DN1008 - Simple, Efficient, All-in-One USB Power Management IC Solution

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Simple, Efficient, All-in-One USB Power Management IC Solution
Design Note 1008
John Shannon
Introduction
Linear Technology offers a variety of devices that simplify
converting power from a USB cable, but the LTC®3455
represents the highest level of functional integration yet
achieved. The LTC3455 seamlessly manages power flow
between an AC adapter, USB cable and Li-ion battery,
while complying with USB power standards, all from a
4mm × 4mm QFN package. In addtion, two high efficiency
synchronous buck converters generate low voltage rails
which most USB-powered peripherals require. The
LTC3455 also provides power-on reset signals for the
microprocessor, a Hot SwapTM output for powering
memory cards as well as an uncommitted gain block
suitable for use as a low-battery comparator or an LDO
controller. The PCB real estate required for the entire USB
power control circuit and two DC/DC converters is only
225mm2.
The simplified block diagram of the LTC3455 in Figure 1
shows the major components that allow the LTC3455 to
accomplish multiple functions. The DC/DC conversion is
WALL
ADAPTER
VMAX
CURRENT SOURCE
USB
INPUT
~LTC3405
LTC4053
CC/CV
Li-Ion
BATTERY
CHARGER
LTC4411
IDEAL
DIODE
SW1
~LTC3406
SW2
DN1008 F01
+
Li-Ion
Figure 1. LTC3455 Simplified Block Diagram
08/04/1008
a relatively straightforward buck converter function. Each
of the DC/DC converters takes as its input a voltage called
VMAX, which is derived from one of the three available
power sources (explained below) and converts it to the
desired output voltage. The DC/DC converters provided
are high frequency, high efficiency synchronous converters with characteristics that are roughly equivalent to the
LTC3405 and the LTC3406. VMAX represents an intermediate voltage bus.
Operation with Wall Adapter Present
If the wall adapter is present, the intermediate VMAX bus
is powered regardless of other available power sources,
providing power to both the DC/DC converters and the
battery charger. The battery, if also present, is charged via
the LTC3455’s constant current, constant voltage, timer
terminated, linear charger (similar in features and characteristics to the LTC4053).
Operation with No Wall Adapter Present,
But Available USB Power
If the source of power is a USB input, then the USB power
is switched through to VMAX via a current limiting circuit
in order to comply with USB current limits. The voltage at
VMAX is used to run both DC/DC converters. If the required
load power is less than the available USB power, then the
remaining power is used to simultaneously charge the
battery. As the DC/DC load increases, the battery charge
current decreases in such a way as to maintain USB input
current compliance. Eventually, as the load power exceeds the available USB power, battery charging ceases
and the battery begins to source supplemental power
through the internal ideal diode into VMAX. In this mode,
some of the load current is provided from the available
USB power, with the balance coming from the battery.
This intermediate bus power architecture has important
benefits at the system level. In contrast with so-called
,LTC and LT are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation.
“charger fed” systems in which the DC/DC loads are
placed in parallel with the battery, this system is capable
of running without a battery or with a fully discharged
battery. Also, in a charger fed system, power is wasted by
first linearly regulating the USB voltage down to the
battery voltage. The benefits of the intermediate bus
architecture are illustrated in Figure 3. An intermediate
POWER
INPUTS
bus voltage yields faster charging and lower heat dissipation than a charger-fed architecture.
Operation when Unplugged
When both the adapter and USB input are not present, the
LTC3455 powers VMAX through an ideal diode connected
to the Li-ion cell.
POWER
OUTPUTS
STATUS OUTPUTS
D5
ADAPTER
VMAX
C1
10µF
1Ω
4.7µF
R6
3.32k
10
8
USB
11
1Ω
17
4.7µF
16
1-CELL
Li-Ion
BATTERY
9
2
C4
1µF
R8
1.24k
R16
2.49k
3
C6
1µF
25
VMAX
USB
20
23
4
RST PBSTAT CHRG
HSO
HSI
WALLFB
14
13
AO
SW2
AI
VBAT
FB2
LTC3455
12
L1
4.7µH
R9
237k
C3
10µF
18
R10
80.6k
PROG
TIMER
SW1
FB1
GND (EXPOSED PAD)
7
L2
4.7µH
1
R12
100k
ON SUSPEND USBHP MODE PWRON ON2 HSON
24
6
5
3.15V
Hot Swap OUT
3.15V
600mA
21
22
19
R14
80.6k
15
1.8V
400mA
C5
10µF
CONTROL INPUTS
DN1008 F02
Figure 2. Minimum LTC3455 Implementation
CHARGER-FED TOPOLOGY
5V 500mA (2.5W)
LTC3455
INTERMEDIATE BUS
ARCHITECTURE
VUSB
VOLTAGE
BATTERY
CHARGER
USB 5V 500mA (2.5W)
I≅
PO
VUSB
I≅
VUSB
PASS
ELEMENT
TO GND
0.5A
PO
VBAT
1.5W
LOAD
1.5W
LOAD
ICHRG
ICHRG
VBAT
VBAT
+
DN1008 F03
+
VBAT ICHRG PLOSS
4.0
3.6
3.0
2.5
200
200
200
50
0.20W
0.28W
0.40W
0.13W
VBAT ICHRG
4.0 0.125
3.6 0.083
3.0
0
2.5 –0.100
PLOSS
0.50W
0.70W
1.00W
1.25W
Figure 3. Charge Rate and Dissipation Advantage of Intermediate Bus vs Charger Fed Topologies
Data Sheet Download
http://www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 2759
dn1008f LT 0804 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 2004
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