APL1117
Application Information
Output Voltage
Rp
PARASITIC
LINE RESISTANCE
The APL1117 develops a 1.25V reference voltage between
the output and the adjust terminal. By placing a resistor
between these two terminals, a constant Current is caused
to flow through R1 and down through R2 to set the overall
output voltage. Normally, this current is chosen to be the
specified minimum load current of 10mA. For fixed volt-
age devices R1 and R2 are included in the device.
APL1117
IN OUT
ADJ
VIN
R1
RL
CONNECT
R1 TO CASE
R2
APL1117
VIN
VOUT
IN
OUT
ADJ
CONNECT
R2 TO LOAD
VREF
R1
R2
Figure 2. Connections for Best Load Regulation
IADJ
60mA
Input Capacitor
R2
R1
VOUT = VREF(1+
)+IADJR2
An input capacitor of 10mF 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.
Figure 1. Basic Adjustable Regulator
Load Regulation
Output Capacitor
When the adjustable regulator is used, load regulation
will be limited by the resistance of the wire connecting the
regulator to the load. The data sheet specification for load
regulation is measured at the output pin of the device.
Best load regulation is obtained when the top of the re-
sistor divider (R1) is tied directly to the output pin of the
device not 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, RP is multiplied by the divider ratio,
the effective resistance between the regulator and the
load would be:
The APL1117 requires an output capacitor to maintain sta-
bility and improve transient response. Proper capacitor
selection is important to ensure proper operation. The
APL1117 output capacitor selection is dependent upon
the ESR (equivalent series resistance) of the output ca-
pacitor to maintain stability. When the output capacitor is
10mF or greater, the output capacitor should have an ESR
less than 1W. This will improve transient response as
well as promote stability. A low-ESR solid tantalum ca-
pacitor works extremely well and provides good transient
response and stability over temperature.
Aluminum electrolytics can also be used, as long as the
ESR of the capacitor is <1W. The value of the output ca-
pacitor can be increased without limit. Higher capacitance
values help to improve transient response and ripple re-
jection and reduce output noise.
R2
Rp X (1+
), Rp = Parasitic Line Resistance
R1
Copyright ã ANPEC Electronics Corp.
9
www.anpec.com.tw
Rev. C.10 - Sep., 2010
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
