LM3679 datasheet(8/22 Pages) TI1

8 / 22 page

LM3679

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

www.ti.com

TYPICAL PERFORMANCE CHARACTERISTICS (continued)

LM3679TL/UR, Circuit of Figure 1, VIN = 3.6V, VOUT = 1.8V, TA = 25°C, unless otherwise noted.

Mode Change by Load Transients

Load Transient Response

VOUT = 1.8V (PWM to PFM)

VOUT = 1.8V (PWM Mode)

Start Up into PWM Mode

Start Up into PFM Mode

VOUT = 1.8V (Output Current = 300mA)

VOUT = 1.8V (Output Current = 1mA)

OPERATION DESCRIPTION

DEVICE INFORMATION

The LM3679, a high efficiency step down DC-DC switching buck converter, delivers a constant voltage from a

single Li-Ion battery and input voltage rails from 2.5V to 5.5V such as cell phones and PDAs. Using a voltage

mode architecture with synchronous rectification, the LM3679 has the ability to deliver up to 350mA depending

on the input voltage and output voltage, ambient temperature, and the inductor chosen.

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 current of

approximately 80 mA or higher, having a voltage precision of ±2.5% with 90% efficiency or better. Lighter load

current causes the device to automatically switch into PFM mode for reduced current consumption (IQ = 16 µA

typ) and a longer battery life. Shutdown mode turns off the device, offering the lowest current consumption

(ISHUTDOWN = 0.01 µA typ).

Additional features include soft-start, under voltage protection, current overload protection, and thermal shutdown

protection. As shown in Figure 1, only three external power components are required for implementation.

Using the UR package allows for a low profile solution size (0.55mm max height, including external components).

The recommended external components are stated within the application information. The max output current is

300mA when these specific low profile external components are used.

The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the

input voltage exceeds 2.5V.

CIRCUIT OPERATION

The LM3679 operates as follows. During the first portion of each switching cycle, the control block in the LM3679

turns on the internal PFET switch. This allows current to flow from the input through the inductor to the output

filter capacitor and load. The inductor limits the current to a ramp with a slope of (VIN–VOUT)/L, by storing energy

in a magnetic field.

Submit Documentation Feedback

Product Folder Links: LM3679

LM3679 Datasheet(PDF) 8 Page - Texas Instruments