LM138 Ver la hoja de datos (PDF) - Linear Technology
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LM138 Datasheet PDF : 12 Pages
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TYPICAL PERFOR A CE CHARACTERISTICS
LT138A/LT338A
LM138/LM338
Current Limit
16
PEAK CURRENT LIMIT
DC CURRENT LIMIT
TCASE = 25°C
12
PRELOAD = 0A
PRELOAD = 5A
8
PRELOAD = 1A
4
0
0
10
20
30
40
INPUT-OUTPUT DIFFERENTIAL (V)
138A/338A G10
Current Limit
14
12
PRELOAD = 0A
10
PRELOAD = 5A
8
6
4
2
VIN = 10V
VOUT = 5V
TCASE = 25°C
0
0.1
1
10
TIME (ms)
100
138A/338A G11
Current Limit
14
12
PRELOAD CURRENT = 0
TCASE = 25°C
10
VIN – VOUT = 10V
8
VIN – VOUT = 15V
6
4
VIN – VOUT = 20V
2
VIN – VOUT = 30V
0
0.1
1
10
TIME (ms)
100
138A/338A G12
Line Transient Response
1.5
VOUT = 10V
1.0 IOUT = 50mA
TJ = 25°C
0.5
CL = 1µF
CL = 10µF
0
– 0.5
– 1.0
COUT = 0
CADJ = 0
– 1.5
1.0
0.5
0
0
10
20
TIME (µs)
30
40
138A/338A G13
Load Transient Response
3
2 CL = 1µF
1 CADJ = 10µF
CL = 0
CADJ = 0
0
–1
–2
VIN = 15V
VOUT = 10V
–3
TCASE = 25°C
PRELOAD = 100mA
6
4
2
0
0
10
20
30
40
TIME (µs)
138A/338A G14
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 con-
stant 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 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 accuracy
of VREF. 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, add-
ing additional error to the output voltage tolerance.
VIN
+
LT338A
VIN
VOUT
VOUT
ADJ
VREF R1
IADJ
50µA
R2
( ) VOUT = VREF
1
+
R2
R1
+ IADJ • R2
138A/338A F01
Figure 1. Basic Adjustable Regulator
138afb
5
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