LINEAR TECHNOLOGY LTC3446 Technical data

One IC Generates Three Sub-2V Power Rails from a Li-Ion Cell

.

Design Note 419

Frank Lee

Introduction

Shrinking geometries in IC technology have pushed the
operating voltages of today’s electronics well below 2V,
presenting a number of design challenges. One common
problem is the need for multiple supply voltages: for
example, one voltage for a CPU core, another for I/O and
still others for peripherals. Sensitive RF, audio and analog
circuitry may require additional dedicated quiet supplies,
separate from less noise-sensitive digital circuits. As the
number of supplies increases, it becomes impractical
to use a separate power supply IC for each voltage and
special-requirements subsystem. Board area would be
quickly consumed by power supplies. One solution to
the space crunch is power supply integration, provided

®

by a triple regulator like the LTC

3446—three voltages

from a single IC.

Triple Supply in a Tiny Package

The LTC3446 combines a 1A synchronous buck regula-

TM

tor with two 300mA very low dropout (VLDO

) linear
regulators to provide up to three stepped-down out­put voltages from a single input voltage, all in a tiny
3mm × 4mm DFN. The 2.7V to 5.5V input voltage range
is ideally suited for Li-Ion/Polymer battery-powered ap-

plications, and for powering low voltage logic from 5V
or 3.3V rails. The output voltage range extends down
to 0.4V for the VLDO regulators and 0.8V for the buck
converter.

Each output is independently enabled or shut down via
its own enable pin. When all outputs are shut down,

quiescent current drops to 1μA or less, conserving

V

IN

battery power. The regulation voltage for each output
is programmed by external resistor dividers. The buck
regulator loop response can be tailored to the load by
adjusting the RC network at the I

TH

pin.

High Effi ciency and Low Noise

The 1A synchronous buck provides the main output with
high effi ciency, up to 90%. This buck converter features
constant-frequency curren t-mode operation at 2.25MHz,
allowing small capacitors and inductor to be used. The
two 300mA VLDO regulators can be connected to run
off the buck output to provide two additional lower volt­age outputs. This way, the buck performs the bulk of the
step-down at the high effi ciencies typical of switching

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
Burst Mode and VLDO are registered trademarks of Linear Technology Corporation
All other trademarks are the property of their respective owners.

L1

2.9V TO 5.5V

V

IN

DIGITAL

CONTROL

1000pF

22μF

X7R

3.3k

V

IN

LTC3446

PGOOD
MODESEL
ENBUCK
ENLDO1
ENLDO2

I

TH

GND

BUCKFB

LV

LV

OUT1

LV

LV

OUT2

LV

DN419 F01

1.8μH

SW

59k

47.5k

IN

110k

FB1

40.2k

80.6k

FB2

40.2k

L1: TOKO A960AW-1R8M

22μF
X7R

2.2μF
X7R

2.2μF
X7R

V

OUT

1.8V
400mA MAX

V

OUT

1.5V
300mA MAX

V

OUT

1.2V
300mA MAX

Figure 1. Schematic Showing the LTC3446 Power Supply Confi gured to Deliver 1.8V from the 1A Buck, and 1.5V and

1.2V from the 300mA VLDO Regulators. The VLDO Regulators Are Powered from the Buck Output via the LVIN Pin.

07/07/419

regulators, while the VLDO regulators provide additional
lower voltages with good effi ciency at the extremely low
noise levels typical of linear regulators.

The schematic in Figure 1 shows the LTC3446 confi gured
to deliver 1.8V from the buck, 1.5V from the fi rst VLDO
regulator, and 1.2V from the second VLDO regulator.
Figure 2 shows the Figure 1 circuit assembled onto a
printed-circuit board.

Selectable Burst Mode® Operation or Pulse­Skipping at Light Load

The LTC3446’s buck regulator f eatures Burst Mode opera­tion for optimum effi ciency when operating at light loads,
at the cost of increased output ripple and the introduction
of switching noise below the 2.25MHz clock frequency.
Burst Mode operation can be defeated by bringing the
MODESEL pin high, which commands the LTC3446 to
continue to switch at the 2.25MHz clock frequency down
to very light loads, whereupon pulses are skipped as
needed to maintain regulation. Figure 3, which shows
the effi ciency of the buck regulator vs load current, also
illustrates the typical effi ciency gains from using Burst
Mode operation at load currents below 100mA.

Very Low Dropout (VLDO) Linear Regulators

The VLDOs in the LTC3446 employ an NMOS source-fol­lower architecture to overcome the traditional tradeoff
between dropout voltage, quiescent current and load
transient response inherent in most PMOS- and PNP­based LDO regulator architectures. The V

pin (refer

IN

to Figure 1), supplies only the micropower bias needed
by the VLDO control and reference circuits, typically at
single-cell Li-Ion voltages. The actual load current is

sourced from the LV

pin, which can be connected to

IN

the buck regulator output.

Each VLDO regulator provides a high accuracy output
that is capable of supplying 300mA of output current
with a typical dropout voltage of only 70mV from LV
to LV

. VIN should exceed the LV

OUT

regulation point

OUT

IN

by 1.4V to provide suffi cient gate drive to the internal
NMOS pass device. Typical single-cell Li-Ion operating
voltages extend down to 3.2V, supporting VLDO output
voltages of up to 1.8V.

A single ceramic c apacitor between 1μF and 2.2μF is all tha t
is required for output bypassing. A low reference voltage
of 400mV allows the VLDO regulators to be programmed
to much lower voltages than are commonly available in
LDO regulators.

Power Good Detection

The LTC3446 includes a built-in supply monitor. The
PGOOD open-drain output pin is pulled low while any
enabled output is mor e than ±8% from its regula tion value.
Once all enabled outputs are within this tolerance window,
the PGOOD pin becomes high impedance. A microproc es­sor can monitor this open drain output pin to assess when
a recently enabled output has completed startup.

Conclusion

The LTC3446 packs an effi cient 1A buck regulator and
two 300mA VLDO regulators in a tiny 3mm × 4mm DFN
package. With an output voltage range extending down
to 0.4V for the VLDO regulators and 0.8V for the buck,
and an input voltage range covering the single-cell Li­Ion range up to 5.5V, the LTC3446 is ideal for powering
today’s multi-voltage, sub-2V systems.

100

Burst Mode

OPERATION

90

PWM

MODE

Figure 2. The LTC3446 Triple Power Supply
Assembled on a Printed Circuit Board

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

80

70

EFFICIENCY (%)

60

1

VIN = 2.7V

= 3.6V

V

IN

= 4.2V

V

IN

10 100

LOAD CURRENT (mA)

1000

DN419 F03

50

40

Figure 3. Effi ciency of the LTC3446’s
Buck Regulator vs Load

For applications help,

call [phone], Ext. 2759

dn419f LT/TP 0707 409K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2007