LX8585-00 MICROSEMI [Microsemi Corporation], LX8585-00 Datasheet - Page 5

• Manufacturer Part Number: LX8585-00
• Description: 4.6A LOW DROPOUT POSITIVE REGULATORS
• Manufacturer: MICROSEMI [Microsemi Corporation]

Copyright © 1997
Rev. 2.2
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.
series. Please see the table below for recommended minimum
capacitor values.
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.
adjust (ADJ) pin and ground, ripple rejection will be improved
(please see the section entitled
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
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.
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.
values may be increased without limit.
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.
Use of an output capacitor is REQUIRED with the LX8585/85A
These regulators offer a more tightly controlled reference voltage
If a bypass capacitor is connected between the output voltage
The circuit shown in Figure 1 can be used to observe the transient
12/97
INPUT
10µF
10µF
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
OUTPUT
“RIPPLE
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
4 . 6 A L
REJECTION”). When ADJ
P
R O D U C T I O N
Output capacitance
O W
A P P L I C A T I O N N O T E S
D
R O P O U T
None
15µF
ADJ
Such an
P
O S I T I V E
D
Power Supply
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.
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.
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.
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.
A T A
Under some conditions, a correctly operating SOA circuit may
When the power system is started “cold”, both the input and
Now consider the case where the regulator is supplying regulated
S
R
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
E G U L A T O R S
H E E T
LX8585-xx/8585A-xx
IN
LX8585/85A
ADJ
Star Ground
OUT
Minumum Load
(Larger resistor)
Full Load
(Smaller resistor)
R
10ms
DSON
<< R
1 sec
L
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