LM3679

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SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

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

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

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 YPD 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

in a magnetic field.

During the second portion of each cycle, the controller turns the PFET switch off, blocking current flow from the

input, and then turns the NFET synchronous rectifier on. The inductor draws current from ground through the

The output filter stores charge when the inductor current is high, and releases it when inductor current is low,

smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at the SW pin to a low-pass filter formed by the inductor and output filter capacitor. The

output voltage is equal to the average voltage at the SW pin.

PWM OPERATION

During PWM operation, the converter operates as a voltage-mode controller with input voltage feed forward. This

allows the converter to achieve good load and line regulation. The DC gain of the power stage is proportional to

the input voltage. To eliminate this dependence, feed forward inversely proportional to the input voltage is

introduced.

While in PWM mode, the output voltage is regulated by switching at a constant frequency and then modulating

the energy per cycle to control power to the load. At the beginning of each clock cycle the PFET switch is turned

on and the inductor current ramps up until the comparator trips and the control logic turns off the switch. The

current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then the

NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock turning off

the NFET and turning on the PFET.

Copyright © 2007–2008, Texas Instruments Incorporated

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