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LM338 Scheda tecnica(PDF) 6 Page - Linear Technology

Il numero della parte LM338
Spiegazioni elettronici  5A Positive Adjustable Voltage Regulator
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Produttore elettronici  LINER [Linear Technology]
Homepage  http://www.linear.com
Logo LINER - Linear Technology

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LT138A/LT339A
LM138/LM338
6
138afd
For more information www.linear.com/LT138A
Typical perForMance characTerisTics
Line Transient Response
Load Transient Response
applicaTions inForMaTion
General
The LT138A develops a 1.25V reference voltage between
the output and the adjustable terminal (see Figure 1). 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 5mA
or 10mA. Because IADJ is very small and constant when
comparedwiththecurrentthroughR1,itrepresentsasmall
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 ac-
curacyofVREF.Earlieradjustableregulatorshadareference
tolerance of ± 4% which is dangerously close to the
±5% supply tolerance required in many logic and analog
systems. Further, even 1% resistors can drift 0.01%/°C,
adding additional error to the output voltage tolerance.
For example, using 2% resistors and ±4% tolerance for
VREF, calculations will show that the expected range of a
5V regulator design would be 4.66V ≤ VOUT ≤ 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,usuallyatrimpot.Thissolutionisbothexpensive
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 VREF . 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.
TIME (μs)
0
0
0.5
–1.5
–1.0
–0.5
20
30
1.5
138A/338A G13
1.0
10
40
0
0.5
1.0
CL = 1μF
CL = 10μF
COUT = 0
CADJ = 0
VOUT = 10V
IOUT = 50mA
TJ = 25°C
TIME (μs)
0
2
6
–3
–2
3
0
20
40
138A/338A G14
4
1
2
–1
0
10
30
VIN = 15V
VOUT = 10V
TCASE = 25°C
PRELOAD = 100mA
CL = 1μF
CADJ = 10μF
CL = 0
CADJ = 0
Figure 1. Basic Adjustable Regulator
LT338A
VIN
R1
138A/338A F01
VOUT
IADJ
50μA
ADJ
VREF
VOUT
VIN
R2
VOUT = VREF
R2
R1
1 +
+ IADJ • R2
( )
+


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