LM3687 Datasheet, PDF (1/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
www.ti.com
SNVS473A – DECEMBER 2007 – REVISED JULY 2008
LM3687 Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup
Mode
Check for Samples: LM3687
FEATURES
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2DC-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
DESCRIPTION
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.
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2
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2007–2008, Texas Instruments Incorporated