The LM3687 is a step-down DC-DC converter with an integrated low dropout Linear Regulator optimized for powering ultra-low voltage circuits from a single Li-Ion cell or 3 cell NiMH/NiCd batteries. It provides a dual output with fixed output voltages and combined load current up to 750mA in post regulation mode or 1100mA in independent mode of operation, over an input voltage range from 2.7V to 5.5V. There are several different fixed output voltage combinations available (refer to table 'Voltage Options'). The Linear Regulator being driven from the fixed output voltage of the buck converter (post regulation) translates to high efficiency. The device offers superior features and performance for mobile phones and similar portable applications with complex power management systems. Automatic intelligent switching between PWM low-noise and PFM low-current mode offers improved efficiency over the full load current range. During full-power operation, a fixed-frequency 1.8MHz (typ.) PWM mode drives loads from ~80mA to 750mA max. Hysteretic PFM mode extends the battery life through reduction of the quiescent current during light loads and system standby. The LM3687 also features internal protection against over-temperature, current overload and under-voltage conditions. Two enable pins allow the separate operation of either the DC-DC or the Linear Regulator.
The LM3687 is a step‑down DC‑DC converter with an integrated low‑dropout linear regulator optimized for powering ultra‑low voltage circuits from a single Li‑ion or 3‑cell NiMH/NiCd battery. It offers dual fixed outputs, up to 750 mA in post‑regulation mode or 1100 mA in independent mode, and features automatic PWM/PFM mode switching for high efficiency.
The document provides a general description, feature list, typical application circuits, connection diagrams, pin descriptions, voltage options, enable combinations, ordering information, absolute maximum and electrical ratings, recommended external components, physical dimensions, and legal notes. It serves as a reference for designers integrating the LM3687 into portable devices such as mobile phones and hand‑held radios.
| Maximum DC‑DC load capability | 750 mA |
|---|---|
| Switching frequency | 1.8 MHz (typ.) |
| Quiescent current | 27 µA (typ.) |
| Low dropout voltage | 82 mV (typ.) at 350 mA load |
The combined load capability is 750 mA (400 mA from the DC‑DC converter plus 350 mA from the linear regulator).
Only four external components are needed: one inductor and three ceramic capacitors.