APL1117
Application Information
Output Voltage
The APL1117 develops a 1.25V reference voltage
between the output and the adjust terminal. By plac-
ing a resistor between these two terminals, a con-
stant 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 voltage devices R1
and R2 are included in the device.
APL1117
IN
OUT
ADJ
I
ADJ
60
µ
A
CONNECT
R 2 TO LO A D
V
IN
A P L1117
IN
OUT
ADJ
Rp
P A R A S IT IC
LIN E R E S IS TA N C E
R1
CONNECT
R 1 TO C A S E
R2
R
L
V
IN
V
OUT
V
REF
R1
Figure 2. Connections for Best Load Regulation
Input Capacitor
A n i n p u t c a p a c i t o r o f 1 0
µF
o r g r e a t e r i s
recommended. Tantalum, or aluminum electrolytic
capacitors can be used for bypassing. Larger Val-
R2
V
O UT
= V
REF
(1+
R2
)+I
ADJ
R2
R1
Figure 1. Basic Adjustable Regulator
ues will improve ripple rejection by bypassing the in-
put to the regulator.
Load Regulation
When the adjustable regulator is used. Load regula-
tion will be limited by the resistance of the wire con-
necting the regulator to the load. The data sheet speci-
fication for load regulation is measured at the output
pin of the device. Best load regulation is obtained
when the top of the resistor divider (R1) is tied di-
rectly 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 were con-
nected to the load, RP is multiplied by the divider
ratio, the effective resistance between the regulator
and the load would be:
R2
Rp X (1+ ), Rp = Parasitic Line Resistance
R1
Copyright
ANPEC Electronics Corp.
Rev. B.8 - Jun., 2003
9
Output Capacitor
The APL1117 requires an output capacitor to main-
tain stability 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 capacitor to maintain stability.
When the output capacitor is 10uF or greater, the
output capacitor should have an ESR less than 1Ω.
This will improve transient response as well as pro-
mote stability. A low-ESR solid tantalum capacitor
works extremely well and provides good transient re-
sponse and stability over temperature.
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