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LX8415-33CST

P1-P3P4-P6P7-P7
0.5A LOW DROPOUT POSITIVE REGULATORS
LX8415-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 0.5
4
P
RELIMINARY DATA SHEET
APPLICATION NOTES
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.
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.
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.
INPUT OUTPUT ADJ
10µF 15µF Tantalum, 100µF Aluminum None
10µF 47µF Tantalum, 220µF Aluminum 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. 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.
RECOMMENDED CAPACITOR VALUES
FIGURE 1 — DYNAMIC INPUT and OUTPUT TEST
LX8415-xx
Power Supply
OUT
IN
ADJ
Star Ground
1 sec
10ms
R
DSON
<< R
L
Full Load
(Smaller resistor)
Minumum Load
(Larger resistor)
C
1
C
2
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
*
F
R
*
R1)
where: C ≡ the value of the capacitor in Farads;
select an equal or larger standard value.
F
R
≡ 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 = V
OUT
/V
REF
where: M ≡ a multiplier for the ripple seen when the
ADJ pin is optimally bypassed.
V
REF
= 1.25V.
For example, if V
OUT
= 2.5V the output ripple will be:
M = 2.5V/1.25V= 2
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.
0.5A LOW DROPOUT POSITIVE REGULATORS
LX8415-xx
PRODUCT DATABOOK 1996/1997
5
Copyright © 1999
Rev. 0.5
P RELIMINARY DATA SHEET
APPLICATION NOTES
FIGURE 2 — BASIC ADJUSTABLE REGULATOR
R2+R1
R1


LX8415-xx
OUT
IN
ADJ
V
OUT
V
IN
R1
R2
V
REF
I
ADJ
50µA
V
OUT
= V
REF
1 + + I
ADJ
R2
R2
R1
OUTPUT VOLTAGE
The LX8
415
ICs develop a 1.25V reference voltage between the output
and the adjust terminal (See Figure 2). By placing a resistor, R1,
between these two terminals, a constant current is caused to flow
through R1 and down through R2 to set the overall output voltage.
Normally this current is the specified minimum load current of 10mA.
Because I
ADJ
is very small and constant when compared with the current
through R1, it represents a small error and can usually be ignored.
LX8415-xx
OUT
IN
ADJ
V
IN
R1
R2
R
L
R
P
Parasitic
Line Resistance
Connect
R1 to Case
of Regulator
Connect
R2
to Load
FIGURE 3 — CONNECTIONS FOR BEST LOAD REGULATION
LOAD REGULATION
Because the LX8415 regulators are three-terminal devices, it is not
possible to provide true remote load sensing. Load regulation will
be limited by the resistance of the wire connecting the regulator to
the load. The data sheet specification for load regulation is
measured at the bottom of the package. Negative side sensing is a
true Kelvin connection, with the bottom of the output divider
returned to the negative side of the load. Although it may not be
immediately obvious, best load regulation is obtained when the top
of the resistor divider, (R1), is connected directly to the case of the
regulator, not to the load. This is illustrated in Figure 3. If R1 were
connected to the load, the effective resistance between the regulator
and the load would be:
R
Peff
= R
P
*
where: R
P
≡ Actual parasitic line resistance.
When the circuit is connected as shown in Figure 3, the parasitic
resistance appears as its actual value, rather than the higher R
Peff
.
Even when the circuit is optimally configured, parasitic resistance
can be a significant source of error. A 100 mil (2.54 mm) wide PC
trace built from 1 oz. copper-clad circuit board material has a
parasitic resistance of about 5 milliohms per inch of its length at
room temperature. If a 3-terminal regulator used to supply 2.50 volts
is connected by 2 inches of this trace to a load which draws 5 amps
of current, a 50 millivolt drop will appear between the regulator and
the load. Even when the regulator output voltage is precisely
2.50 volts, the load will only see 2.45 volts, which is a 2% error. It
LOAD REGULATION (continued)
is important to keep the connection between the regulator output
pin and the load as short as possible, and to use wide traces or
heavy-gauge wire.
The minimum specified output capacitance for the regulator
should be located near the reglator package. If several capacitors
are used in parallel to construct the power system output capaci-
tance, any capacitors beyond the minimum needed to meet the
specified requirements of the regulator should be located near the
sections of the load that require rapidly-changing amounts of
current. Placing capacitors near the sources of load transients will
help ensure that power system transient response is not impaired
by the effects of trace impedance.
To maintain good load regulation, wide traces should be used on
the input side of the regulator, especially between the input
capacitors and the regulator. Input capacitor ESR must be small
enough that the voltage at the input pin does not drop below V
IN (MIN)
during transients.
V
IN (MIN)
= V
OUT
+ V
DROPOUT (MAX)
where: V
IN (MIN)
≡ the lowest allowable instantaneous
voltage at the input pin.
V
OUT
≡ the designed output voltage for the
power supply system.
V
DROPOUT (MAX)
≡ the specified dropout voltage
for the installed regulator.
THERMAL CONSIDERATIONS
The LX8415 regulators have internal power and thermal limiting
circuitry designed to protect each device under overload conditions.
For continuous normal load conditions, however, maximum junc-
tion temperature ratings must not be exceeded. It is important to
give careful consideration to all sources of thermal resistance from
junction to ambient. This includes junction to case, case to heat sink
interface, and heat sink thermal resistance itself.
0.5A LOW DROPOUT POSITIVE REGULATORS
LX8415-xx
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 0.5
6
P
RELIMINARY DATA SHEET
APPLICATION NOTES
THERMAL CONSIDERATIONS (continued)
Example
Given: V
IN
= 5.0V ±5%, V
OUT
= 2.5V ±3%
I
OUT
= 0.5A, T
A
= 55°C, T
J
= 125°C
R
θJT
= 15°C/W, R
θTS
= 5°C/W
Find: The size of a square area of 1oz. copper circuit-
board trace-foil that will serve as a heatsink,
adequate to maintain the junction temperature of the
LX8415 in the ST (SOT-223) package within
specified limits.
Solution: The junction temperature is:
T
J
= P
D
(R
θJT
+ R
θCS
+ R
θSA
) + T
A
where: P
D
≡ Dissipated power.
R
θJT
≡ Thermal resistance from the junction to the
mounting tab of the package.
R
θTS
≡ Thermal resistance through the interface
between the IC and the surface on which
it is mounted.
R
θSA
≡
Thermal resistance from the mounting surface
of the heatsink to ambient.
T
S
≡ Heat sink temperature.
T
J
T
C
T
S
T
A
R
q
JT
R
q
CS
R
q
SA
T
J
- T
A
P
D
3.1°C/W
R
θSA
- 22.3°C/W
First, find the maximum allowable thermal resistance of the
heat sink:
P
D
= [[V
IN
*
(1 + Tol
VIN
)] - [V
OUT
*
(1 - Tol
VOUT
)]]
*
I
OUT
P
D
= 1.4W
R
θSA
= - (R
θJT
+ R
θTS
) , R
θSA
= 29.6°C/W
A test was conducted to determine the thermal characteristics of
1 oz. copper circuit-board trace material. The following equation
describes the observed relationship between the area of a square
copper pad, and the thermal resistance from the tab of a SOT-223
package soldered at the center of the pad to ambient.
Area
SINK
= in
2
Substituting the value for R
θSA
calculated above, we find that a
square pad with area:
Area
SINK
= 0.43 in
2
(0.66" x 0.66"), 280mm
2
(17 x 17 mm)
will be required to maintain the LX8415 junction temperature
within specified limits.
P1-P3P4-P6P7-P7

LX8415-33CST

Mfr. #:
Buy LX8415-33CST
Manufacturer:
Microchip / Microsemi
Description:
IC REG LIN 3.3V 500MA SOT223 PWR
Lifecycle:
New from this manufacturer.
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