LT138A/LT339A
LM138/LM338
TYPICAL PERFORMANCE CHARACTERISTICS
Line Transient Response
1.5
OUTPUT VOLTAGE
DEVIATION (V)
1.0
0.5
0
–0.5
–1.0
–1.5
INPUT VOLTAGE
CHANGE (V)
1.0
0.5
0
0
10
20
TIME (μs)
30
40
C
OUT
= 0
C
ADJ
= 0
LOAD CURRENT (A)
OUTPUT VOLTAGE
DEVIATION (V)
V
OUT
= 10V
I
OUT
= 50mA
T
J
= 25°C
3
2
1
0
–1
–2
–3
6
4
2
0
0
10
20
TIME (μs)
30
40
138A/338A G14
Load Transient Response
C
L
= 1μF
C
ADJ
= 10μF
C
L
= 0
C
ADJ
= 0
C
L
= 1μF
C
L
= 10μF
V
IN
= 15V
V
OUT
= 10V
T
CASE
= 25°C
PRELOAD = 100mA
138A/338A G13
APPLICATIONS INFORMATION
General
The LT138A develops a 1.25V reference voltage between the
output and the adjustable terminal (see Figure 1). By plac-
ing 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 5mA or 10mA.
Because I
ADJ
is very small and constant when compared
with the current through R1, it represents a small error and
can usually be ignored. It is easily seen from the output
voltage equation, that even if the resistors were of exact
value, the accuracy of the output is limited by the accu-
racy of V
REF
. Earlier adjustable regulators had a reference
tolerance of ± 4% which is dangerously close to the ±5%
supply tolerance required in many logic and analog sys-
tems. Further, even 1% resistors can drift 0.01%/°C, adding
additional error to the output voltage tolerance.
LT338A
V
IN
V
IN
ADJ
V
OUT
V
REF
R1
V
OUT
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 expensive
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 inexpensive 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.
+
I
ADJ
50μA
R2
V
OUT
= V
REF
1 + R2 + I
ADJ
• R2
R1
( )
138A/338A F01
Figure 1. Basic Adjustable Regulator
138afc
6
LINER [ LINEAR TECHNOLOGY ]
