LM1117I National Semiconductor Corporation, LM1117I Datasheet - Page 9

• Manufacturer Part Number: LM1117I
• Description: 800ma Low-dropout Linear Regulator
• Manufacturer: National Semiconductor Corporation

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 (C
pass capacitor prevents ripple from being amplified as the
output voltage is increased. At any ripple frequency, the
impedance of the C
ripple from being amplified:
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=124
11µF.
1.3 Output Capacitor
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 be greater than 0.5
the case of the adjustable regulator, when the C
a larger output capacitance (22µf tantalum) is required.
2.0 Output Voltage
The LM1117 adjustable version develops a 1.25V reference
voltage, V
As shown in Figure 1 , this voltage is applied across resistor
R1 to generate a constant current I1. The current I
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.
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.
(2 *f
RIPPLE
FIGURE 1. Basic Adjustable Regulator
REF
*C
, between the output and the adjust terminal.
and f
ADJ
ADJ
)
ADJ
<
RIPPLE
) to improve ripple rejection. This by-
should be less than R1 to prevent the
R1
=120Hz, the C
and less than 5 . In
ADJ
should be
ADJ
DS100919-17
ADJ
is used,
from
>
9
Figure 2 , shows a typical application using a fixed output
regulator. The Rt1 and Rt2 are the line resistances. It is
obvious that the V
the voltage drops along the line resistances. In this case, the
load regulation seen at the R
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.
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 regu-
lation. For example, a 5V regulator with 0.05
between the regulator and load will have a load regulation
due to line resistance of 0.05
nected near the load, the effective line resistance will be
0.05
addition, the ground side of the resistor R2 can be returned
near the ground of the load to provide remote ground sens-
ing and improve load regulation.
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
adjust pin can take a transient signal of
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
FIGURE 3. Best Load Regulation using Adjustable
FIGURE 2. Typical Application using Fixed Output
(1+R2/R1) or in this case, it is 4 times worse. In
LOAD
Output Regulator
is less than the V
Regulator
LOAD
x I
would be degraded from
L
. If R1 (=125 ) is con-
±
25V with respect to
OUT
by the sum of
DS100919-18
DS100919-19
www.national.com
resistance