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LM1117-ADJ-252 View Datasheet(PDF) - First Components International

Part Name
Description
Manufacturer
LM1117-ADJ-252
Fci
First Components International Fci
LM1117-ADJ-252 Datasheet PDF : 14 Pages
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LM1117 800mA Low-dropout Linear Regulator
APPLICATION NOTE
1.0 External Capacitors/Stability
1.1 Input Bypass Capacitor
An input capacitor is recommended. A 10µF tantalum on the
input is a suitable input bypassing for almost all applications.
1.2 Adjust Terminal Bypass Capacitor
The adjust terminal can be bypassed to ground with a by-
pass capacitor (CADJ) to improve ripple rejection. This by-
pass capacitor prevents ripple from being amplified as the
output voltage is increased. At any ripple frequency, the im-
pedance of the CADJ should be less than R1 to prevent the
ripple from being amplified:
(2π*fRIPPLE*CADJ) < R1
The R1 is the resistor between the output and the adjust pin.
Its value is normally in the range of 100-200. For example,
with R1=124and fRIPPLE=120Hz, the CADJ should be >
11µF.
1.3 Output Capacitor
The output capacitor is critical in maintaining regulator stabil-
ity, and must meet the required conditions for both minimum
amount of capacitance and ESR (Equivalent Series Resis-
tance). The minimum output capacitance required by the
LM1117 is 10µF, if a tantalum capacitor is used. Any increase
of the output capacitance will merely improve the loop stabil-
ity and transient response. The ESR of the output capacitor
should be less than 0.5. In the case of the adjustable regu-
lator, when the CADJ is used, a larger output capacitance
(22µf tantalum) is required.
2.0 Output Voltage
The LM1117 adjustable version develops a 1.25V reference
voltage, VREF, between the output and the adjust terminal.
As shown in Figure 1, this voltage is applied across resistor
R1 to generate a constant current I1. The current IADJ from
the adjust terminal could introduce error to the output. But
since it is very small (60µA) compared with the I1 and very
constant with line and load changes, the error can be ig-
nored. The constant current I1 then flows through the output
set resistor R2 and sets the output voltage to the desired
level.
For fixed voltage devices, R1 and R2 are integrated inside
the devices.
Figure 2, shows a typical application using a fixed output
regulator. The Rt1 and Rt2 are the line resistances. It is ob-
vious that the V LOAD is less than the VOUT by the sum of the
voltage drops along the line resistances. In this case, the
load regulation seen at the RLOAD would be degraded from
the data sheet specification. To improve this, the load should
be tied directly to the output terminal on the positive side and
directly tied to the ground terminal on the negative side.
FIGURE 2. Typical Application using Fixed Output
Regulator
When the adjustable regulator is used (Figure 3), the best
performance is obtained with the positive side of the resistor
R1 tied directly to the output terminal of the regulator rather
than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regula-
tion. For example, a 5V regulator with 0.05resistance be-
tween the regulator and load will have a load regulation due
to line resistance of 0.05x IL. If R1 (=125) is connected
near the load, the effective line resistance will be 0.05
(1+R2/R1) or in this case, it is 4 times worse. In addition, the
ground side of the resistor R2 can be returned near the
ground of the load to provide remote ground sensing and im-
prove load regulation.
FIGURE 1. Basic Adjustable Regulator
3.0 Load Regulation
The LM1117 regulates the voltage that appears between its
output and ground pins, or between its output and adjust
pins. In some cases, line resistances can introduce errors to
the voltage across the load. To obtain the best load regula-
tion, a few precautions are needed.
FIGURE 3. Best Load Regulation using Adjustable
Output Regulator
4.0 Protection Diodes
Under normal operation, the LM1117 regulators do not need
any protection diode. With the adjustable device, the internal
resistance between the adjust and output terminals limits the
current. No diode is needed to divert the current around the
regulator even with capacitor on the adjust terminal. The ad-
just pin can take a transient signal of ±25V with respect to
the output voltage without damaging the device.
When a output capacitor is connected to a regulator and the
input is shorted to ground, the output capacitor will discharge
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