LX8117B Microsemi Corporation, LX8117B Datasheet - Page 7

• Manufacturer Part Number: LX8117B
• Description: 0.8, 1 & 1.2a Low Dropout Positive Regulators
• Manufacturer: Microsemi Corporation

Copyright © 1999
Rev. 1.5a
OVERLOAD RECOVERY (continued)
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.
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.
OUTPUT VOLTAGE
The LX8117 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
through R1, it represents a small error and can usually be ignored.
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
ADJ
is very small and constant when compared with the current
OUT
F
R1
/V
V
R
REF
REF
= 1.25V.
OUT
R
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.
= 2.5V the output ripple will be:
* R1)
0.8, 1 & 1.2A L
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
P
R O D U C T I O N
A P P L I C A T I O N N O T E S
O W
D
LX8117-xx/8117A-xx/8117B-xx
R O P O U T
D
LOAD REGULATION
Because the LX8117 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.
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
P
A T A
When the circuit is connected as shown in Figure 3, the parasitic
V
O S I T I V E
IN
R
where: R
FIGURE 3 — CONNECTIONS FOR BEST LOAD REGULATION
V
Peff
IN
= R
S
FIGURE 2 — BASIC ADJUSTABLE REGULATOR
V
The data sheet specification for load regulation is
H E E T
OUT
P
IN
R
*
P
E G U L AT O R S
= V
LX8117-xx
R2+R1
Actual parasitic line resistance.
REF
ADJ
IN
R1
50µA
1 +
I
LX8117-xx
ADJ
OUT
ADJ
R2
R1
+ I
Line Resistance
OUT
ADJ
Parasitic
R1
R2
R2
R
P
Connect
R1 to Case
of Regulator
V
REF
R1
R2
V
Connect
R2
to Load
OUT
R
L
Peff
.
7