7.3.1 Ripple Rejection

Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 13 and Figure 16). The LM1086 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than V_REF, the ripple rejection decreases as function of adjustment gain: (1+R1/R2) or V_O/V_REF. Therefore a 5-V adjustment decreases ripple rejection by a factor of four (−12dB); Output ripple increases as adjustment voltage increases.

However, the adjustable version allows this degradation of ripple rejection to be compensated. The adjust terminal can be bypassed to ground with a capacitor (C_ADJ). The impedance of the C_ADJ should be equal to or less than R1 at the desired ripple frequency. This bypass capacitor prevents ripple from being amplified as the output voltage is increased.