LM337L: 10uF C2 capacitor's role

Hi Takeshi,

the LM337 generates a stable reference voltage of 1.25V. To provide an output voltage of more than 1.25V a voltage divider (R1/R2) is used. R1 and R2 make that the 1.25V reference voltage appears amplified by a factor determined by this voltage divider. Some of the regulating parameters like noise, temperature stability, load regulation, output impedance and PSRR of LM337 get worse by nearly the same factor. That's why many parameters in the datasheet are specified in % Vo (% of output voltage) and not in absolute numbers.

The effect of C2 is to reduce the factor determined by the voltage divider R1/R2 for higher frequencies which will heavily improve many of the parameters. For instance, the noise of a -12V regulator can be decreased by about a factor of ten! PSRR and output impedance are also improved by about a factor of ten.

Kai