LM3281 Datasheet, PDF (10/26 Pages) Texas Instruments – 3.3-V, 1.2-A, 6-MHz Miniature Step-Down DC-DC Converter

LM3281
SNVSA38 – NOVEMBER 2014
7 Detailed Description
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7.1 Overview
The LM3281 is a size- and performance-optimized step-down DC-DC converter for powering power amplifiers,
front-end modules, wireless connectivity solutions, and a wide variety of other applications. The device
complements the portfolio of SuPA (Supply for PA) products by combining small solution size, low dropout
analog bypass with smooth mode transitions, very low standby current for always-on applications, very low ripple
with “forced PWM” mode operation, high maximum output current, ability to drive large load capacitance while
retaining transient performance, and soft start to limit start-up current.
7.2 Functional Block Diagram
EN MODE
VIN
ECO COMP
OVERVOLTAGE
DETECTOR
BYPASS
CONTROL
FB
OLP
Ref2
Ref1
PWM
ERROR
COMP.
CONTROL LOGIC
DRIVER
AMP
FB
SW
RAMP
GENERATOR
NCP
Ref3
OSCILLATOR
OUTPUT
SHORT
PROTECTION
SOFT
START
THERMAL
SHUTDOWN
Ref4
LIGHT-LOAD
CHECK COMP
GND
7.3 Feature Description
7.3.1 Small Solution Size
Solution size less than 7.5 mm² is possible using the LM3281 in combination with only three small passive
components.
7.3.2 Automatic Analog Bypass with Low Dropout
An internal bypass transistor under analog control automatically engages as V_IN falls below the V_OUT target.
Output stays regulated in analog bypass mode until full dropout. The parallel impedance of this additional bypass
transistor with normal DC-DC output path reduces V_OUT voltage drop-out, maximizing V_OUT supply voltage to the
load at low V_IN conditions. The analog implementation provides a smooth transition among regulation and bypass
modes, avoiding V_OUT distortion.
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