LT138A/LT338A
LM138/LM338
APPLICATIONS INFORMATION
Table 1. 0.5% and 1% Standard Resistance Values
1.00
1.02
1.05
1.07
1.10
1.13
1.15
1.18
1.21
1.24
1.27
1.30
1.33
1.37
1.40
1.43
1.47
1.50
1.54
1.58
1.62
1.65
1.69
1.74
1.78
1.82
1.87
1.91
1.96
2.00
2.05
2.10
2.15
2.21
2.26
2.32
2.37
2.43
2.49
2.55
2.61
2.67
2.74
2.80
2.87
2.94
3.01
3.09
3.16
3.24
3.32
3.40
3.48
3.57
3.65
3.74
3.83
3.92
4.02
4.12
4.22
4.32
4.42
4.53
4.64
4.75
4.87
4.99
5.11
5.23
5.36
5.49
5.62
5.76
5.90
6.04
6.19
6.34
6.49
6.65
6.81
6.98
7.15
7.32
7.50
7.68
7.87
8.06
8.25
8.45
8.66
8.87
9.09
9.31
9.53
9.76
C
ADJ
10μF
R2
V
IN
V
IN
ADJ
LT338A
V
OUT
R1
NOT
NEEDED
V
OUT
If a very large output capacitor is used, such as a 100μF
shown in Figure 2, the regulator could be damaged or
destroyed if the input is accidentally shorted to ground
or crowbarred, due to the output capacitor discharging
into the output terminal of the regulator. To prevent this, a
diode D1 as shown, is recommended to safely discharge
the capacitor.
D1
1N4002
+
C
OUT
100μF
138A/338A F02
Figure 2
Standard resistance values are obtained from the Decade Table by
multiplying by multiples of 10. As an example, 1.21 can represent
1.21Ω, 12.1Ω, 121Ω, 1.21k etc.
Load Regulation
Because the LT138A is a three-terminal device, it is not
possible to provide true remote load sensing. Load regula-
tion 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 resis-
tor divider, R1, is connected
directly
to the case
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
P
R2
+
R1
,R
P
=
Parasitic Line Resistance
R1
Bypass Capacitors
Input bypassing using a 1μF tantalum or 25μf electrolytic
is recommended when the input filter capacitors are more
than 5 inches from the device. Improved ripple rejection
(80dB) can be accomplished by adding a 10μF capacitor
from the ADJ pin to ground. Increasing the size of the
capacitor to 20μF will help ripple rejection at low output
voltage since the reactance of this capacitor should be
small compared to the voltage setting resistor, R2. For
improved AC transient response and to prevent the pos-
sibility of oscillation due to unknown reactive load, a 1μF
capacitor is also recommended at the output. Because of
their low impedance at high frequencies, the best type of
capacitor to use is solid tantalum.
Protection Diodes
The LT138A/LT338A do not require a protection diode
from the adjustment terminal to the output (see Figure
2). Improved internal circuitry eliminates the need for
this diode when the adjustment pin is bypassed with a
capacitor to improve ripple rejection.
Connected as shown, R
P
is not multiplied by the divider
ratio. R
P
is about 0.004Ω per foot using 16 gauge wire. This
translates to 4mV/ft at 1A load current, so it is important to
keep the positive lead between regulator and load as short
as possible, and use large wire or PC board traces.
138afc
7
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