The LM3686 is a step-down DC-DC converter with a
very low-dropout linear regulator and a low-noise linear regulator optimized
for powering ultra-low voltage circuits.
It provides three outputs with
combined load current up to 900 mA over an input voltage range from 2.7 V to 5.5 V.
The device offers superior features and performance for many applications. Automatic
intelligent switching between PWM low-noise and PFM low-current mode offers improved system
control. During full-power operation, a fixed-frequency 3 MHz (typical), PWM mode drives loads from
approximately 70 mA to 600 mA maximum. Hysteretic PFM mode extends the battery life through
reduction of the quiescent current to 28 μA (typical) at light load and system standby. Internal
synchronous rectification provides high efficiency.
For all available packages, see the orderable addendum at the
end of the data sheet.
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Three enable (EN_x) pins allow the separate operation of either the DC-DC,
post-regulation linear regulator, or the linear regulator alone. If the DC-DC is not enabled during
start-up of the post-regulation linear regulator, a parallel small-pass transistor supplies the
linear regulator from VBATT with maximal 50 mA. In the combined operation
where both enables are raised together, the small-pass transistor is deactivated and the big pass
transistor provides 350 mA output current. In shutdown mode (EN_x pins pulled low), the device
turns off and reduces battery consumption to 2.5 μA (typical).
The LM3686 is available in a 12-pin DSBGA package. A high-switching frequency of 3 MHz
(typical) allows the use of a few tiny surface-mount components. Only six external surface-mount
components, an inductor and five ceramic capacitors, are required to establish a 15.66
mm2 total solution size.
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