LX8415 Microsemi Corporation, LX8415 Datasheet - Page 5

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

Copyright © 1999
Rev. 0.4
OUTPUT VOLTAGE
The
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
through R1, it represents a small error and can usually be ignored.
LOAD REGULATION
Because the
possible to provide true remote load sensing. Load regulation will
be limited by the resistance of the wire connecting the regulator to
the load.
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:
resistance appears as its actual value, rather than the higher R
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
When the circuit is connected as shown in Figure 3, the parasitic
Even when the circuit is optimally configured, parasitic resistance
1/99
LX8415
R
where: R
V
Peff
IN
ADJ
= R
is very small and constant when compared with the current
V
ICs develop a 1.25V reference voltage between the output
FIGURE 2 — BASIC ADJUSTABLE REGULATOR
The data sheet specification for load regulation is
LX8415
OUT
P
*
= V
P
R2+R1
Actual parasitic line resistance.
REF
R1
regulators are three-terminal devices, it is not
IN
50µA
1 +
I
LX8415-xx
ADJ
ADJ
R2
R1
+ I
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
OUT
ADJ
R2
P
V
REF
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
R1
R2
D
R O P O U T
V
OUT
Peff
.
P
O S I T I V E
D
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.
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.
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
during transients.
THERMAL CONSIDERATIONS
The
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.
A T A
The minimum specified output capacitance for the regulator
To maintain good load regulation, wide traces should be used on
V
IN
LX8415
FIGURE 3 — CONNECTIONS FOR BEST LOAD REGULATION
V
where: V
IN (MIN)
R
S
regulators have internal power and thermal limiting
E G U L A T O R S
= V
H E E T
IN
V
V
IN (MIN)
OUT
DROPOUT (MAX)
OUT
LX8415-xx
+ V
ADJ
DROPOUT (MAX)
OUT
the lowest allowable instantaneous
voltage at the input pin.
the designed output voltage for the
power supply system.
the specified dropout voltage
for the installed regulator.
Line Resistance
Parasitic
R1
R2
R
P
Connect
R1 to Case
of Regulator
LX8415-xx
Connect
R2
to Load
R
L
IN (MIN)
5