Microsemi

Microsemi Microelectronics Division

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

Page 5

Copyright

© 2005

Rev. 1.1, 2005-03-09

LX8821

TM

®

APPLICATION INFORMATION

The LX8821 is part of a family of Dual LDO (Low Drop-Out)

linear regulators in Microsemi’s S-PAK power package which offer

maximum power dissipation in a low profile surface mount

technology. The family includes combination fixed and adjustable

versions. Each channel can supply up to 2.5A independently with a

regulator design optimized for system efficiency by consuming

minimal ground current and directing quiescent current to the load.

INPUT CAPACITOR

To improve load transient response and noise rejection an input

bypass capacitor is of at least 10µF is required. Generally we

recommend a 10µF ceramic or tantalum or 22µF electrolytic

capacitor.

OUTPUT CAPACITOR

The regulator requires output capacitors connected between each

output (V1, V2) to GND to stabilize the internal control loop. Many

types of capacitors are available, with different capacitance values

tolerances, temperature coefficients and equivalent series resistance.

We recommend a minimum of 10µF. To ensure good transient

response from the power supply system under rapidly changing

current load conditions, designers generally use additional output

capacitors connected in parallel. Such an arrangement serves to

minimize the effects of the parasitic resistance (ESR) and

inductance (ESL) that are present in all capacitors. The regulator

has been tested stable with capacitor ESR’s in the range of 0.05 to 2

ohms. We have found it best to use the same type of capacitor for

both input and output bypass.

ADJUSTABLE OUTPUT VOLTAGE

The LX8821 develops a 1.25V reference voltage between the

adjust terminal and GND (See Figure 2). By placing a resistor, R2,

between these two terminals, a constant current is caused to flow

through R1 and down through R2 to set the overall output voltage.

current through R2, it represents a small error and can usually be

ignored.

R1

R2

V

IN

ADJ

V

2

V

REF

1

2

1

1

2

R

I

R

R

V

V

ADJ

REF

+

⎜⎜

⎝

⎛

⎟

⎠

⎞

+

=

I

ADJ

1µA

V

1

Part

LX8821

Figure 2 –

Basic Adjustable Regulator

MINIMUM LOAD REQUIREMENT

The LX8821 has a minimum load requirement for proper output

regulation. This typical current is specified at 0mA for the fixed

output and 1ma for the adjustable output regulators.

TEMPERATURE PROTECTION

The thermal protection shuts the LX8821 down when the junction

temperature exceeds 160°C. Each output has independent thermal

shutdown capability.

Exposure to absolute maximum rated

conditions for extended periods may affect device reliability: see

Thermal Considerations below.

CURRENT LIMIT PROTECTION

The LX8821 includes over current protection; when the output

load current exceeds typically 3A the circuit forces the regulator

decrease in output.

THERMAL CONSIDERATIONS

Thermal shutdown protects the integrated circuit from thermal

overload caused from a rise in junction temperature during power

dissipation. This means of protection is intended for fault protection

only and not as a means of current or power limiting during normal

application usage. Proper thermal evaluation should be done to

ensure that the junction temperature dose not exceed it’s maximum

rating.

reliability.

Due to variation in individual device electrical

characteristics and thermal resistance, the built in thermal overload

protection may be activated at power levels slightly above or below

the rated dissipation. Also peak output power should be considered

for each individual output.

Power dissipation for regulator can be calculated using the

following equation:

2

2

IN(MAX)

1

1

IN(MAX)

D

I

)

V

(V

I

)

V

(V

P

×

−

+

×

−

=

(Note: power dissipation resulting from quiescent (ground) current

is negligible)

Thermal resistance,

on mounting technique using a FR4 copper clad PCB. Junction

temperature of the integrated circuit can be calculated using:

AMB

AMB

TAB

TAB

JUNCTION

JUNCTION

T

T

T

T

+

+

=

−

−

JT

DMAX

TAB

θ

P

T

×

=

;

PCB

2

DREG

1

DREG

AMB

TAB

θ

)

P

(P

T

×

+

=

−

Calculated values:

C

3.3

C/W

4.5

(0.735)

C/W

4.5

(0.21A)

1.5V)

(5V

T

REG1

TAB

J

°

=

°

×

=

°

×

×

−

=

−

C

C/W

C/W

V

V

7.7

4.5

(1.7)

4.5

(1A)

)

3.3

(5

T

REG2

TAB

J

°

=

°

×

=

°

×

×

−

=

−

C

73.1

C/W

30

1.7W)

(0.735W

T

AMB

TAB

°

=

°

×

+

=

−

C

130.8

C

50

C

73.1

C

7.7

T

MAX

-

JUNCTION

°

=

°

+

°

+

°

=

It is important to note that although each output of the regulator

will produce up to 2.5A 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 the maximum junction

temperature is not exceeded.