Microsemi

Linfinity Microelectronics Division

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

Page 5

Copyright

© 2000

Rev. 2.1d, 2001-03-15

LX8384x-xx

5A Low Dropout Positive Regulators

A

M I CROS E M I

COM P A N Y

 

 

The LX8384/84A/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

LX8384/84A/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 (<400m

Ω ESR) 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 operation.

  !"#$ %&'






(

10µ

15µF Tantalum, 100µF Aluminum

None

10µ

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 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 under-shoot, can be compared quickly in order to

develop an optimum solution.

LX 8384 x

Pow er

Supp ly

IN

AD J

OU T

S ta r G rou nd

M in im u m Lo ad

(L arg er re sistor)

F u ll Load

(S m alle r

re sisto r)

R

10m s

1 sec

F IG URE 1

- DY NA M IC IN P U T A ND O U T P UT T E S T

OVERLOAD RECOVERY

Like almost all IC power regulators, the LX8384/84A/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 LX8384/84A/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 opera-

tion 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