December 2009 - All-in-One Power for Portables: Single IC Replaces Battery Charger, Pushbutton Controller, LED Driver and Five Voltage Regulator ICs

L DESIGN IDEAS
All-in-One Power for Portables:
Single IC Replaces Battery Charger,
Pushbutton Controller, LED Driver and
Five Voltage Regulator ICs by Marty Merchant
Introduction
Pushbutton Control
The LTC3577/LTC3577-1 integrates
a number of portable device power
management functions into one IC,
reducing complexity, cost and board
area in handheld devices. The major
functions include:
qFive voltage regulators to power
memory, I/O, PLL, CODEC, DSP
or a touch-screen controller
qA battery charger and
PowerPath™ manager
qAn LED driver for backlighting
an LCD display, keypad and/or
buttons
qPushbutton control for
debouncing the on/off button,
supply sequencing and allowing
end-users to force a hard reset
when the microcontroller is not
responding
By combining these functions,
the LTC3577/LTC3577-1 does more
than just reduce the number of required ICs; it solves the problems of
ADAPTER
PC USB
LTC3577/LTC3577-1
5V
5V
OVP
VOUT
INPUT
POWER
CC/CV
POWER ON
MICRO
functional interoperability—where
otherwise separate features operate
together for improved end-product
performance. For instance, when the
power input is from USB, the limited
input current is logically distributed
among the power supply outputs and
the battery charger.
The LTC3577/LTC3577-1 offers
other important features, including
PowerPath control with instant-on
operation, input overvoltage protection
for devices that operate in harsh environments and adjustable slew rates on
the switching supplies, making it possible to reduce EMI while optimizing
efficiency. The LTC3577-1 features a
4.1V battery float voltage for improved
battery cycle life and additional high
temperature safety margin, while the
LTC3577 includes a standard 4.2V
battery float voltage for maximum
battery run time.
STATUS
I2C BUS
PUSHBUTTON
4.2V/4.1V
SEQUENTIAL
BUCK
LOGIC
CONTROL
VOUT1
MICROCONTROLLER
VOUT2
MEMORY
VOUT3
I/0
I2C
CONTROLLED
LED DRIVER
LCD BACKLIGHTING
I2C
CONTROLLED
LDO1
CODEC/DSP
LDO2
PERIPHERAL POWER
Figure 1. Portable device power distribution block diagram featuring the LTC3577/LTC3577-1
34
The built in pushbutton control circuitry of the LTC3577/LTC3577-1
eliminates the need to debounce the
pushbutton and includes power-up
sequence functionality. A PB Status
output indicates when the pushbutton
is depressed, allowing the microprocessor to alter operation or begin the
power-down sequence. Holding the
pushbutton down for five seconds
produces a hard reset. The hard reset
shuts down the three bucks, the two
LDOs and the LED driver, allowing
the user to power down the device
when the microprocessor is no longer
responding.
Battery, USB, Wall and High
Voltage Input Sources
The LTC3577/LTC3577-1 is designed
to direct power from two power supply
inputs and/or a Li-Ion/Polymer battery. The VBUS input has selectable
input current limit control, designed
to deliver 100mA or 500mA for USB
applications, or 1A for higher power
applications. A high power voltage
source such as a 5V supply can be
connected via an externally controlled
FET. The voltage control (VC) pin can
be used to regulate the output of a
high voltage buck, such as the LT3480,
LT3563 or LT3505 at a voltage slightly
above the battery for optimal battery
charger efficiency.
Figure 1 shows a system block diagram of the LTC3577/LTC3577-1. An
overvoltage protection circuit enables
one or both of the input supplies to be
protected against high voltage surges.
The LTC3577/LTC3577-1 can provide
power from a 4.2V/4.1V Li-Ion/Polymer battery when no other power is
available or when the VBUS input current limit has been exceeded.
Linear Technology Magazine • December 2009
DESIGN IDEAS L
Battery Charger
The LTC3577/LTC3577-1 battery
charger can provide a charge current
up to 1.5A via VBUS or wall adapter
when available. The charger also has
an automatic recharge and a trickle
charge function. The battery charge/
no-charge status, plus the NTC status
can be read via the I2C bus. Since
Li-Ion/Polymer batteries quickly lose
capacity when both hot and fully
charged, the LTC3577/LTC3577-1
reduces the battery voltage when the
battery heats up, extending battery
life and improving safety.
Three Bucks, Two LDOs
and a Boost/LED Driver
VOUT
2.7V TO 5.5V
ILED_FS
LTC3577/
LTC3577-1
C1
22µF
6.3V
L2
47nH
IFC-0805-47
L1
10µH
LPS4018-103M
D2
BAT54S
C4
10µF
25V
C2
10µF
25V
SW
D1
BAT54S
C3
10µF
25V
ILED
R1
301k
VBOOST
–12V
35mA
VBOOST
12V
35mA
R2
21.5k
LED_OV
DN470 F02
Figure 2. Dual polarity boost converter
A patented switching slew rate control
feature, set via the I2C interface, allows
the reduction of EMI noise in exchange
for efficiency.
The LTC3577/LTC3577-1 LED
boost driver can be used to drive up
to 10 series white LEDs at up to 25mA
or be configured as a constant voltage
boost converter. As a LED driver, the
current is controlled by a 6-bit, 60dB
logarithmic DAC, which can be further
reduced via internal PWM control. The
LED current smoothly ramps up and
down at one of four different rates.
Overvoltage protection prevents the
internal power transistor from damage if an open circuit fault occurs.
Alternatively, the LED boost driver can
be configured as a fixed voltage boost,
providing up to 0.75W at 36V.
Many circuits require a dual polarity
voltage to bias op amps or other analog
devices. A simple charge pump circuit,
as shown in Figure 2, can be added
to the boost converter switch node to
provide a dual polarity supply. Two
forward diodes are used to account
for the two diode voltage drops in the
inverting charge pump circuit and
provide the best cross-regulation.
For circuits where cross-regulation is
not important, or with relatively light
negative loads, using a single forward
diode for the boost circuit provides the
best efficiency.
Multiphase Operation
Thermal Performance
Conclusion
For a 4-phase, 4-rail output voltage system, use two LTM4619s and
drive their MODE_PLLIN pins with a
LTC6908-2 oscillator, such that the
two µModule devices are synchronized
90° out of phase. Reference Figure 21 in
the LTM4619 data sheet. Synchronization also lowers voltage ripple, reducing the need for high voltage capacitors
whose bulk size consumes board
space. The design delivers four different output voltage rails (5V, 3.3V,
2.5V and 1.8V) all with 4A maximum
load.
Exceptional thermal performance
is shown in Figure 5 where the
unit is operating in parallel output mode; single 12VIN to a single
1.5VOUT at 8A. Both outputs tied
together create a combined output
current of 8A with both channels
running at full load (4A each). Heat
dissipation is even and minimal, yielding good thermal results. If additional
cooling is needed, add a heat sink on
top of the part or use a metal chassis
to draw heat away.
The LTM4619 dual output µModule
regulator makes it easy to convert
a wide input voltage range (4.5V to
26.5V) to two or more 4A output
voltage rails (0.8V to 5V) with high
efficiency and good thermal dissipation. Simplicity and performance are
achieved through dual output voltage
regulation from a single package,
making the LTM4619 an easy choice
for system designs needing multiple
voltage rails. L
The LTC3577/LTC3577-1 contains
five resistor-adjustable step-down
regulators: two bucks, which can provide up to 500mA each, a third buck,
which can provide up to 800mA, and
two LDO regulators, which provide
up to 150mA each and are enabled
via the I2C interface. Individual LDO
supply inputs allow the regulators to be
connected to low voltage buck regulator outputs to improve efficiency. All
regulators are capable of low voltage
operation, adjustable down to 0.8V.
The three buck regulators are sequenced at power up (VOUT1, VOUT2
then VOUT3) via the pushbutton controller or via a static input pin. Each
buck can be individually selected to
run in Burst Mode operation to optimize efficiency or pulse-skipping mode
for lower output ripple at light loads.
Conclusion
The high level of integration of the
LTC3577/LTC3577-1 reduces the
number of components, required
board real estate and overall cost of
power systems for portable electronics.
It greatly simplifies power path design
with built-in solutions to a number of
complex power flow logic and control
problems. L
LTM4619, continued from page 33
Linear Technology Magazine • December 2009
35
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