LM26480 datasheet(14/30 Pages) NSC

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DC/DC Converters

OVERVIEW

The LM26480 provides the DC/DC converters that supply the

various power needs of the application by means of two linear

low dropout regulators, LDO1 and LDO2, and two buck con-

verters, SW1 and SW2. The table here under lists the output

characteristics of the various regulators.

Supply Specification

Supply

Load

Output

OUT Range (V)

MAX

Maximum Output

Current (mA)

LDO1

analog

1.0 to 3.5

300

LDO2

analog

1.0 to 3.5

300

SW1

digital

0.8 to 2.0

1500

SW2

digital

1.0 to 3.3

1500

LINEAR LOW DROPOUT REGULATORS (LDOs)

LDO1 and LDO2 are identical linear regulators targeting ana-

log loads characterized by low noise requirements. LDO1 and

LDO2 are enabled through the ENLDO pin.

30040404

NO-LOAD STABILITY

The LDOs will remain stable and in regulation with no external

load. This is an important consideration in some circuits, for

example, CMOS RAM keep-alive applications.

SW1, SW2: Synchronous Step-

Down Magnetic DC/DC Converters

FUNCTIONAL DESCRIPTION

The LM26480 incorporates two high-efficiency synchronous

switching buck regulators, SW1 and SW2, that deliver a con-

stant voltage from a single Li-Ion battery to the portable

system processors. Using a voltage mode architecture with

synchronous rectification, both bucks have the ability to de-

liver up to 1500 mA depending on the input voltage and output

voltage (voltage head room), and the inductor chosen (max-

imum current capability).

There are three modes of operation depending on the current

required - PWM, PFM, and shutdown. PWM mode handles

current loads of approximately 70 mA or higher, delivering

voltage precision of +/-3% with 90% efficiency or better.

Lighter output current loads cause the device to automatically

switch into PFM for reduced current consumption (I

Q = 33 µA

typ.) and a longer battery life. The Standby operating mode

turns off the device, offering the lowest current consumption.

PWM or PFM mode is selected automatically or PWM mode

can be forced through the setting of the buck control register.

Both SW1 and SW2 can operate up to a 100% duty cycle

(PMOS switch always on) for low drop out control of the output

voltage. In this way the output voltage will be controlled down

to the lowest possible input voltage.

Additional features include soft-start, under-voltage lock-out,

current overload protection, and thermal overload protection.

CIRCUIT OPERATION DESCRIPTION

A buck converter contains a control block, a switching PFET

connected between input and output, a synchronous rectify-

ing NFET connected between the output and ground

(BCKGND pin) and a feedback path. During the first portion

of each switching cycle, the control block 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

by storing energy in a magnetic field. During the second por-

tion of each cycle, the control block 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 slope of

The output filter stores charge when the inductor current is

high, and releases it when low, smoothing the voltage across

the load.

SYNC FUNCTION

The LM26480SQ-BF is the only version of the part that has

the ability to use an external oscillator. The source must be

13 MHz nominal and operate within a range of 15.6 MHz and

10.4 MHz, proportionally the same limits as the 2.0 MHz in-

ternal oscillator. The LM26480SQ-BF has an internal divider

which will divide the speed down by 6.5 to the nominal 2MHz

and use it for the regulators. This SYNC function replaces the

internal oscillator and works in forced PWM only. The buck

regulators no longer have the PFM function enabled. When

the LM26480SQ-BF is sold with this feature enabled, the part

will not function without the external oscillator present.

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 voltage

inversely proportional to the input voltage is introduced.

INTERNAL SYNCHRONOUS RECTIFICATION

While in PWM mode, the buck uses an internal NFET as a

synchronous rectifier to reduce rectifier forward voltage drop

and associated power loss. Synchronous rectification pro-

vides a significant improvement in efficiency whenever the

output voltage is relatively low compared to the voltage drop

across an ordinary rectifier diode.

LM26480 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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