LX8585-xx/8585A-xx

PRODUCT DA T ABOOK 1996/1997

5

Copyright © 1997

Rev. 2.2

12/97

APPLICA TION NOTES

The LX8585/85A 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 LX8585/85A

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 compen-

sation 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

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 Tantalum, 100µF Aluminum

None

10µF

47µF Tantalum, 220µF Aluminum

15µF

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 gener-

ally 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 under-

shoot, can be compared quickly in order to develop an optimum

solution.

RECOMMENDED CAPACITOR VALUES

FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST

LX8585/85A

Power Supply

OUT

IN

ADJ

Star Ground

1 sec

10ms

R

Full Load

(Smaller resistor)

Minumum Load

(Larger resistor)

OVERLOAD RECOVERY

Like almost all IC power regulators, the LX8585/85A regulators are

equipped with Safe Operating Area (SOA) protection. The SOA

circuit limits the regulator's maximum output current to progres-

sively 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 LX8585/85A 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.