LX8940

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.3

3/99

4

APPLICA TION NOTES

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 recom-

mended 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.

Conse-

quently, 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 electro-

lytic capacitors change values at cold temperatures. The effective

capacitance is reduced and regulator stability is effected. Tanta-

lum capacitors are a good choice for these types of environments.

OUTPUT PROTECTION

The output features fault protection against overvoltage 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.

PRODUCTION DATA - Information contained in this document is proprietary to Lin Finity, and is current as of publication date. This document

may not be modified in any way without the express written consent of LinFinity. Product processing does not necessarily include testing of

all parameters. Linfinity reserves the right to change the configuration and performance of the product and to discontinue product at any time.