LT138A/LT338A
LM138/LM338
APPLICATIONS INFORMATION
For example, using 2% resistors and
±4%
tolerance for
V
REF
, calculations will show that the expected range of a
5V regulator design would be 4.66V
≤
V
OUT
≤
5.36V or
approximately
±7%.
If the same example were used for a
15V regulator, the expected tolerance would be
±8%.
With
these results most applications required some method of
trimming, usually a trim pot. This solution is both expen-
sive and not conductive to volume production.
One of the enhancements of Linear Technology’s adjust-
able regulators over existing devices is the tightened
initial tolerance of V
REF
. This allows relatively inexpen-
sive 1% or 2% film resistors to be used for R1 and R2 to
set the output voltage within an acceptable tolerance.
With a guaranteed 1% reference, a 5V power supply
design, using
±2%
resistors, would have a worst-case
manufacturing tolerance of
±4%.
If 1% resistors are used,
the tolerance will drop to
±2.5%.
A plot of the worst-case
output voltage tolerance as a function of resistor tolerance
is shown on the front page of this data sheet.
For convenience, a table of standard 1% resistor values is
shown in Table 1.
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
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.
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
6
U
W
U
U
(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 possi-
bility 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 capaci-
tor to improve ripple rejection.
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
LT338A
V
IN
V
IN
ADJ
V
OUT
R1
NOT
NEEDED
V
OUT
C
ADJ
10µF
+
R2
C
OUT
100µF
138A/338A F02
Figure 2
Load Regulation
Because the LT138A is a three-terminal device, it is not
possible to provide true remote load sensing. Load regu-
lation will be limited by the resistance of the wire connect-
ing 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 connec-
tion, with the bottom of the output divider returned to the
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