LT137A/LM137
LT337A/LM337
APPLICATIONS INFORMATION
Output Voltage
The output voltage is determined by two external resis-
tors, R1 and R2 (see Figure 1). The exact formula for the
output voltage is:
⎛
R2
⎞
V
OUT
=
V
REF
⎜
1
+ ⎟ +
I
ADJ
(
R2
)
⎝
R1
⎠
Where: V
REF
= Reference Voltage, I
ADJ
= Adjustment Pin
Current. In most applications, the second term is small
enough to be ignored, typically about 0.5% of V
OUT
. In
more critical applications, the exact formula should be
used, with I
ADJ
equal to 65μA. Solving for R2 yields:
R2
=
V
OUT
– V
REF
V
REF
+
I
R1
ADJ
EXAMPLES:
1. A precision 10V regulator to supply up to 1A load
current.
a. Select R1 = 100Ω to minimize effect of I
ADJ
b. Calculate R2 =
V
OUT
– V
REF
10 V – 1.25V
=
=
696.4
Ω
V
REF
1.25V
+
65μA
+
I
R1
ADJ
100
Ω
Use R2 = 698Ω
2. A 15V regulator to run off batteries and supply
50mA. V
IN MAX
= 25V
a. To minimize battery drain, select R1 as high as
possible
R1
=
1.25V
=
417
Ω
, use 402
Ω
, 1%
3mA
Smaller values of R1 and R2 will reduce the influence
of I
ADJ
on the output voltage, but the no-load current
drain on the regulator will be increased. Typical values
for R1 are between 100Ω and 300Ω, giving 12.5mA and
4.2mA no-load current respectively. There is an additional
consideration in selecting R1, the minimum load current
specification of the regulator. The operating current of the
LT137A flows from input to output. If this current is not
absorbed by the load, the output of the regulator will rise
above the regulated value. The current drawn by R1 and
R2 is normally high enough to absorb the current, but
care must be taken in no-load situations where R1 and
R2 have high values.
The maximum value for the operating current, which must
be absorbed, is 5mA for the LT137A. If input-output volt-
age differential is less than 10V, the operating current that
must be absorbed drops to 3mA.
+
+
C2
5μF
ADJ
–V
IN
V
IN
LT137A V
OUT
C1
10μF
I
ADJ
R2
b. The high value for R1 will exaggerate the error due
to I
ADJ
, so the exact formula to calculate R2 should
be used.
R2
=
V
OUT
– V
REF
15V – 1.25V
=
=
4331
Ω
V
REF
1.25V
+
65μA
+
I
R1
ADJ
402
Ω
Use R2 = 4320Ω
Capacitors and Protection Diodes
An output capacitor, C3, is required to provide proper fre-
quency compensation of the regulator feedback loop. A 1μF
or larger solid tantalum capacitor is generally sufficient for
this purpose if the 1MHz impedance of the capacitor is 2Ω
or less. High Q capacitors, such as Mylar, are not recom-
mended because they tend to reduce the phase margin at
light load currents. Aluminum electrolytic capacitors may
also be used, but the minimum value should be 10μF to
ensure a low impedance at 1MHz. The output capacitor
should be located within a few inches of the regulator to
keep lead impedance to a minimum. The following caution
should be noted: if the output voltage is greater than 6V
and an output capacitor greater than 20μF has been used,
it is possible to damage the regulator if the input voltage
137afb
+
R1
C3
1μF
V
REF
–V
OUT
137A F01
Figure 1
7
LINER [ LINEAR TECHNOLOGY ]
