LX8942IDT Microsemi Corporation, LX8942IDT Datasheet - Page 5

• Manufacturer Part Number: LX8942IDT
• Description: Low Dropout Dual Regulator
• Manufacturer: Microsemi Corporation

Copyright © 2005
Rev. 1.2, 2006-01-20
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.
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.
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%.
E
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.
value above the minimum. One possible advantage of this
would be to maintain the output voltage during brief
periods of negative input transients.
XTERNAL
The advantages of using a low-dropout regulator such as
In addition to a low dropout voltage, an important
In some applications the regulator output voltage is used
To stabilize the outputs and prevent oscillation (perhaps
The size of the output capacitor can be increased to any
TM
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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.
S
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.
quiescent current to the IC is very low (<1.5mA) when the
other regulator output is off.
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).
The output capacitors chosen should be rated for the full
TANDBY
The LX8942 differs from most fixed voltage regulators
The standby regulator circuit is designed so that the
If the standby output is not required it can be disabled.
Figure 1 – Disabling Standby Output to Eliminate C3
Low Dropout Dual Regulator
LX8942
Part
O
P
UTPUT
RODUCTION
VIN
D
ATA
10K
R
C3
D
S
HEET
Standby
Output
LX8942
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