LM3668_15 Datasheet, PDF (13/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

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8 Detailed Description
LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
8.1 Overview
The LM3668, a high-efficiency buck or boost DC-DC converter, delivers a constant voltage from either a single
Li-Ion or three cell NIMH/NiCd battery to portable devices such as mobile phones and PDAs. Using a voltage
mode architecture with synchronous rectification, the device has the ability to deliver up to 1 A, depending on the
input voltage, output voltage, ambient temperature and the chosen inductor.
In addition, the device incorporates a seamless transition from buck-to-boost or boost-to-buck mode. The internal
error amplifier continuously monitors the output to determine the transition from buck-to-boost or boost-to-buck
operation. Figure 40 shows the four switches network used for the buck and boost operation. Table 1
summarizes the state of the switches in different modes.
There are three modes of operation depending on the current required: Pulse Width Modulation (PWM), Pulse
Frequency Modulation (PFM), and shutdown. The device operates in PWM mode at load currents of
approximately 80 mA or higher to improve efficiency. Lighter load current causes the device to automatically
switch into PFM mode to reduce current consumption and extend battery life. Shutdown mode turns off the
device, offering the lowest current consumption.
V_IN
V_OUT
MODE
Buck
Boost
P1
SW1
P2
SW2
N1
N2
Figure 40. Simplified Diagram of Switches
Table 1. State of Switches in Different Modes
ALWAYS ON
SW P2
SW P1
ALWAYS OFF
SW N2
SW N1
SWITCHING
SW P1 & N1
SW N2 & P2
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