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 ).
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
alone or both. If the power input voltage for the Linear Regulator VIN_LIN
is not sufficiently high (e.g. the DC-DC converter is not enabled or starting up) a startup LDO
supplies the Linear Regulator Output from VBATT for 50mA rated load current
(Startup Mode). If VIN_LIN is at the required voltage level, the startup LDO
is deactivated and the main regulator provides 350mA output current. In shutdown mode (Enable pins
pulled low) the device turns off and reduces battery consumption to 0.1µA (typ.).
The LM3687 is available in a tiny, lead-free (NO PB) 9-bump DSBGA package. A high
switching frequency of 1.8MHz (typ.) allows the use of tiny surface-mount components. Only four
external components -one inductor and three ceramic capacitors- are required.
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