LX8940

Microsemi

Integrated Products Division

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

Page 4

Copyright

© 2000

Rev. 1.3a, 2005-11-18

5V Low Dropout Regulator

TM

®

APPLICATION NOTE

The advantages of using a low-dropout regulator such as

the LX8940 is the need for less “headroom” for full

regulation, and the inherent reverse polarity protection

provided by the PNP output device. A typical NPN

regulator design requires an input to output differential of

approximately two volts minimum. This is due to the 2Vbe

+ Vcesat of the NPN Darlington used in the output, coupled

with the voltage drop across the current limit resistor. In

contrast, the “PNP Regulator” uses a single series pass

transistor with its single Vcesat, thus the lower input to

output voltage differential or dropout voltage. In some

applications the regulator output voltage is used not only as

a power supply but also as a voltage reference for control

systems. In such cases not just the temperature stability of

the output is important but also the initial accuracy.

LX8940 fills this need as the internal bandgap reference is

trimmed allowing a typical output voltage tolerance of

±1%.

EXTERNAL CAPACITORS

To stabilize the outputs and prevent oscillation (perhaps

by many volts) external capacitors are required. The

minimum recommended value for the output capacitors is

10μF, although the actual size and type will likely vary

according to the particular application, e.g., operating

temperature range and load. Another consideration is the

effective series resistance (ESR) of the

capacitor. Capacitor ESR will vary by manufacturer.

Consequently, some evaluation may be required to

determine the minimum value of the output capacitors.

Generally worst case occurs at the maximum load and

minimum ambient temperature. The size of the output

capacitor can be increased to any value above the

minimum. One possible advantage of this would be to

maintain the output voltage during brief periods of

negative input transients The output capacitors chosen

should be rated for the full range of ambient temperature

over which the circuit will be exposed and expected to

operate. For example, many aluminum type electrolytic

capacitors change values at cold temperatures. The

effective capacitance is reduced and regulator stability is

affected. Tantalum capacitors are a good choice for these

types of environments.

OUTPUT PROTECTION

The output features fault protection against over voltage as

well as a thermal shutdown feature. If the input voltage

rises above 33V (load dump), the output shuts down

automatically. The internal circuitry is thus protected and

the IC is able to survive higher voltage transients than

might otherwise be expected. The thermal shutdown output

effectively guards against overheating of the die and

protects the device from being damaged.