Datasheet

LD1117 adjustable: application note LD1117
24/44 DocID2572 Rev 33
7 LD1117 adjustable: application note
The LD1117 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 9 we obtain:
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 application R
2
value is in the range of few kΩ, so the R
2
x I
ADJ
product could not
be considered in the V
OUT
calculation; then the above expression becomes:
V
OUT
= V
REF
(1 + R
2
/ R
1
).
In order to have the better load regulation it is important to realize a good Kelvin connection
of R
1
and R
2
resistors. In particular R
1
connection must be realized very close to OUT and
ADJ pin, while 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 10).
Figure 10. Adjustable output voltage application
Figure 11. Adjustable output voltage application with improved ripple rejection