Datasheet

VIN

Load

SW1

SW2

SW2SW1

OUT

LM3668

SNVS449M –JUNE 2007–REVISED MAY 2013

www.ti.com

FUNCTIONAL DESCRIPTION

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 LM3668 has the ability to deliver up to 1A 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 46 shows the four switches network used for the buck and boost operation. Table 2

summarizes the state of the switches in different modes.

There are three modes of operation depending on the current required: PWM (Pulse Width Modulation), PFM

(Pulse Frequency Modulation), 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 46. Simplified Diagram of Switches

Table 2. 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

Buck Operation

When the input voltage is greater than the output voltage, the device operates in buck mode where switch P2 is

always ON and P1 & N1 control the output. Figure 47 shows the simplified circuit for buck mode operation.

Figure 47. Simplified Circuit for Buck Operation

Product Folder Links: LM3668