LX8942

Microsemi

Integrated Products Division

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

Page 5

Copyright

© 2005

Rev. 1.2, 2006-01-20

Low Dropout Dual Regulator

TM

®

APPLICATION NOTE

The advantages of using a low-dropout regulator such as

the LX8942 are 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 addition to a low dropout voltage, an important

advantage of the LX8942 series is low quiescent current in

the standby mode. When the high current or primary

regulator is shut off, the regulator enters a micropower

mode. Here all but the most essential circuitry to power the

standby output is deactivated. This allows the lowest

possible quiescent current (typical around 1.2mA), a vital

factor when used in a battery powered system.

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. The LX8942 fills this need as the internal

bandgap reference is trimmed allowing a typical output

voltage tolerance of ±1%.

To stabilize the outputs and prevent oscillation (perhaps

by many volts) external capacitors are required. The

minimum recommended value for the output capacitors is

47μF for the 8.2V output and 100µF for the 5V output. 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 will freeze at -30°C.

The effective capacitance is reduced to zero in such a

situation. Capacitors rated for -40°C operation must be

used in order to maintain regulator stability at that

temperature. Tantalum capacitors satisfy this requirement.

The LX8942 differs from most fixed voltage regulators

in that it is equipped with two regulator outputs instead of

one. The additional output is intended for use in systems

requiring standby memory circuits. While the high current

regulator output can be controlled with the ON/OFF pin

described below, the standby remains on under all

conditions as long as sufficient input voltage is applied to

the IC. Thus, memory and other circuits powered by this

output remain unaffected by positive line transients,

thermal shutdown, etc.

The standby regulator circuit is designed so that the

quiescent current to the IC is very low (<1.5mA) when the

other regulator output is off.

If the standby output is not required it can be disabled.

This is accomplished by connecting a resistor from the

standby output to the supply voltage, thereby also

eliminating the requirement for a more expensive output

capacitor to prevent unwanted oscillations. The resistor

value depends upon the minimum input voltage expected

for a given system. Since the standby output is shunted

with an internal 5.6V zener, the current through the

external resistor should be sufficient to bias internal

resistors up to this point. Approximately 60mA will

suffice, resulting in a 10k external resistor for most

applications (Figure 1).

R

D

10K

C3

Part

LX8942

VIN

Standby

Output

Figure 1 –

Disabling Standby Output to Eliminate C3