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LD1117 adjustable: application note

LD1117

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DocID2572 Rev 33

LD1117 adjustable: application note

The LD1117 adjustable has a thermal stabilized 1.25 ± 0.012 V reference voltage between

the OUT and ADJ pins. IADJ is 60 µA typ. (120 µA max.) and ΔIADJ is 1 µA typ. (5 µA max.).

R1 is normally fixed to 120 Ω. From Figure 9 we obtain:

VOUT = VREF + R2 (IADJ +IR1) = VREF + R2 (IADJ +VREF /R1) = VREF (1 + R2 / R1) + R2 x IADJ.

In normal application R2 value is in the range of few kΩ, so the R2 x IADJ product could not

be considered in the VOUT calculation; then the above expression becomes:

VOUT = VREF (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 10 µF electrolytic capacitor

placed in parallel to the R2 resistor (see Figure 10).

Figure 10. Adjustable output voltage application

Figure 11. Adjustable output voltage application with improved ripple rejection

LD33CV Datasheet(PDF) 24 Page - STMicroelectronics