4 Strategies on Reducing the Noise of the Parallel LDO System

From sections 2 and 3, any variation between Z_OUT of the individual LDOs degrades the overall noise performance predicted by equation 3. The following design practices help equalize the effective resistance (“R”) seen at each output of the regulator in a parallel LDO architecture.

1. Common input voltage – Feed all regulators from the same input voltage source so that each device experiences the same junction temperature and input voltage.
2. Matched output capacitors – Use identical, tight‑tolerance capacitors (both value and ESR) on every LDO to keep the capacitive component of Z_OUT consistent.
3. Matched ballast resistors – Select discrete ballast resistors with the same nominal value and low tolerance (≤ 1 %).
4. Remote sensing – Connect the remote‑sense pin of the LDO directly to the V_OUT pad of the ballast resistor as a kelvin sense trace to eliminate lead‑wire and trace resistance between V_OUT and R_B.
5. PDN impedance matching – Verify that the power‑distribution network between each ballast resistor and the load, as well as the return path from the load to the RTN pin of the LDO, has matched impedance for every regulator.
6. Common reference node – Tie the reference pins of all LDOs together so that the pins share the same reference voltage.
7. Prefer unity‑gain architecture – When possible, select an LDO that operates in unity‑gain mode (no external feedback divider), eliminating one source of mismatch.
8. Matched feedback network (if required) – If the device does not support unity gain, use a feedback resistor network with identical, low‑tolerance parts for each regulator to set the output voltage.

By applying these eight design practices, the output impedances of the parallel LDOs become closely matched, allowing the system to achieve the noise reduction indicated by equation 3.