IOUT2

Substituting the known values yields a maximum junction

temperature of:

PD(REG1) = (5.25V − 1.5V)⋅ 0.5A = 1.75W

PD(REG2) = (5.25V − 2.5V)⋅ 0.5A = 1.375W

LAYOUT CONSIDERATION

The layout must be done with low impedance paths for

VIN, VOUT1, VOUT2 and GND by using sufficiently

wide traces to avoid voltage drops and noise pick up. The

output capacitors must be placed as close as possible to the

voltage regulator output pins.

The input capacitor should be connected between VIN

and the ground plane with short leads.

Although it may not be immediately obvious, best load

regulation for the adjustable output is obtained when the

top of the voltage feedback resistor divider (R1) is

connected as close as possible to the case of the regulator;

not to the load.

The PCB copper can be used as a heatsink for the

surface mounted package. Using the minimum size as

shown in the recommended pad layout (Figure 3), limits

the usable power to about 1W for an ambient temperature

of 50°C. Since most applications require greater than 2W

there is the need to provide additional heatsinking. This

can be accomplished by using additional copper area both

on the PCB surface, as shown in the possible heat sink

layout below, or to an embedded ground plane. Since the

die pad (copper tab) is in electrical contact with ground,

the designer can use thermal vias, on the surface of the

PCB, taking advantage of the heat-spreading (Cu) layer of

an internal ground plane.

Board Level

Ground

Plane

( ) TJ = TA + P + P D(REG1) D(REG2) ⋅ θJA

TJ

=

50°C

+

(1.75W

+ 1.375W)⋅ 30

°C

W

TJ = 140°C

Component

Trace

A

B

This value is within the safe operating range of the

device under worst case conditions

It is important to note that although each output of the

regulator will produce up to 1A in current, the individual or

total power dissipation may limit the useful total current

draw. The junction temperature should be calculated for

each individual output as well as the combined outputs to

insure that maximum junction temperature in not exceeded.

B

Figure 3 – Recommended Layout

Component

Component Trace

VIAS

A

Ground Plane

Inner Power Plane

Backside Trace

Figure 4 – Ground Plane Heatsink

Copyright © 2003

Revision 1.0a, 4/29/2005

Microsemi

Integrated Products Division

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

Page 6

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