LX8584x-xx
PRODUCTION DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
Copyright © 1997
Rev. 1.3, 2005-11-11
WWW.Microsemi .COM
7A Low Dropout Positive Regualtors
TM
®
THEORY OF OPERATION
The LX8584/84A/84B series ICs are easy to use Low-Dropout
(LDO) voltage regulators. They have all of the standard self-
protection features expected of a voltage regulator: short circuit
protection, safe operating area protection and automatic thermal
shutdown if the device temperature rises above approximately
165°C.
Use of an output capacitor is REQUIRED with the LX8584 /
84A / 84B series. Please see the table below for recommended
minimum capacitor values.
These regulators offer a more tightly controlled reference voltage
tolerance and superior reference stability when measured against
the older pin-compatible regulator types that they replace.
Stability
The output capacitor is part of the regulator’s frequency
compensation system. Many types of capacitors are available, with
different capacitance value tolerances, capacitance temperature
coefficients, and equivalent series impedances. For all operating
conditions, connection of a 220μF aluminum electrolytic capacitor
or a 47μF solid tantalum capacitor between the output terminal
and ground will guarantee stable operation.
If a bypass capacitor is connected between the output voltage
adjust (ADJ) pin and ground, ripple rejection will be improved
(please see the section entitled “RIPPLE REJECTION”). When
ADJ pin bypassing is used, the required output capacitor value
increases. Output capacitor values of 220μF (aluminum) or 47μF
(tantalum) provide for all cases of bypassing the ADJ pin. If an
ADJ pin bypass capacitor is not used, smaller output capacitor
values are adequate. The table below shows recommended
minimum capacitance values for stable operation.
Recommended Capacitor Values
Input Output Adj
10µF 15µF Tantalum, 100µF Aluminum None
10µF 47µF Tantalum, 200µF Aluminum 15µF
In order to ensure good transient response from the power supply
system under rapidly changing current load conditions, designers
generally use several 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. Cost-effective solutions that sufficiently limit ESR and
ESL effects generally result in total capacitance values in the
range of hundreds to thousands of microfarads, which is more than
adequate to meet regulator output capacitor specifications. Output
capacitance values may be increased without limit.
The circuit shown in Figure 1 can be used to observe the
transient response characteristics of the regulator in a power
system under changing loads. The effects of different capacitor
types and values on transient response parameters, such as
overshoot and undershoot, can be quickly compared in order to
develop an optimum solution.
LX8584/84A
/84B
Power Supply
OUT
IN
ADJ
Star Ground
1 sec
10ms
R
DSON
<< R
L
Full Load
(Smaller resistor)
Minumum Load
(Larger resistor)
Figure 1 – Dynamic Input and Output Test
Overload Recovery
Like almost all IC power regulators, the LX8584/84A/84B
regulators are equipped with Safe Operating Area (SOA)
protection. The SOA circuit limits the regulator's maximum
output current to progressively lower values as the input-to-
output voltage difference increases. By limiting the maximum
output current, the SOA circuit keeps the amount of power that is
dissipated in the regulator itself within safe limits for all values of
input-to-output voltage within the operating range of the
regulator. The LX8584/84A/84B SOA protection system is
designed to be able to supply some output current for all values of
input-to-output voltage, up to the device breakdown voltage.
Under some conditions, a correctly operating SOA circuit may
prevent a power supply system from returning to regulated
operation after removal of an intermittent short circuit at the
output of the regulator. This is a normal mode of operation which
can be seen in most similar products, including older devices such
as 7800 series regulators. It is most likely to occur when the
power system input voltage is relatively high and the load
impedance is relatively low.
When the power system is started “cold”, both the input and
output voltages are very close to zero. The output voltage closely
follows the rising input voltage, and the input-to-output voltage
difference is small. The SOA circuit therefore permits the
regulator to supply large amounts of current as needed to develop
the designed voltage level at the regulator output. Now consider
the case where the regulator is supplying regulated voltage to a
resistive load under steady state conditions. A moderate input-to-
output voltage appears across the regulator but the voltage
difference is small enough that the SOA circuitry allows
sufficient current to flow through the regulator to develop the
designed output voltage across the load resistance. If the output
resistor is short-circuited to ground, the input-to-output voltage
difference across the regulator suddenly becomes larger by the
amount of voltage that had appeared across the load resistor. The
SOA circuit reads the increased input-to output voltage, and cuts
back the amount of current that it will permit the regulator to
supply to its output terminal. When the short circuit across the
output resistor is removed, all the regulator output current will
again flow through the output resistor. The maximum current that
the regulator can supply to the resistor will be limited by the SOA
circuit, based on the large input-to-output voltage across the
regulator at the time the short circuit is removed from the output.
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