LD1117 series LD1117 Adjustable: Application note
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6 LD1117 Adjustable: Application note
The LD1117 Adjustable has a thermal stabilized 1.25±0.012V reference voltage between
the OUT and ADJ pins. I
ADJ
is 60µA typ. (120µA max.) and ∆I
ADJ
is 1µA typ. (5µA max.).
R1 is normally fixed to 120W. From figure 10 we obtain:
V
OUT
= V
REF
+ R2 (I
ADJ
+I
R1
) = V
REF
+ R2 (I
ADJ
+V
REF
/R
1
) = V
REF
(1 + R2 / R
1
) + R2 x
I
ADJ
.
In normal application R2 value is in the range of few kohm, so the R2 x I
ADJ
product could
not be considered in the V
OUT
calculation; then the above expression becomes:
V
OUT
= V
REF
(1 + R2 / R1).
In order to have the better load regulation it is important to realize a good Kelvin connection
of R1 and R2 resistors. In particular R1 connection must be realized very close to OUT and
ADJ pin, while R2 ground connection must be placed as near as possible to the negative
Load pin. Ripple rejection can be improved by introducing a 10µF electrolytic capacitor
placed in parallel to the R2 resistor (see Fig. 11).
Figure 10. Adjustable output voltage application
Figure 11. Adjustable output voltage application with improved ripple rejection
