LM3691

SNVS506H – MAY 2008 – REVISED SEPTEMBER 2011

www.ti.com

OPERATION DESCRIPTION

DEVICE INFORMATION

The LM3691, a high-efficiency, step-down DC-DC switching buck converter, delivers a constant voltage from

either a single Li-Ion or three cell NiMH/NiCd battery to portable devices such as cell phones and PDAs. Using a

voltage mode architecture with synchronous rectification, the LM3691 has the ability to deliver up to 1000 mA

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), ECO,

and shutdown. The device operates in PWM mode at load currents of approximately 50 mA (typ.) or higher.

Lighter output current loads cause the device to automatically switch into ECO mode for reduced current

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

overload protection, and thermal shutdown protection. As shown in Figure 1, only three external power

components are required for implementation.

CIRCUIT OPERATION

The LM3691 operates as follows. During the first portion of each switching cycle, the control block in the LM3691

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 NFET to the output filter capacitor and load, which ramps the inductor current down with a

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

Figure 45. Typical PWM Operation

12

Submit Documentation Feedback

Copyright © 2008–2011, Texas Instruments Incorporated

Product Folder Links: LM3691