APL1117A
Application Information
Output Voltage
The output voltage of the APL1117A-ADJ can be adjusted
with a resistive divider from output voltage to ADJ pin to
ground (see Figure1), and the internal reference voltage
is 1.25V. The following equation can be used to calculate
the output voltage:
V
OUT
=
1.25 V
×
(1
+
R2
)
+
I
ADJ
×
R2
R1
CONNECT
R2 TO LOAD
V
IN
APL1117A
IN
OUT
ADJ
Rp
PARASITIC
LINE RESISTANCE
R1
R2
CONNECT
R1 TO CASE
RL
Where the I
ADJ
is a constant current flows through R2 and
represents a small error. Because the I
ADJ
is very small, it
can usually be ignored. Note that the APL1117A-ADJ needs
a minimum load current of 10mA to present the output
voltage. For fixed voltage devices, the R1 and R2 are in-
cluded in the device.
V
IN
APL1117A
VIN
VOUT
ADJ
I
ADJ
60µA
V
OUT
V
REF
R1
Figure 2. Connections for The Best Load Regulation
Input Capacitor
An input capacitor of 10µF or greater is recommended.
Tantalum or aluminum electrolytic capacitors can be used
for bypassing. Larger Values will improve ripple rejection
by bypassing the input to the regulator.
Output Capacitor
R2
The APL1117A requires an output capacitor to maintain
stability and improve transient response. Proper capaci-
tor selection is important to ensure proper operation. The
APL1117A output capacitor selection depends upon the
ESR (equivalent series resistance) of the output capaci-
tor to maintain stability. When the output capacitor is 10µF
or greater, the output capacitor should have an ESR be-
tween 0.15Ω to 20Ω. This will improve transient response
as well as promote stability. A low-ESR solid tantalum
capacitor works extremely well and provides good tran-
sient response and stability over temperature.
Aluminum electrolytics can also be used, as long as the
ESR of the capacitor is less than1Ω. The value of the
output capacitor can be increased without limit. Higher
capacitance values help to improve transient response
and ripple rejection and reduce output noise.
Ripple Rejection
Figure 1. Basic Adjustable Regulator
Load Regulation
When the adjustable regulator is used, load regulation
will be limited by the resistance of the wire connecting the
regulator to the load. The datasheet specification for load
regulation is measured at the output pin of the device.
The best load regulation is obtained when the top of the
resistor divider (R1) is tied directly to the output pin of the
device, but to the load. For fixed voltage devices, the top of
R1 is internally connected to the output, and the ground
pin can be connected to low side of the load. If R1 is
connected to the load, R
P
is multiplied by the divider ratio,
the effective resistance between the regulator and the
load would be:
Rp X (1+
R2
), Rp = Parasitic Line Resistance
R1
The curves for Ripple Rejection were generated by using
an adjustable device with the adjust pin bypassed. With a
22µF bypassing capacitor, 60dB ripple rejection is ob-
tainable at any output level. The impedance of the adjust
pin capacitor, at the ripple frequency, should be < R1. R1
Copyright
©
ANPEC Electronics Corp.
Rev. A.4 - May., 2010
6
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
