3 LDO Output Impedance

Using the TPS7A57 as an example [2], the output impedance Z_OUT decreases with increasing load current (Figure 3-1). At very low frequencies, Z_OUT is dominated by the drain-source resistance (R_DS) of the internal pass MOSFET, the PCB trace impedance, and the internal bond wire impedance. In the mid‑band region, the performance of the internal error amplifier becomes the dominant factor. At frequencies above the mid‑band, the series impedance of the output capacitor, together with the PCB parasitics, dictates Z_OUT. To satisfy the condition that R falls out of equation 2, the output impedances of all parallel‑connected LDOs must be identical.

Figure 3-1 TPS7A57 Z_OUT Variation with Load Current