SNVSB79A November 2018 – July 2019 LM25180
The LM25180 has an external bias supply feature that reduces input quiescent current and increases efficiency. When the voltage at SS/BIAS exceeds a rising threshold of 5.5 V, bias power for the internal LDO regulator can be derived from an external voltage source or from a transformer auxiliary winding as shown in Figure 22. With a bias supply connected, the LM25180 then uses its internal soft-start ramp to control the primary current during start-up.
When using a transformer auxiliary winding for bias power, the total leakage current related to diodes DBIAS1 and DBIAS2 in Figure 22 should be less than 1 µA across the full operating temperature range.