LX8382A 데이터 시트보기 (PDF) - Microsemi Corporation
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LX8382A Datasheet PDF : 7 Pages
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LX8382-xx
PRODUCT DATABOOK 1996/1997
10A LOW DROPOUT POSITIVE REGULATORS
PRODUCTION DATA SHEET
APPLICATION NOTES
The LX8382/8382A 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 LX8382/82A
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.
RECOMMENDED CAPACITOR VALUES
INPUT
10µF
10µF
OUTPUT
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
ADJ
None
15µF
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. Out-
put 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.
Power Supply
IN LX8382-xx OUT
/82A-xx
ADJ
Minumum Load
(Larger resistor)
Full Load
(Smaller resistor)
RDSON << RL
Star Ground
1 sec
10ms
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
OVERLOAD RECOVERY
Like almost all IC power regulators, the LX8382 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 LX8382 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.
4
Copyright © 1997
Rev. 1.0 8/97
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