LX8586 Microsemi Corporation, LX8586 Datasheet - Page 5

• Manufacturer Part Number: LX8586
• Description: Low Drop Out Regulator - Positive Adjustable
• Manufacturer: Microsemi Corporation

Copyright © 1996
Rev. 1.0a, 2005-11-10
O
If this limited current is not sufficient to develop the designed
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.
R
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:
At a ripple frequency of 120Hz, with R1 = 100Ω:
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:
Where:
For example, if V
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.
O
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
constant when compared with the current through R1, it represents
a small error and can usually be ignored.
Where:
IPPLE
VERLOAD
UTPUT
The LX8586/86A ICs develop a 1.25V reference voltage
TM
R
V
EJECTION
C = 1 / (6.28 * F
OLTAGE
R
ECOVERY
C ≡ the value of the capacitor in Farads; select an
equal or larger standard value.
F
R1 ≡ the value of resistor R1 in ohms
C = 1 / (6.28 * 120Hz * 100Ω) = 13.3μF
M = V
pin is optimally bypassed.
V
M = 2.5V/1.25V = 2
M ≡ a multiplier for the ripple seen when the ADJ
R
REF
≡ the ripple frequency in Hz
OUT
= 1.25V.
OUT
= 2.5V the output ripple will be:
/V
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
(continued)
REF
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
L
Because the LX8586/86A 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:
where: R
When the circuit is connected as shown in Figure 3, the parasitic
resistance appears as its actual value, rather than the higher R
V
V
6A Low Dropout Positive Regulators
OAD
IN
IN
R
V
EGULATION
OUT
P
Figure 3 - Connections for Best Load Regulation
≡ Actual parasitic line resistance.
R
= V
IN
Peff
P
Figure 2 - Basic Adjustment Regulator
LX8586/86A
RODUCTION
REF
=
IN
50µA
R
ADJ
LX8586/86A
1 +
P
I
ADJ
*
⎛
⎜
⎝
ADJ
R2
R1
OUT
R2
R1
+ I
+
D
R1
ATA
OUT
Line Resistance
LX8586 / LX8586A
ADJ
⎞
⎟
⎠
Parasitic
R2
S
R1
R2
R
HEET
Connect
R1 to Case
of Regulator
P
V
REF
R1
R2
V
Connect
R2
to Load
OUT
R
L
PEFF
Page 5