LM3686 Datasheet, PDF (1/20 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Post Linear Regulators System and Low-Noise Linear Regulator

December 17, 2008
LM3686
Step-Down DC-DC Converter with Integrated Post Linear
Regulators System and Low-Noise Linear Regulator
General Description
The LM3686 is a step-down DC-DC converter with one inte-
grated very low dropout linear regulator and a low noise linear
regulator optimized for powering ultra-low voltage circuits
from a single Li-Ion cell or 3 cell NiMH/NiCd batteries. It pro-
vides three outputs with combined load current up to 900 mA
over an input voltage range from 2.7V to 5.5V.
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 (typ.), PWM mode drives loads from ~70 mA to 600
mA max. Hysteretic PFM mode extends the battery life
through reduction of the quiescent current to 28 μA (typ.) at
light load and system standby. Internal synchronous rectifi-
cation provides high efficiency.
Three enable 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 startup of the post-
regulation linear regulator, a parallel small pass transistor
supplies the linear regulator from V_BATT with maximal 50 mA.
In the combined operation where both enables are raised to-
gether, the small pass transistor is deactivated and the big
pass transistor provides 350 mA output current. In shutdown
mode (Enable pins pulled low), the device turns off and re-
duces battery consumption to 2.5 μA (typ.).
The LM3686 is available in a 12-bump micro SMD package.
A high-switching frequency of 3 MHz (typ.) allows the use of
a few tiny surface-mount components. Only six external sur-
face-mount components, an inductor and five ceramic capac-
itors, are required to establish a 15.66 mm² total solution size.
Features
DC-DC REGULATOR
■ ^V_{OUT_DCDC} ^{= 1.2V to 2.5V ( in 100 mV steps - factory}
programmed )
■ ^{600 mA maximum load capability (LILO = OFF)}
■ ^{3 MHz PWM fixed switching frequency (typ.)}
■ ^{Automatic PFM/PWM mode switching}
■ ^{Internal synchronous rectification for high efficiency}
■ ^{Internal soft start}
DUAL RAIL LINEAR REGULATOR: LILO
■ ^{Load transients < 50 mV peak typ.}
■ ^{Line transients < 1 mV peak typ.}
■ ^V_{OUT_LILO} ^{= 0.7V to 2.0V ( in 50 mV steps - factory}
programmed )
■ ^{70 µA typical I}_Q ^{from V}_{IN_LILO}
■ ^{350 mA Maximum Load Capability (Large FET)}
LINEAR REGULATOR: LDO
■ ^{Load transients < 80 mV peak typ.}
■ ^{Line transients < 1 mV peak typ.}
■ ^V_{OUT_LDO} ^{= 1.5V to 3.3V ( in 100 mV steps - factory}
programmed )
■ ^{50 µA typical I}_Q
■ ^{300 mA Maximum Load Capability}
COMBINED GLOBAL FEATURES
■ ^V_BATT ^{≥ V}_{OUT_LILO} ^{+ 1.5V or 2.7V, whichever is higher}
■ ^{900 mA maximum combined load capability}
■ ^{Operates from a single Li-Ion cell or 3 cell NiMH/NiCd}
batteries
■ ^{Only six tiny surface-mount external components required}
(one inductor, five ceramic capacitors)
■ ^{12–bump micro SMD}
■ ^{100 µA Iq from V}_BATT
■ ^{15.66 mm2 total solution size}
Applications
■ ^{Moblie TV}
■ ^{Hand-Held Radios}
■ ^{Personal Digital Assistants}
■ ^{Palm-top PCs}
■ ^{Portable Instruments}
■ ^{Battery-Powered Devices}
■ ^{Portable Personal Clients}
© 2008 National Semiconductor Corporation 300255
www.national.com