LA8385 Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – 3A LOW DROPOUT POSITIVE REGULATORS

PRODUCT DATABOOK 1996/1997
LX8385-xx/8385A-xx/8385B-xx
3A LOW DROPOUT POSITIVE REGULATORS
P R O D U C T I O N D ATA S H E E T
APPLICATION NOTES
The LX8385/85A/85B series ICs are easy to use Low-Dropout (LDO)
voltage regulators. They have all of the standard self-protection
features expected of voltage regulators: 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 LX8385/85A/
85B 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
(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.
In order to ensure good transient response from the power supply
RECOMMENDED CAPACITOR VALUES
INPUT
OUTPUT
10µF
10µF
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
ADJ
None
15µF
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 quickly compared in order to develop an optimum
solution.
Power Supply
IN
LX8385 _OUT
/85A/85B
ADJ
Minumum Load
(Larger resistor)
Full Load
(Smaller resistor)
R_DSON << R_L
Star Ground
1 sec
10ms
FIGURE 1 — Dynamic Input And Output Test
OVERLOAD RECOVERY
Like almost all IC power regulators, the LX8385/85A/85B 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 LX8385/85A/85B 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.
Copyright © 1999
Rev. 2.0a 10/00
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