LX8415 Microsemi Corporation, LX8415 Datasheet - Page 4

• Manufacturer Part Number: LX8415
• Description: 0.5a Low Dropout Positive Regulators
• Manufacturer: Microsemi Corporation

LX8415-xx
4
The LX8415 series ICs are easy to use Low-Dropout (LDO) voltage
regulators. They have the standard self-protection features ex-
pected of a voltage regulator: short circuit protection and automatic
thermal shutdown if the device temperature rises above approxi-
mately 165°C.
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
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. Output
capacitance 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 quickly compared in order to develop an optimum
solution.
Use of an output capacitor is REQUIRED with the LX8415 series.
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
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
0 . 5 A L
REJECTION”). When ADJ
P
R E L I M I N A R Y
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
P
Power Supply
O S I T I V E
D
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:
case, 15µF.
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:
were to be bypassed to ground with a properly selected capacitor.
A T A
At a ripple frequency of 120Hz, with R1 = 100 :
The closest equal or larger standard value should be used, in this
When an ADJ pin bypass capacitor is used, output ripple
For example, if V
Output ripple will be twice as bad as it would be if the ADJ pin
C = 1 / (6.28 * F
where: C
C = 1 / (6.28 * 120Hz * 100 ) = 13.3µF
M = V
where: M
M = 2.5V/1.25V= 2
R
S
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
OUT
E G U L A T O R S
H E E T
F
R1
V
/V
C
R
REF
IN
1
REF
= 1.25V.
OUT
LX8415-xx
the value of the capacitor in Farads;
select an equal or larger standard value.
the ripple frequency in Hz
the value of resistor R1 in ohms
a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
R
= 2.5V the output ripple will be:
ADJ
* R1)
Star Ground
OUT
C
2
Minumum Load
(Larger resistor)
Full Load
(Smaller resistor)
Copyright © 1999
R
10ms
DSON
Rev. 0.4
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