LX8586 / LX8586A

Microsemi

Integrated Products Division

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

Page 4

Copyright

© 1996

Rev. 1.0a, 2005-11-10

6A Low Dropout Positive Regulators

TM

®

APPLICATION NOTE

The LX8586/86A 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 LX8586/86A

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.

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.

Input

Output

Adj

10µF

15µF Tant, 100µF Alum.

None

10µF

47µF Tant, 220µF Alum

15µF

Chart –

Recommended Capacitor Values

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.

LX8586/86A

Power Supply

OUT

IN

ADJ

Star Ground

1 sec

10ms

R

Full Load

(Smaller resistor)

Minumum Load

(Larger resistor)

Figure 1 -

Dynamic Input and Output Test

Like almost all IC power regulators, the LX8586/86A

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 LX8586/86A 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.