Typical Performance Characteristics (Continued)
LM1117-5.0 Line Transient Response
10091911
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
impedance of the CADJ should be less than R1 to prevent the
ripple from being amplified:
<
R1
1/(2π*fRIPPLE*CADJ
)
10091917
The R1 is the resistor between the output and the adjust pin.
Its value is normally in the range of 100-200Ω. For example,
FIGURE 1. Basic Adjustable Regulator
3.0 Load Regulation
>
with R1 = 124Ω and fRIPPLE = 120Hz, the CADJ should be
11µF.
1.3 Output Capacitor
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.
The output capacitor is critical in maintaining regulator sta-
bility, and must meet the required conditions for both mini-
mum amount of capacitance and ESR (Equivalent Series
Resistance). 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 stability and transient response. The ESR of the output
capacitor should range between 0.3Ω - 22Ω. In the case of
the adjustable regulator, when the CADJ is used, a larger
output capacitance (22µf tantalum) is required.
Figure 2, shows a typical application using a fixed output
regulator. The Rt1 and Rt2 are the line resistances. It is
obvious that the VLOAD 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.
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.
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