SNOS412M – FEBRUARY 2000 – REVISED MARCH 2013
APPLICATION INFORMATION
EXTERNAL CAPACITORS/STABILITY
Input Bypass Capacitor
An input capacitor is recommended. A 10µF tantalum on the input is a suitable input bypassing for almost all
applications.
Adjust Terminal Bypass Capacitor
The adjust terminal can be bypassed to ground with a bypass capacitor (C
ADJ
) to improve ripple rejection. This
bypass capacitor prevents ripple from being amplified as the output voltage is increased. At any ripple frequency,
the impedance of the C
ADJ
should be less than R1 to prevent the ripple from being amplified:
1/(2π*f
RIPPLE
*C
ADJ
) < R1
(1)
The R1 is the resistor between the output and the adjust pin. Its value is normally in the range of 100-200Ω. For
example, with R1 = 124Ω and f
RIPPLE
= 120Hz, the C
ADJ
should be > 11µF.
Output Capacitor
The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both
minimum amount of capacitance and ESR (Equivalent Series Resistance). The minimum output capacitance
required by the LM1117-N 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 C
ADJ
is used, a larger output capacitance (22µf
tantalum) is required.
OUTPUT VOLTAGE
The LM1117-N adjustable version develops a 1.25V reference voltage, V
REF
, between the output and the adjust
terminal. As shown in
this voltage is applied across resistor R1 to generate a constant current I1. The
current I
ADJ
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 ignored. 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 19. Basic Adjustable Regulator
LOAD REGULATION
The LM1117-N 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 regulation, a few precautions are needed.
shows a typical application using a fixed output regulator. The Rt1 and Rt2 are the line resistances. It
is obvious that the V
LOAD
is less than the V
OUT
by the sum of the voltage drops along the line resistances. In this
case, the load regulation seen at the R
LOAD
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.
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