LX8585-XX MICROSEMI [Microsemi Corporation], LX8585-XX Datasheet - Page 6

• Manufacturer Part Number: LX8585-XX
• Description: 4.6A Low Dropout Positive Regulators
• Manufacturer: MICROSEMI [Microsemi Corporation]

Copyright © 1997
Rev. 2.2a, 2005-11-10
OVERLOAD RECOVERY (continued)
voltage across the output resistor, the voltage will stabilize at some
lower value, and will never reach the designed value. Under these
circumstances, it may be necessary to cycle the input voltage
down to zero in order to make the regulator output voltage return
to regulation.
RIPPLE REJECTION
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:
where: C ≡ the value of the capacitor in Farads; select an equal
At a ripple frequency of 120Hz, with R1 = 100Ω:
The closest equal or larger standard value should be used, in this
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:
where: M ≡ a multiplier for the ripple seen when the ADJ pin is
For example, if V
were to be bypassed to ground with a properly selected capacitor.
OUTPUT VOLTAGE
between the output and the adjust terminal (See Figure 6). 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
constant when compared with the current through R1, it represents
a small error and can usually be ignored.
If this limited current is not sufficient to develop the designed
Ripple rejection can be improved by connecting a capacitor
When an ADJ pin bypass capacitor is used, output ripple
Output ripple will be twice as bad as it would be if the ADJ pin
The LX8585/85A ICs develop a 1.25V reference voltage
TM
C = 1 / (6.28 * F
F
R1 ≡ the value of resistor R1 in ohms
C = 1 / (6.28 * 120Hz * 100Ω) = 13.3μF
V
M = 2.5V/1.25V= 2
M = V
R
REF
≡ the ripple frequency in Hz
or larger standard value.
optimally bypassed.
= 1.25V.
OUT
OUT
/V
REF
= 2.5V the output ripple will be:
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
R
* R1)
ADJ
is very small and
A P P L I C A T I O N N O T E
Integrated Products Division
®
Microsemi
4.6A Low Dropout Positive Regulators
LOAD REGULATION
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 7. If R1 were connected to the load, the
effective resistance between the regulator and the load would be:
where: R
resistance appears as its actual value, rather than the higher R
V
Because the LX8585/85A regulators are three-terminal devices,
When the circuit is connected as shown in Figure 7, the parasitic
IN
V
IN
V
OUT
R
Figure 7 – Connections For Best Load Regulation
P
Peff
≡ Actual parasitic line resistance.
= V
P
Figure 6 – Basic Adjustable Regulator
IN
RODUCTION
=
REF
LX8585/85A
IN
R
50µA
P
LX8585/85A
1 +
ADJ
I
*
ADJ
⎛
⎜
⎝
LX8585-xx / LX858A-xx
ADJ
R2
R2
R1
OUT
R1
+
D
+ I
R1
ATA
OUT
ADJ
⎞
⎟
⎠
Line Resistance
Parasitic
R2
S
R1
R2
HEET
R
P
Connect
R1 to Case
of Regulator
V
REF
R1
R2
Connect
R2
to Load
V
R
OUT
L
Peff
Page 6