LT1038 Datasheet, PDF (5/8 Pages) Linear Technology – 10Amp Positive Adjustable Voltage Regulator

LT1038
APPLICATIO S I FOR ATIO
General
The LT1038 develops a 1.25V reference voltage between
the output and the adjustment 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 10mA or
20mA. 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.
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
(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.
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.
Load Regulation
Because the LT1038 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 can be a true Kelvin
connection if the bottom of resistor R2 is returned to the
negative side of the load. Although it may not be immedi-
ately obvious, best load regulation is obtained when the
top of the resistor divider, R1, is connected directly to the
case, not to the load. This is illustrated in Figure 3. If R1
were connected to the load, the effective resistance be-
tween the regulator and the load would be:
R_p
•


R2 + R1^
R1 
R_p = Parasitic Line Resistance
Protection Diodes
The LT1038 does not require a protection diode from the
adjustment terminal to the output (see Figure 2). Im-
proved internal circuitry eliminates the need for this diode
when the adjustment pin is bypassed with a capacitor to
improve ripple rejection.
Connected as shown, R_p is not multiplied by the divider
ratio. R_p is about 0.004Ω per foot using 16 gauge wire.
This translates to 4mV/ft at 1A load current, so it is
important to keep the lead between the regulator and the
load as short as possible, and use large wire or PC board
traces.
LT1038
V_IN
V_IN
V_OUT
V_OUT
ADJ
I_ADJ
50µA
V_{REF R1}
V_OUT
=
V_REF
(1
+
R2
R1
)
+
I_ADJ
R2
R2
1038 F01
D1
1N4002
LT1038
V_IN
V_IN
V_OUT
+ ^VOUT
ADJ
R1 _*
C_OUT
100µF
C_ADJ ⁺
10µF
R2
*NOT NEEDED
1038 F02
R_p
LT1038
PARASITIC
LINE RESISTANCE
V_IN VIN
V_OUT
ADJ
CONNECT
R1 TO CASE
R1
R_L
R2
CONNECT R2
TO LOAD
1038 F03
Figure 1. Basic Adjustable Regulator
Figure 2
Figure 3. Connections for
Best Load Regulation
5