SW2

SW1

P2

N2

P1

N1

LM3668

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SNVS449O – JUNE 2007 – REVISED APRIL 2015

8 Detailed Description

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.

Figure 40. Simplified Diagram of Switches

Table 1. State of Switches in Different Modes

MODE

ALWAYS ON

ALWAYS OFF

SWITCHING

Buck

SW P2

SW N2

SW P1 & N1

Boost

SW P1

SW N1

SW N2 & P2

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Product Folder Links: LM3668