Datasheet

LD1117A adjustable: application note LD1117A
12/24 DocID7194 Rev 26
7 LD1117A adjustable: application note
The LD1117A adjustable has a thermal stabilized 1.25 ± 0.012 V reference voltage between
the OUT and ADJ pins. I
ADJ
is 60 µA typ. (120 µA max.) and ΔI
ADJ
is 1 µA typ. (5 µA max.).
R
1
is normally fixed to 120 Ω. From Figure 6 the following is obtained:
V
OUT
= V
REF
+ R
2
(I
ADJ
+ I
R1
) = V
REF
+ R
2
(I
ADJ
+V
REF
/R
1
) = V
REF
(1 + R
2
/ R
1
) + R
2
x
I
ADJ
.
In normal applications the R
2
value is in the range of a few kΩ, so the R
2
x I
ADJ
product can
not be considered in the V
OUT
calculation; the above expression then becomes:
V
OUT
= V
REF
(1 + R
2
/ R
1
).
In order to have a better load regulation it is important to realize a good Kelvin connection of
R
1
and R
2
resistors. In particular, the R
1
connection must be realized very close to the OUT
and ADJ pins, while the R
2
ground connection must be placed as near as possible to the
negative load pin. Ripple rejection can be improved by introducing a 10 µF electrolytic
capacitor placed in parallel to the R
2
resistor (see Figure 8).
Figure 8. Adjustable output voltage application
LD1117A
Figure 9. Adjustable output voltage application with improved ripple rejection
LD1117A