DN470 - One Device Replaces Battery Charger, Pushbutton Controller, LED Driver and Voltage Regulator ICs in Portable Electronics

One Device Replaces Battery Charger, Pushbutton Controller,
LED Driver and Voltage Regulator ICs in Portable Electronics
Design Note 470
Marty Merchant
Introduction
The LTC®3577/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:
• Five voltage regulators to power memory, I/O, PLL,
CODEC, DSP or a touch-screen controller
• A battery charger and PowerPath™ manager
• An LED driver for backlighting an LCD display, keypad
and/or buttons
• Pushbutton 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 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.
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.
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and μModule are
registered trademarks of Linear Technology Corporation. PowerPath is a trademark of
Linear Technology Corporation. All other trademarks are the property of their respective
owners.
LTC3577/LTC3577-1
ADAPTER
OVP
PC USB
Pushbutton Control
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
09/09/470
VOUT
INPUT
POWER
5V
4.2V/4.1V
CC/CV
POWER ON
MICRO
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.
5V
SEQUENTIAL
BUCK
STATUS
2
I C BUS
PUSHBUTTON
LOGIC
CONTROL
VOUT1
MICROCONTROLLER
VOUT2
MEMORY
VOUT3
I/0
I2C
CONTROLLED
LED DRIVER
LCD BACKLIGHTING
I2C
CONTROLLED
LDO1
CODEC/DSP
LDO2
PERIPHERAL POWER
DN470 F01
Figure 1. Portable Device Power Distribution Block
Diagram Featuring the LTC3577/LTC3577-1
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.
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.
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.
VOUT
2.7V TO 5.5V
ILED_FS
Three Bucks, Two LDOs and a Boost/LED Driver
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. A patented switching slew rate control
feature, set via the I2C interface, allows the reduction of
EMI noise in exchange for efficiency.
Data Sheet Download
www.linear.com
C1
22μF
6.3V
LTC3577/
LTC3577-1
L2
47nH
IFC-0805-47
L1
10μH
LPS4018-103M
C4
10μF
25V
D2
BAT54S
C2
10μF
25V
SW
D1
BAT54S
C3
10μF
25V
R1
301k
VBOOST
–12V
35mA
VBOOST
12V
35mA
ILED
R2
21.5k
LED_OV
DN470 F02
Figure 2. Dual Polarity Boost Converter
Conclusion
The high level of integration of the LTC3577/LTC3577-1
reduces the number of components, required board real
estate and overall cost; and greatly simplifies design
by solving a number of complex power flow logic and
control problems.
For applications help,
call (978) 656-4778
Linear Technology Corporation
dn470f LT/TP 0909 116K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2009
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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