LX8415-00(2005) Просмотр технического описания (PDF) - Microsemi Corporation
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LX8415-00 Datasheet PDF : 7 Pages
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LX8415-xx
PRODUCT DATABOOK 1996/1997
0.5A LOW DROPOUT POSITIVE REGULATORS
PR E L I M I N A RY D ATA S H E E T
APPLICATION NOTES
The LX8415 series ICs are easy to use Low-Dropout (LDO) voltage Power Supply
regulators. They have the standard self-protection features ex-
IN LX8415-xx OUT
Minumum Load
(Larger resistor)
Full Load
pected of a voltage regulator: short circuit protection and automatic
ADJ
(Smaller resistor)
thermal shutdown if the device temperature rises above approxi-
mately 165°C.
Use of an output capacitor is REQUIRED with the LX8415 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.
e STABILITY
The output capacitor is part of the regulator’s frequency compen-
t 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
le 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
o capacitor is not used, smaller output capacitor values are adequate.
The table below shows recommended minimum capacitance values
for stable operation.
RECOMMENDED CAPACITOR VALUES
s INPUT
10µF
10µF
OUTPUT
15µF Tantalum, 100µF Aluminum
47µF Tantalum, 220µF Aluminum
ADJ
None
15µF
b 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
O capacitors. Cost-effective solutions that sufficiently limit ESR and
C1
C2
RDSON << RL
Star Ground
1 sec
10ms
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
RIPPLE REJECTION
Ripple rejection can be improved by connecting a capacitor
between the ADJ pin and ground. The value of the capacitor should
be chosen so that the impedance of the capacitor is equal in
magnitude to the resistance of R1 at the ripple frequency. The
capacitor value can be determined by using this equation:
C = 1 / (6.28 * FR * R1)
where: C ≡ the value of the capacitor in Farads;
select an equal or larger standard value.
FR ≡ the ripple frequency in Hz
R1 ≡ the value of resistor R1 in ohms
At a ripple frequency of 120Hz, with R1 = 100Ω:
C = 1 / (6.28 * 120Hz * 100Ω) = 13.3µF
The closest equal or larger standard value should be used, in this
case, 15µF.
When an ADJ pin bypass capacitor is used, output ripple
amplitude will be essentially independent of the output voltage. If
an ADJ pin bypass capacitor is not used, output ripple will be
proportional to the ratio of the output voltage to the reference
voltage:
M = VOUT/VREF
where: M ≡ a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
VREF = 1.25V.
For example, if V = 2.5V the output ripple will be:
OUT
M = 2.5V/1.25V= 2
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
Output ripple will be twice as bad as it would be if the ADJ pin
were to be bypassed to ground with a properly selected capacitor.
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.
4
Copyright © 1999
Rev. 0.5
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