Step-Down Converter Includes LDO Regulator

Linear Technology’s LTC3541 is a 500-mA 2.25-MHz, synchronous buck regulator and a 300-mA very low dropout (VLDO) regulator in a single package. The device provides two high-efficiency voltage rails from a single input requiring only a single inductor. The synchronous buck regulator delivers up to 95% efficiency, whereas the VLDO output offers very low noise.

The input voltage range of 2.7 V to 5.5 V makes the device ideal for single cell Li-ion and Li-polymer or multi-cell alkaline/Ni-Cad/Ni-MH powered applications. The LTC3541 can generate output voltages as low as 0.8 V from the buck regulator and 0.4 V from the VLDO, enabling it to power the latest generation of microcontrollers. The buck regulator's 2.25-MHz switching frequency and the VLDO design enable the use of small external components.

All versions of the LTC3541 can be programmed via an external pin to have the synchronous buck power up independently or simultaneously with the VLDO. Additionally, when both the synchronous switcher and the VLDO are enabled, an automatic start-up sequencing feature brings up either the switcher or the VLDO in a specific order.

The LTC3541 and LTC3541-2 have an automatic start-up feature that brings the buck output into regulation prior to enabling the VLDO regulator. Alternatively, the LTC3541-1 and LTC3541-3 bring the VLDO into regulation prior to enabling the switching regulator. The VLDO can either be operated directly from the output of the switching regulator, for maximum efficiency, or from a separate input from 0.9 V to 4.1 V.

An automatic Burst Mode feature reduces quiescent current in the switcher to only 85 µA, thereby maximizing battery run-time. For applications that are particularly noise sensitive, the Burst Mode feature can be disabled and replaced with either a pulse-skipping or continuous-switching operation. Other features include a power-good output and over-temperature protection.