Microsemi

Integrated Products

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

Page 6

Copyright

 2000

Rev. 1.2a, 2002-11-01

LX8816

Dual Channel 1A Low Dropout Regulator

I N T E GR A T ED

PRO DUC T S

APPLICATION INFORMATION

Description

The LX8816 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 one amp 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 a input bypass

capacitor is of at least 2.2uF is required. Generally we recommend a

20uF ceramic or tantalum or 22uF electrolytic capacitor.

Output Capacitor

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 4.7uF. 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 LX8816 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

very small and constant when compared with the current through R2, it

represents a small error and can usually be ignored.

LX8816-04

R1

R2

V

IN

ADJ

V

1

FIGURE 2 - BASIC ADJUSTABLE

REGULATOR

V

REF

1

2

1

1

1

R

I

R

R

V

V

ADJ

REF

+













+

=

I

ADJ

1µA

V

2

Minimum Load Requirement

The LX8816 has a minimum load is requirement for proper output

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

output and 2ma for the adjustable output regulators.

Temperature Protection

The thermal protection shuts the LX8816 down when the junction

shutdown capability. Exposure to absolute maximum rated conditions

for extended periods may affect device reliability, see Thermal

Considerations below.

Current Limit Protection

The LX8816 includes over current protection, when the output load

current exceeds typically 1.4A 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.

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:

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

For the S-PAK package, thermal resistance,

depending on mounting technique when mounted on a FR4 copper

clad PCB. Junction temperature of the integrated circuit can be

calculated using:

Calculated values:

= (5V-2.5V) * (210mA) * 4.5°C/W = (0.525) * 4.5°C/W = 2.4°C

= (5V-3.3V) * (1.0A) * 4.5°C/W = (1.7) * 4.5°C/W = 7.7°C

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 the maximum junction temperature in not

exceeded.