LM3687 Datasheet, PDF (1/22 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

December 4, 2007
LM3687
Step-Down DC-DC Converter with Integrated Low Dropout
Regulator and Startup Mode
General Description
The LM3687 is a step-down DC-DC converter with an inte-
grated 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 out-
put voltages and combined load current up to 750mA in post
regulation mode or 1100mA in independent mode of opera-
tion, 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 volt-
age of the buck converter (post regulation) translates to high
efficiency.
The device offers superior features and performance for mo-
bile 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 V_{IN_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
V_BATT for 50mA rated load current (Startup Mode). If V_{IN_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
micro SMD 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.
Features
DC-DC Converter:
■ ^{750mA maximum load capability}
■ ^{1.8MHz PWM fixed switching frequency (typ.)}
■ ^{Automatic PFM/PWM mode switching}
■ ^{27µA typ. Quiescent Current}
■ ^{Internal synchronous rectification for high efficiency}
■ ^{Internal soft start}
Dual Rail Linear Regulator:
■ ^{Startup Mode}
■ ^{Load transients < 25mVpeak typ.}
■ ^{Line transients < 1mVpeak typ.}
■ ^{Very Low Dropout Voltage: 82mV typ. at 350mA load}
current
■ ^{0.7V ≤ V}_{IN_LIN} ^{≤ 4.5V}
■ ^{10µA typical I}_Q ^{from V}_{IN_LIN}
■ ^{350mA maximum load capability}
Combined Common Features:
■ ^{65µA typical Quiescent Current from V}_BATT ^{if both}
regulators are enabled
■ ^{750mA maximum combined load capability in post}
regulation setup (DC-DC 400mA + Linear Regulator
350mA)
■ ^{1100mA maximum total load capability in independent}
mode of operation (DC-DC: 750mA, Linear Regulator:
350mA)
■ ^{Operates from a single Li-Ion cell or 3 cell NiMH/NiCd}
batteries
■ ^{Only four tiny surface-mount external components}
required (one inductor, three ceramic capacitors)
■ ^{Small 9-bump micro SMD package}
■ ^{Over-temperature, current overload and under-voltage}
protection
Applications
■ ^{Mobile Phones}
■ ^{Hand-Held Radios}
■ ^{Personal Digital Assistants}
■ ^{Palm-top PCs}
■ ^{Portable Instruments}
■ ^{Battery Powered Devices}
© 2007 National Semiconductor Corporation 202101
www.national.com