LTC3455EUF-1#PBF [Linear]
LTC3455/LTC3455-1 - Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger; Package: QFN; Pins: 24; Temperature Range: -40°C to 85°C;
| 型号： | LTC3455EUF-1#PBF |
| 厂家： | 
Linear |
| 描述： | LTC3455/LTC3455-1 - Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger; Package: QFN; Pins: 24; Temperature Range: -40°C to 85°C 电池 开关 |
| 文件： | 总28页 (文件大小：337K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3455/LTC3455-1
Dual DC/DC Converter
with USB Power Manager
and Li-Ion Battery Charger
FEATURES
DESCRIPTION
The LTC®3455/LTC3455-1 are complete power manage-
ment solutions for a variety of portable applications.
These devices contain two synchronous step-down
DC/DC converters, a USB power controller, a full-featured
Li-Ion battery charger, a Hot Swap output, a low-battery
indicator, and numerous internal protection features. The
LTC3455/LTC3455-1 provide a small, simple solution for
obtaining power from three different power sources: a
single-cell Li-Ion battery, a USB port, and a wall adapter.
Current drawn from the USB bus is accurately limited
under all conditions. Whenever a USB or a wall adapter
is present, the battery charger is enabled and all internal
powerforthedeviceisdrawnfromtheappropriateexternal
powersource.Alloutputsaredischargedtogroundduring
shutdown to provide complete output disconnect. These
devices are available in a 4mm × 4mm 24-pin exposed-
pad QFN package.
n
Seamless Transition between Input Power Sources:
Li-Ion Battery, USB, and 5V Wall Adapter
n
Accurate USB Current Limiting (500mA/100mA)
n
Two High Efficiency DC/DC Converters: Up to 96%
n
Thermal Regulation Maximizes Battery Charge
Rate without Risk of Overheating
n
Full-Featured Li-Ion Battery Charger with 4.2V Float
Voltage for LTC3455 and 4.1V for LTC3455-1
n
4.1V Float Voltage (LTC3455-1) Improves Battery Life
and High Temperature Safety Margin
n
Hot Swap™ Output for SDIO and Memory Cards
Pin-Selectable Burst Mode® Operation
n
n
Output Disconnect: All Outputs Discharged to Ground
During Shutdown
n
Available in a 4mm × 4mm × 0.75mm 24-Pin
QFN Package
APPLICATIONS
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents including 6522118.
n
Handheld Computers
n
Digital Cameras
n
MP3 Players
TYPICAL APPLICATION
USB 5V
1Ω
USB
MODE
HSON
ON2
SUSPEND
USB
CONTROLLER
5.6V
4.7μF
μC
USBHP
PWRON
RST
Efficiency
WALL 5V
V
MAX
100
95
90
85
80
75
70
65
60
55
1000
100
10
PBSTAT
SWITCHER 2
= 3.3V
10μF
1Ω
4.7μF
V
OUT2
1M
1M
ON/OFF
LTC3455/LTC3455-1
3.32k
1.24k
1k
1.8V
CHRG
SWITCHER 1
= 1.8V
ON
V
OUT1
WALLFB
0.1μF
3.3V, HS
1μF
HSO
TIMER
PROG
HSI
2.49k
POWER LOSS FOR
BOTH OUTPUTS
4.7μH
3.3V
0.5A
SW2
V
BAT
10pF
249k
4.7μF
1M
SINGLE
CELL Li-ION
3.3V TO 4.2V
V
= 3.6V
BAT
1.8V
10μF
FB2
+
1
1000
1
10
100
80.6k
LOAD CURRENT (mA)
LBO
AO
AI
3455 TA01b
4.7μH
10pF
V
BAT
1.8V
0.4A
SW1
2.49M
100k
10μF
FB1
806k
80.6k
GND
3455 TA01a
3455fc
1
LTC3455/LTC3455-1
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
Transient (t < 1ms and Duty Cycle < 1%):
V
USB Voltages.................................. –0.3V to 7V
MAX
Steady State:
, V
SW1, SW2 Voltages................... –0.3V to (V
TIMER Voltage ........................... –0.3V to (V
PWRON, ON, ON2, HSON Voltages ............. –0.3V to 6V
PBSTAT, RST, CHRG, AO Voltages ............... –0.3V to 6V
HSI, HSO Voltages ....................................... –0.3V to 6V
MODE, USBHP, SUSPEND Voltages.............. –0.3V to 6V
WALLFB, AI, PROG Voltages........................ –0.3V to 2V
FB1, FB2 Voltages........................................ –0.3V to 2V
Junction Temperature ........................................... 125°C
Operating Temperature Range (Note 2)....–40°C to 85°C
Storage Temperature Range- .................–65°C to 125°C
24 23 22 21 20 19
V
, USB Voltages........................ –0.3V to 6V
BAT MAX
FB1
PROG
1
2
3
4
5
6
18 FB2
+ 0.3V)
+ 0.3V)
MAX
MAX
AO
AI
17
16
TIMER
25
CHRG
15 HSON
HSO
USBHP
SUSPEND
14
13 HSI
7
8
9 10 11 12
UF PACKAGE
24-LEAD (4mm s 4mm) PLASTIC QFN
= 125°C, θ = 37°C/W, θ = 4.3°C/W
T
JMAX
JA
JC
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3455EUF#PBF
LTC3455EUF-1#PBF
TAPE AND REEL
PART MARKING
3455
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3455EUF#TRPBF
LTC3455EUF-1#TRPBF
–40°C to 85°C
–40°C to 85°C
24-Lead (4mm × 4mm) Plastic QFN
24-Lead (4mm × 4mm) Plastic QFN
34551
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON2 = 0V,
VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
3.0
MAX
UNITS
V
Battery Undervoltage Lockout Voltage
Battery Undervoltage Lockout Hysteresis
V
BAT
Rising
2.9
3.2
450
mV
V
BAT
Pin Quiescent Current (Note 3)
Burst Mode, Battery Powered
PWM Mode, Battery Powered
USB Powered
V
V
V
= V
= 1V, Not Switching
MODE
110
500
10
160
800
20
μA
μA
μA
μA
μA
ON2
ON2
USB
= 1V, V
= 0V, Not Switching
MODE
= 5V, Charger Off
Wall Powered
Shutdown
V
= 1.5V, V
= 4.5V, Charger Off
= 0V
10
20
WALL
V
MAX
MAX
= 0V, V
2
4
PWRON
ON Pin Threshold
0.8
0.8
0.8
0.8
1.23
60
1.1
1.0
V
V
PWRON Pin Threshold
ON2 Pin Threshold
1.0
V
MODE Pin Threshold
WALLFB Pin Threshold Voltage
WALLFB Pin Hysteresis
1.0
V
l
WALLFB Rising
1.20
1.26
V
mV
3455fc
2
LTC3455/LTC3455-1
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON2 = 0V,
VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
2.5
2.5
2.5
2.5
1
MAX
UNITS
μA
ON Pin Pullup Current
V
V
V
V
= 1V
ON
PWRON Pin Pulldown Current
ON2 Pin Pulldown Current
MODE Pin Pullup Current
WALLFB Pin Input Bias Current
PBSTAT Pin Low Voltage
= 1V
= 1V
μA
PWRON
μA
ON2
= 1V
μA
MODE
l
V
= 1.35V
30
nA
WALLFB
V
= 0V, I
PBSTAT
= 0V, I
PBSTAT
= 100μA
= 1mA
0.02
0.20
0.10
0.35
V
V
ON
ON
V
RST Pin Low Voltage
I
I
= 100μA
= 1mA
0.02
0.20
0.10
0.35
V
V
RST
RST
RST Pulse Duration
After FB1 and FB2 in Regulation
200
ms
Battery-V
PMOS
MAX
V
V
V
PMOS Switch On-Resistance
Switch Current Limit
0.15
4.0
Ω
A
MAX
MAX
MAX
2.5
0.4
Switch Current Limit at Startup
With V
Rising, V
= 3V, V = 3.6V
0.9
A
MAX
MAX
BAT
Gain Block
l
l
AI Pin Threshold Voltage
AI Pin/FB2 Pin Voltage Difference
AI Pin Input Bias Current
AO Pin Sink Current
0.784
–8
0.805
0
0.826
8
V
mV
nA
mA
V
V
– V
AI
FB2
V
AI
= 0.85V
1
25
V
AI
= 0.6V, V = 1.5V
1.0
1.8
0.8
2.5
1.2
AO
AO Pin Voltage
V = 0.6V, I = 1mA
AI AO
Switching Regulators
l
l
FB1, FB2 Voltage
0.784
0.805
0.01
8
0.826
V
%/V
mV
nA
FB1, FB2 Voltage Line Regulation
FB1, FB2 Voltage Burst Mode Hysteresis
FB1, FB2 Pin Input Bias Current
Switching Frequency
V
V
V
= 3V to 5V
MAX
= 2V
MODE
= V = 0.85V
1
25
FB1
FB2
Both Switchers
Both Switchers
Both Switchers
1.2
1.5
1.8
MHz
Ω
PMOS Switch On-Resistance
NMOS Switch On-Resistance
PMOS Switch Current Limit
0.35
0.45
Ω
Switcher 1
Switcher 2
450
700
600
900
850
1300
mA
mA
USB Power Manager
USB Undervoltage Lockout Voltage
USB Undervoltage Lockout Hysteresis
USB Minimum Voltage to Charge Battery
USB PMOS Switch On-Resistance
USB Current Limit
From Low to High
3.75
3.90
150
4.0
4.10
V
mV
V
V
USB
= 5V
0.5
Ω
l
l
V
USB
V
USB
= 5V, V
= 5V, V
= 2V
= 0V
440
60
475
80
500
100
mA
mA
USBHP
USBHP
USB Suspend Mode Bias Current
SUSPEND Pin Threshold
V
= 5V, V
= 2V
4
20
1.1
1.1
μA
V
USB
SUSPEND
0.8
0.8
2.5
2.5
USBHP Pin Threshold
V
SUSPEND Pin Pulldown Current
USBHP Pin Pulldown Current
V
V
= 0.5V
μA
μA
SUSPEND
= 0.5V
USBHP
3455fc
3
LTC3455/LTC3455-1
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VBAT = 3.6V, VMAX = 3.6V, VPWRON = 2V, VON is open, VON2 = 0V,
VUSB = 0V, VWALLFB = 0V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Hot Swap Output
Hot Swap PMOS Switch On-Resistance
Hot Swap PMOS Switch Current Limit
HSON Pin Threshold
V
V
= 3.3V
0.9
160
0.8
2.5
Ω
mA
V
HSI
= 3.3V, V
= 2.5V
120
HSI
HSO
1.1
HSON Pin Pulldown Current
Battery Charger
μA
Regulated Charger V Voltage
0°C ≤ T ≤ 85°C (LTC3455)
4.158
4.058
4.200
4.1
4.242
4.142
V
V
BAT
A
0°C ≤ T ≤ 85°C (LTC3455-1)
A
Charger Current Limit (USB Powered)
R
PROG
R
PROG
= 2.49kΩ, V
= 2.49kΩ, V
= 2V, V
= 0V, V
= 5V, 0°C ≤ T ≤ 85°C
400
50
470
90
mA
mA
USBHP
USBHP
USB
USB
A
= 5V, 0°C ≤ T ≤ 85°C
425
A
Charger Current Limit (Wall Powered)
Recharge Battery Voltage Threshold
Trickle Charge Trip Threshold
Trickle Charge Trip Hysteresis
Trickle Charge Current
R
= 2.49kΩ, V
= 4.5V, 0°C ≤ T ≤ 85°C
500
150
2.85
60
575
mA
mV
V
PROG
MAX
A
V
– V
RECHARGE
BAT(REGULATED)
Battery Voltage Rising
mV
mA
μA
V
R
PROG
= 2.49kΩ, V = 2V
65
BAT
PROG Pin Current
Internal Pull-Up Current, No R
2
PROG
PROG Pin Voltage
R
PROG
= 2.49kΩ
= 5mA
1.23
0.75
10
CHRG Pin Output Low Voltage
Timer Accuracy
I
V
CHRG
C
TIMER
= 0.1μF
%
Junction Temperature in Constant
Temperature Mode
105
°C
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
expected to meet these extended temperature limits, but is not 100%
tested at –40°C and 85°C.
Note 3: Quiescent current is pulled from the V pin when neither USB
BAT
or wall power is present, and from the V
power is present.
pin when either USB or Wall
MAX
Note 2: The LTC3455/LTC3455-1 are guaranteed to meet specified
performance from 0°C to 85°C and is designed, characterized and
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Quiescent Current
PWM Mode Quiescent Current
Shutdown Quiescent Current
120
100
80
60
40
20
0
600
500
400
300
200
100
0
5
4
3
2
1
0
V
= 3.6V
BAT
BOTH SWITCHERS ENABLED
BOTH SWITCHERS ENABLED
ONLY SWITCHER 1 ENABLED
ONLY SWITCHER 1 ENABLED
V
= 3.6V
V
= 3.6V
BAT
BAT
NOT SWITCHING
NOT SWITCHING
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
3455 G01
3455 G02
3455 G03
3455fc
4
LTC3455/LTC3455-1
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Pins (FB1, FB2)
and AI Pin Voltage
Switching Regulator Oscillator
Frequency
Switching Regulator Current Limit
815
810
805
800
795
790
785
2.0
1.5
1.0
0.5
0
1000
SWITCHER 2
800
FOR BOTH SWITCHERS
AI
SWITCHER 1
600
400
200
0
FB1
FB2
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (oC)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
3455 G04
3455 G05
3455 G06
USB Pin Current Limit
VMAX Pin Current Limit
HSO Pin Current Limit
500
400
300
200
100
0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
200
150
100
50
USBHP = 2V
NORMAL OPERATION
STARTUP
USBHP = 0V
V
V
= 3.3V
= 2.5V
HSI
HSO
V
= 5V
USB
0
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
3455 G07
3455 G08
3455 G09
Battery Undervoltage Lockout
USB Undervoltage Lockout
WALLFB Trip Voltage
4.00
3.75
3.50
3.25
3.00
2.75
2.50
4.00
3.75
3.50
3.25
3.00
2.75
2.50
1.26
1.24
1.22
1.20
1.18
1.16
1.14
1.12
1.10
RISING
FALLING
RISING
FALLING
RISING
FALLING
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
3455 G10
3455 G11
3455 G12
3455fc
5
LTC3455/LTC3455-1
TYPICAL PERFORMANCE CHARACTERISTICS
Battery Charger Regulation
Voltage
Battery Charger Recharge
Threshold
Battery Charger Trickle-Charge
Threshold
4.30
4.25
4.20
4.15
4.10
4.05
4.00
4.20
4.15
4.10
4.05
4.00
3.95
3.90
3.0
2.9
2.8
2.7
2.6
2.5
LTC3455
RISING
LTC3455
FALLING
LTC3455-1
LTC3455-1
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
3455 G13
3455 G14
3455 G15
Charge Current When
Wall-Powered
Charge Current When
USB-Powered
Battery Current When
USB- or Wall-Powered
600
500
400
300
200
100
0
600
500
400
300
200
100
0
15.0
12.5
10.0
7.5
5.0
2.5
0
V
V
= 3.6V
= 5V
PROG
V
= 4.2V
BAT
BAT
USB
R
CHARGER OFF
= 2.49k
V
= 2V
USBHP
THERMAL CONTROL
LOOP IN OPERATION
THERMAL CONTROL
LOOP IN OPERATION
V
V
= 3.6V
= 4.5V
PROG
BAT
MAX
R
V
= 0V
USBHP
= 2.49k
50
100 125
50
TEMPERATURE (°C)
100 125
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
–50 –25
0
25
75
–50 –25
0
25
75
TEMPERATURE (°C)
3455 G16
3455 G17
3455 G18
PROG Pin Voltage
vs Charge Current
RDS(ON) for Switching Regulator
Power Switches
RDS(ON) for VMAX, USB, and HSO
PMOS Switches
1.50
1.25
1.00
0.75
0.50
0.25
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
V
V
= 3.6V
= 4.5V
= 2.49
V
BAT
= 3.6V
V
V
V
= 3.3V
= 5V
= 3.6V
BAT
MAX
PROG
= 25°C
HSI
USB
BAT
R
T
HSO
USB
A
NMOS
PMOS
V
MAX
50
400
CHARGE CURRENT (mA)
500
50
TEMPERATURE (°C)
100 125
100 125
0
100
200
300
–50 –25
0
25
75
–50 –25
0
25
75
TEMPERATURE (°C)
3455 G19
3455 G20
3455 G21
3455fc
6
LTC3455/LTC3455-1
PIN FUNCTIONS
FB1 (Pin 1): Feedback Pin for Switcher 1. Set the output
voltage by connecting feedback resistors to this pin.
V
(Pin 10): Max Voltage Pin. This pin is used to
MAX
power the two internal step-down DC/DC converters and
is provided externally to power other devices (i.e. LDOs
or Switchers for LCD bias, white LED backlight drive, etc).
When the LTC3455/LTC3455-1 is on and neither USB or
wallpowerareavailable,aninternalPMOSswitchconnects
PROG (Pin 2): Charge Current Program and Charge Cur-
rent Monitor Pin. Connect a resistor, R
to ground to program battery charge current.
, from this pin
PROG
I
= 1000 • 1.23V / R
this pin to the V
pin. When either USB or wall power
BAT
PROG
BAT
is present, they provide power to this pin, and the battery
charger draws power from this pin. In shutdown, this pin
is discharged to ground to provide output disconnect.
In all modes the voltage on the PROG pin can be used to
measurechargecurrent. PROGhasaweakpull-upcurrent
source to turn the charger off if the pin is left open.
WALLFB(Pin11):WallPowerDetectionPin.Thispinisthe
input to a comparator used to signal the presence of a 5V
wall adapter. A resistor divider taken from the wall adapter
inputisconnectedtothispintotelltheLTC3455/LTC3455-1
when the adapter voltage is high enough to provide power
to the LTC3455/LTC3455-1. When this pin is higher than
1.23V, the battery charger is enabled. The 5V wall adapter
TIMER (Pin 3): Timer Capacitor Pin. Connect a capacitor,
TIMER
cycle termination time. The timer starts when USB or wall
power is first present. The timer period is:
C
, between this pin and ground to set the charge
T
(hours) = C
• (3 hours) / (0.1μF)
TIMER
TIMER
Tie TIMER to ground to disable just the internal timer
function. Tie TIMER to V to use the charger in a con-
is connected to the V
pin through a Schottky diode.
MAX
MAX
Tie WALLFB to ground if a wall adapter is not used.
stant-current-onlymode(whichdisablesthetimer,voltage
amplifier and trickle charge function).
SW2 (Pin 12): Switch Pin for Switcher 2. Minimize the
length of the metal trace connected to this pin. Place the
inductor for Switcher 2 as close to this pin as possible.
CHRG (Pin 4): Open-Drain Charge Status Pin. This pin is
pulled low with an internal N-channel MOSFET whenever
the battery charger is enabled, and is forced into a high
impedance state whenever it is disabled.
HSI (Pin 13): Hot Swap Input Pin. This pin is connected
to the HSO pin through a current-limited PMOS switch.
USBHP (Pin 5): USB High Power Mode Pin. This pin is
used to select the appropriate USB current limit (either
500mA or 100mA). Pull high to select 500mA (high power
mode); low to select 100mA (low power mode).
HSO (Pin 14): Hot Swap Output Pin. This output is used
for memory cards or other devices that would appear as
a short if they were hot-plugged directly to one of the
outputs (typically the 3.3V output). The current out of this
pin is limited to 160mA.
SUSPEND (Pin 6): USB Suspend Pin. When this pin is
pulled high, the internal USB power controller is disabled
and the USB pin current reduces to less than 20μA.
HSON (Pin 15): Hot Swap Enable Pin. This pin turns on
the PMOS that connects the HSI and HSO pins.
SW1(Pin7):SwitchPinforSwitcher1.Minimizethelength
of the metal trace connected to this pin. Place the inductor
for Switcher 1 as close to this pin as possible.
AI (Pin 16): Gain Block Input Pin. This pin is the inverting
input to an amplifier that can be used as a low-battery
detector or as an LDO with the addition of an external
PNP or PMOS. The non-inverting input of the gain block
is connected to the 0.8V internal reference.
USB (Pin 8): USB Supply Pin. Input current into this pin
is limited to either 100mA or 500mA based on the state
of the USBHP pin. The charger and Switcher 1 will remain
alive whenever USB power is present (when USB pin is
above 3.9V and SUSPEND is low).
AO (Pin 17): Gain Block Output Pin. This pin is an open-
drain output, and is pulled low when the AI pin is less
than 800mV. This output can be used as a low-battery
detector, or as an LDO with the addition of an external
PNP or PMOS. This pin can sink up to 1mA.
V
(Pin 9): Battery Input Pin. Bypass this pin with a
BAT
capacitor as close to the device as possible.
3455fc
7
LTC3455/LTC3455-1
PIN FUNCTIONS
FB2 (Pin 18): Feedback Pin for Switcher 2. Set the output
PBSTAT (Pin 23): Push-Button Status Pin. This pin is an
open drain output that indicates the state of the ON pin
(whichisusuallyconnectedtoamomentary-onpush-but-
ton) to the microcontroller. This pin follows the state of the
ON pin (PBSTAT goes low when ON is pulled low).
voltage by connecting feedback resistors to this pin.
ON2 (Pin 19): Enable Pin for Switcher 2. This pin turns on
Switcher 2 only if ON is low or PWRON is high. Switcher
2 cannot be turned on by itself.
ON (Pin 24): ON Pin. Pull this pin to ground to turn on
the LTC3455/LTC3455-1. This pin is typically used with
a momentary-on push-button switch to turn on the
LTC3455/LTC3455-1. This pin would be held low until the
PWRON pin is pulled high by a microcontroller to keep the
LTC3455/LTC3455-1 turned on. If a momentary-on switch
is not used, this pin can be held to ground to keep on the
LTC3455/LTC3455-1. Leave ON open if not used. This pin
has a weak pull-up current source.
RST (Pin 20): Reset Pin. This pin is an open-drain output
that provides a 200ms reset signal during power-up to
initialize a microcontroller.
MODE (Pin 21): Burst Mode Enable Pin. Tie this pin high
toallowBurstModeoperationfortheLTC3455/LTC3455-1.
BurstModeoperationwillprovidesuperiorefficiencywhen
both outputs are operating with very low output currents.
Tie this pin to ground to force PWM operation under all
load current conditions. Burst Mode is disabled initially
at startup (for 200ms) and also whenever external power
is available (even if MODE is pulled high).
GND (Pin 25 – Exposed Pad): Ground Pin. The exposed
backside pad is the only ground pin for the LTC3455/
LTC3455-1 and must be soldered to the PC board ground
plane for the device to operate properly.
PWRON (Pin 22): Power-On Pin. Pull this pin high to turn
on the LTC3455/LTC3455-1. This pin is typically used in
conjunction with the ON and PBSTAT pins, and a momen-
tary-on switch. Tie PWRON to ground if not used.
SIMPLIFIED BLOCK DIAGRAM
V
IS CONNECTED TO THE BEST
MAX
5V WALL ADAPTER
AVAILABLE INPUT POWER SOURCE
(WALL ADAPTER, USB OR BATTERY)
USB POWER
MANAGER
USB POWER
3.9V TO 5.3V
V
MAX
USE TO POWER OTHER
DC/DCs AND LDOs
BATTERY
SWITCHER 1
SWITCHER 2
HOT SWAP
Li-Ion BATTERY
3.3V TO 4.2V
V
OUT1
PMOS SWITCH
1.8V TYPICAL
BATTERY
CHARGER
V
OUT2
3.3V TYPICAL
USE FOR LDO
OR LOW BATTERY
INDICATOR
GAIN BLOCK
HOT SWAP OUTPUT
3.3V TYPICAL
3455 SBD
3455fc
8
LTC3455/LTC3455-1
BLOCK DIAGRAM
WALL 5V
3.9V
–
+
3.32k
1Ω
USB POWER MANAGER
WALLFB
EXTPWR
USB
11
+
–
USB
5V
8
1
1000
4.7μF
1.24k
1Ω
1.23V
BATTERY CHARGER
1
5.6V
4.7μF
1000
–
+
REF
BATTERY PMOS SWITCH
V
MAX
10
V
MAX
SUSPEND
USBHP
–
+
+
1.23V
6
5
R
10μF
2.41R
USB
CONTROLLER
4R
1k
CHRG
TIMER
PROG
V
4
3
2
MAX
CHARGE
CONTROL
R
0.1μF
2.49k
GND
25
9
SWITCHER 1
4.7μH
SW1
FB1
PWM
DRIVER
V
7
1
1.8V
10μF
BAT
V
BAT
3.3V to 4.2V
2.43M
806k
100k
AI
4.7μF
16
–
80.6k
0.8V
+
AO
ENABLE
LBO
17
–
+
806k
1.8V
0.8V
ON/OFF
SWITCHER 2
ON
24
23
4.7μH
UVLO
SW2
FB2
PWM
DRIVER
V
+
BAT
12
18
3.3V
PBSTAT
806k
3.0V
–
249k
10μF
EXTPWR
1.8V
–
+
80.6k
PWRON
ON2
μC
0.8V
22
19
ENABLE
RST
MODE
HSON
20
21
15
200ms RESET PULSE
BURST MODE ENABLE
HSI
13
14
HOT SWAP
ENABLE
HSO
3.3V, HS
1μF
3455 BD01
3455fc
9
LTC3455/LTC3455-1
OPERATION
The LTC3455/LTC3455-1 are designed to be a complete
power management solution for a wide variety of portable
systems. The device incorporates two current mode step-
downswitchingregulators, afull-featuredbatterycharger,
a USB power controller, a Hot Swap output, a low-battery
comparator (which can also be configured as an LDO)
and numerous protection features into a single package.
When only battery power is available, the battery PMOS
LTC3455/LTC3455-1 seamlessly transition from battery
power (a single-cell Li-Ion cell) to either the USB supply or
a wall adapter. The battery PMOS switch is turned off, the
charger is activated and all internal power for the device
is drawn from the appropriate external power source.
Maximumchargecurrentandchargetimeareprogrammed
using an external resistor and capacitor, respectively. The
USBpowermanagerprovidesaccuratecurrentlimitingfor
the USB pin under all conditions. The Hot Swap output is
ideal for powering memory cards and other devices that
can be inserted while the system is fully powered.
switch connects the V
pin to the V
pin to provide
MAX
BAT
powertobothswitchingregulators(andanyotherdevices
powered from V ). When external power is applied, the
MAX
APPLICATIONS INFORMATION
Undervoltage Lockout (UVLO)
Whenever the WALLFB pin is above 1.23V, system power
is drawn from the wall adapter via the V
pin, and the
MAX
Ifnoexternalpowerispresent,theLTC3455/LTC3455-1will
startonlyifthebatteryvoltageisabove3.0V. Thisprevents
startingupwithabatterythatistooclosetodeepdischarge.
Once started, the battery must drop below 2.6V before the
LTC3455/LTC3455-1 will shut off. This hysteresis is set
intentionallylargetopreventtheLTC3455/LTC3455-1from
turningoffataninappropriatetime,likeduringtheread-or
write-cycle of a hard-disk drive (which could potentially
damage the drive). The internal UVLO is meant only as a
last chance safety measure to prevent running the battery
voltagetoolowanddamagingit.Anaccurate,user-settable
low-battery threshold can be implemented using the gain
block(seethe“GainBlock”sectionfordetails)whichgives
the microcontroller complete control over the timing of a
shutdown due to a low-battery condition.
battery charger is active. The 5V wall adapter output is
connected to the V pin through a Schottky diode, and
MAX
a resistor divider from the 5V wall input is connected to
the WALLFB pin to signal the LTC3455/LTC3455-1 that
wall power is present. A higher voltage adapter can also
be used, but the 6V maximum rating on the V
pin
MAX
requires the use of an additional regulator to step down
the voltage.
If USB power is present and above 3.9V (and wall power
is not available), system power is drawn from the USB pin.
The battery charger is active, but charge current will be
held off until the USB pin increases above 4.0V to prevent
the battery charger from further loading down an already
low USB supply. As long as the USB pin stays above 3.9V,
the USB port supplies all other system power.
If external power is present and the battery voltage is less
than 3.0V, the V
pin voltage must be greater than 3.9V
If the system needs more power than the USB bus can
supply, the charger turns off completely, the USB power
controller becomes a 500mA (or 100mA) current source
MAX
for the LTC3455/LTC3455-1 to start, and once started, the
V
MAX
pin must stay above 3.1V for the device to continue
running.
andtheV
voltagebeginstodecrease.IfV
continues
MAX
MAX
todecrease,eventuallythebatterywillprovidetheadditional
current needed. This allows the LTC3455/LTC3455-1 to
withstand load current transients that briefly require more
power than the USB power supply can provide.
Selecting the Input Power Source
The priority for supplying power to both DC/DC convert-
ers, all internal circuitry, and the V
USB, battery.
pin is: Wall adapter,
MAX
3455fc
10
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Operation When No Battery Is Present
rest. When wall powered, this operation is intended only
for surviving fault conditions and should not be a normal
mode of operation.
As long as USB or wall power is available, the LTC3455/
LTC3455-1 will operate with no battery present, a crucial
requirement for systems with a removable battery. Keep
in mind, however, that if the LTC3455/LTC3455-1 are USB
powered and the battery is not present, absence of the
batterymeansthatthereisnoreservoirifthesystemneeds
more power than the USB port can supply. This is espe-
cially a problem when starting up the LTC3455/LTC3455-1
in USB low power mode with no battery present, which
is discussed in greater detail on the next page, in the
section entitled “Startup Issues in USB Low Power Mode
When No Battery is Present”. Similarly, if external power
is available, the LTC3455/LTC3455-1 will operate even if
the battery is bad or in deep-discharge.
Concerns When USB Powered
The popularity of USB (Universal Serial Bus) makes it
an attractive choice for transferring data in a variety of
portable devices. Therefore, utilizing the USB port to
power these portable devices while charging their battery
is very desirable, but it is not necessarily an easy task. As
the performance of digital cameras, handheld computers,
and MP3 players increases, the power needed to operate
them also increases. The power available from a single
USB port (maximum 2.5W) is barely enough to support
the peak power needed by many full-featured portable
devices, even without the power needed to quickly charge
their batteries.
TheLTC3455/LTC3455-1arealsoagoodchoiceforsystems
that are always powered by a USB supply or wall adapter.
The charger can then be used to charge a large capacitor
or backup battery, which can briefly provide power to the
system after the external power has been removed. This
gives the microcontroller enough time to follow proper
shutdown procedures even after the main power source
is abruptly removed. If USB powered, the large capacitor
or backup battery will also be used to provide additional
current if the system briefly needs more power than the
USB bus can provide.
To further complicate matters, a USB port is not the ideal
powersource.Eachdevicecandrawamaximumof500mA
(in high power mode), but the voltage provided to the por-
table device can vary quite significantly. Although a USB
power supply has a 5V nominal rating, when you include
normal supply variations, cable losses, and transient
conditions, the USB voltage showing up at the portable
device is typically much lower—often falling to only 4V.
Since the USB port has a strict current limit of 500mA,
this means the amount of power available to the portable
device can be as low as 2W. The reduced USB voltage also
presentsproblemswhentryingtofullychargeasingle-cell
Lithium-Ion battery when the USB voltage may itself be
below or near the float voltage.
Concerns When Wall Adapter Powered
Alwayschooseawalladapterthatcanprovidepowerforall
load and battery charging requirements. Choosing a wall
adapter with a power rating that is too small will result in
verylongchargetimesanderraticsystemoperation. Ifthe
total current needed (load and battery charging) exceeds
The LTC3455/LTC3455-1 are specifically designed to al-
leviate these problems and make the most of the power
the USB port has to offer. See the sections entitled ”Large
TransientLoadswhenUSBpowered”and”SpecialCharger
FeatureswhenUSBpowered”formoredetaileddiscussions
of the LTC3455/LTC3455-1’s special USB features.
what the adapter can provide, the voltage on the V
pin
MAX
will begin to droop. If it droops close enough to the bat-
tery voltage (the V pin), the charge current decreases
BAT
and eventually reduces to zero. If the load current is still
too much for the wall adapter to provide, the wall adapter
will provide what it can and the battery will provide the
3455fc
11
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
USB High Power/Low Power/Suspend Modes
powered and the battery is not present, absence of the
batterymeansthatthereisnoreservoirifthesystemneeds
more power than the USB port can supply. Since the USB
can only provide 100mA maximum current in low power
mode, this gives, at best, only 500mW (5V • 100mA) of
power available to get everything up and running. With
a low USB voltage and a low USB current limit, less than
300mW may be available to start up the device. For some
applications (if the outputs are too heavily loaded), this
is simply not enough power to start up the system. If the
There are three basic modes for the USB power manager:
high power, low power, and suspend. High power mode
allowstheLTC3455/LTC3455-1todrawupto500mAfrom
theUSBport,andisselectedbypullingtheUSBHPpinhigh.
Low power mode reduces the allowable current drawn to
100mA, and is selected by pulling the USBHP pin low. The
USBHP pin has a weak internal pulldown current source
to ensure that the LTC3455/LTC3455-1 always start up in
USB low power mode. The SUSPEND pin will disable the
USB power manager completely, reducing the USB pin
current to under 20μA.
V
pin or the switching regulator outputs are loaded too
MAX
heavily, the LTC3455/LTC3455-1 will be unable to regulate
the outputs (due to insufficient input power), and an in-
ternal protection circuit will turn off the part after 200ms.
This protection feature is discussed in detail in the section
entitled“LoworBadBatteryProtection(200msTimeout)”.
Once this protection circuit is tripped, USB power must
be removed and reapplied to restart the part.
Operation in USB Low Power Mode
Most applications that draw power from the USB bus
should be in low power mode only for a brief amount
of time. Devices should be in low power mode (draw no
more than 100mA of current from the USB bus) upon
power-up, and can transition to high power mode (draw
up to 500mA from the USB bus) after the device has been
given permission to do so by the USB host controller. The
change to high power mode is usually very quick, so the
full 500mA of current is available shortly after connecting
totheUSBbus.WhiletheLTC3455/LTC3455-1willoperate
when in low power mode, the amount of power available
is so small that it is difficult or impossible to charge a
battery or even provide enough current to power the rest
of the system. For this reason, USB high power operation
should always be used with the LTC3455/LTC3455-1.
Several steps can be taken to help lighten the total system
load which will help greatly when the LTC3455/LTC3455-1
must start up in USB low power mode with no battery
present.
1. Minimize the load currents on the V
pin by delaying
MAX
the turn on of all devices that are powered from V
until after USB high power mode is available.
MAX
2. Minimize the load current on the output of Switcher 1
since Switcher 1 turns on automatically whenever USB
power is available.
3. Delay the turn-on of Switcher 2 until after USB high
power mode is available.
Startup Issues in USB Low Power Mode When No
Battery Is Present
For some applications, USB high power mode should
be selected during startup (at least briefly) to allow the
LTC3455/LTC3455-1toturnonproperly.StartupinUSBhigh
power mode is typically not a problem, as more than 2W
of power is available from the USB port in this mode.
For applications that must operate in USB low power
mode when no battery is present, careful attention must
be given to how the V
pin and the output of the two
MAX
switchingregulatorsareloaded, especiallyduringstartup.
Keep in mind that when the LTC3455/LTC3455-1 are USB
3455fc
12
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Handling Large Transient Loads when USB Powered
The oscilloscope photographs in Figure 1 show how the
LTC3455/LTC3455-1 handle load transients when USB
powered. The top photo shows a brief transient load that
Many portable devices have nominal loads that can easily
be supported by the USB supply, but they have brief tran-
sient loads that can exceed the maximum available USB
power. The LTC3455/LTC3455-1 are designed to handle
these overloads while drawing as much power as possible
from the USB port. If the USB bus is providing power but
the LTC3455/LTC3455-1 (or any other devices connected
turns off the charger but does not dip the V
voltage.
MAX
The bottom photo shows a prolonged transient condition
that turns off the charger and completely dips the V
MAX
voltage to the point where the battery must provide cur-
rent. For both cases, normal operation resumes as soon
as the transient passes.
to the V
pin) need more total power than the USB bus
MAX
can supply, the battery charger turns off completely and
the USB power controller becomes a 500mA (or 100mA)
Extra capacitance can be connected to the V
pin to
MAX
act as a reservoir to help support large transient cur-
rents. For most systems this is not necessary, as the
LTC3455/LTC3455-1 cleanly handle heavy transients. For
some designs, however, it may be desirable to use a larger
current source and the V
voltage begins to decrease.
MAX
At this point, the capacitance connected to the V
pin
MAX
provides the additional current needed by the system. As
long as the USB pin stays above 3.9V, the USB bus will
continue to provide as much current as possible. Once the
capacitor connected to V
to act as a larger reservoir.
MAX
Up to 50μF of ceramic capacitance may be connected to
the V pin without difficulty. More than 50μF requires
V
MAX
pin drops just below the V
voltage, the battery
BAT
MAX
will provide the additional current needed. This operation
allowstheLTC3455/LTC3455-1towithstandloadtransients
that briefly demand more power than can be provided by
the USB bus.
usingacapacitorwithsomeESRoraddingsomeresistance
in series with some of the ceramic capacitance. This is
necessary to ensure loop stability in the battery charger
loop when under USB power.
V
MAX
2V/DIV
I
MAX
500mA/DIV
I
USB
500mA/DIV
I
BAT
500mA/DIV
3455 F01a
100μs/DIV
USB Maximum Current Condition
V
MAX
2V/DIV
I
MAX
500mA/DIV
I
USB
500mA/DIV
I
BAT
500mA/DIV
3455 F01b
100μs/DIV
USB Heavy Over-Current Condition
Figure 1. Handling Load Transients when USB Powered
3455fc
13
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Using the V
Pin to Power Other Devices
Startup and Shutdown when Battery-Powered
MAX
The V
pin can be used to provide power for other
When only battery power is available, the LTC3455/
LTC3455-1 turn on when either the ON pin is pulled low
or the PWRON pin is pulled high. Either of these pins will
keep the device running, but typically the ON and PWRON
pins are used together to provide turn-on and turn-off us-
ing a single momentary-on push-button switch. Figure 2
shows the method for using a momentary-on pushbutton
to turn the LTC3455/LTC3455-1 off and on.
MAX
devices within the system. This pin is connected to the
battery when no external power is available, and it is
connected to either the USB bus or the wall adapter when
either are available. This ensures that all devices powered
from V
input power source.
will always draw power from the best available
MAX
TheinternalPMOSconnectingV
tothebatteryiscurrent
MAX
limited to 900mA at startup (to minimize in-rush current)
When the momentary-on switch is first pressed, short-
and to 4A once V
has risen close to the battery voltage.
ing the ON pin to ground, PBSTAT goes low and the
MAX
Because of the reduced startup current limit, the turn-on
of other devices powered from V should always be
LTC3455/LTC3455-1 first bring up the V
pin, then
MAX
enables Switcher 1 to power the microcontroller. Once
up and running, the microcontroller provides the PWRON
signal to keep the LTC3455/LTC3455-1 turned on after the
push-buttonisreleased. Whenthepush-buttonispressed
again to turn off the device, the PBSTAT pin is pulled low
tonotifythemicrocontrollerthatthepush-buttonhasbeen
pressed. The microcontroller prepares for shutdown then
pulls the PWRON signal low. When the push-button is re-
leased, the ON pin goes high and the LTC3455/LTC3455-1
turnoff.TheONandPWRONpinsenableSwitcher1(along
with all the internal circuits needed for normal operation),
and the ON2 pin enables Switcher 2. Switcher 2 can only
operate when Switcher 1 is also enabled. The turn-on of
MAX
delayed to minimize the currrent initially needed from the
V
MAX
pin. The best choice is to enable these devices from
either switcher output, since the turn-on of both switch-
ers is always delayed until the V pin has reached the
MAX
V
BAT
pin voltage. The V
pin is discharged to ground
MAX
when the LTC3455/LTC3455-1 are shut down, so that
any device supplied by V will have its input grounded
MAX
during shutdown. This ensures output disconnect for all
supply voltages within the system.
both switchers is always delayed until the V
pin has
MAX
reached the V pin voltage.
BAT
LTC3455/LTC3455-1
PBSTAT
23
ON
24
PUSH
BUTTON
μC
SWITCHER 1
ENABLED
PWRON
ON2
22
19
SWITCHER 2
ENABLED
3455 F02
Figure 2. Momentary Push-Button Operation
3455fc
14
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
LTC3455/LTC3455-1
19
ON2
23
PBSTAT
ON 24
SWITCHER 2
ENABLED
PWRON 22
V
9
+
–
BAT
SWITCHER 1
ENABLED
3V
WALLFB 11
+
–
CHARGER
ENABLED
1.23V
USB
8
6
+
–
USB POWER
CONTROLLER
ENABLED
3.9V
SUSPEND
3455 F03
Figure 3. Turn-On Logic Diagram
Startup and Shutdown When USB or Wall Powered
the output sequencing when both switchers are enabled
at startup with the ON2 pin tied to V . The turn-on of
MAX
Whenever USB or wall power is present (as sensed by
the USB and WALLFB pins), Switcher 1 and the battery
charger will always be enabled. If the LTC3455/LTC3455-1
are off and external power is applied, both the charger and
Switcher 1 will start independent of the state of the ON and
PWRONpins.Thisprovidesmaximumbatteryrun-timeby
always allowing the battery to charge whenever external
power is available, and ensures that the microcontroller
is always alive when external power is available (this is
important for designs that utilize coulomb-counting or
other battery monitoring techniques). Switcher 2 starts
only if ON2 is also pulled high. Figure 3 shows the turn-on
logic diagram for the LTC3455/LTC3455-1.
both switchers is always delayed until the V
pin has
MAX
reached the V pin voltage.
BAT
Reset Signal (RST)
A200msresetsignal(theRSTpinispulledlow)isprovided
for proper initialization of a microcontroller whenever the
LTC3455/LTC3455-1 are first turned on, either by the ON
or PWR pins, or by the application of external power. The
RSTsignalisalsopulledlowwhenevertheentireLTC3455/
LTC3455-1 are in shutdown, ensuring no false starts for
the microcontroller as the output voltages are rising or
collapsing.
Starting Switcher 2/Power Supply Sequencing
PWRON/ON2
2V/DIV
Switcher 2 can operate only when Switcher 1 is also
enabled and in regulation. The ON2 pin can be driven by
a logic signal for independent control of Switcher 2. If
both outputs always operate together, tie the ON2 pin to
V
MAX
2V/DIV
V
V
(1.8V)
OUT1
2V/DIV
the V
pin. This will enable Switcher 2 after the output
MAX
(3.3V)
2V/DIV
OUT2
of Switcher 1 has reached 90% of its final value. This
power-up delay ensures proper supply sequencing and
reducesthepeakbatterycurrentatstartup.Figure4shows
3455 F04
100μs/DIV
Figure 4. Sequencing for Switcher 1 and 2 Outputs
3455fc
15
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Low or Bad Battery Protection (200ms Timeout)
Three good diode choices are the MBRM110E (1A, 10V),
MBR120ESF (1A, 20V), and the MBRA210E (2A, 10V).
All are available in very small packages from ON Semi-
conductor (www.onsemi.com), have reverse leakage cur-
rents under 1μA at room temperature, and have forward
drops of around 500mV at their maximum rated current
(1A or 2A).
The 200ms reset timer is also used to prevent starting
the LTC3455/LTC3455-1 when there is insufficient exter-
nal power or insufficient battery voltage to regulate the
outputs. When first turned on, the internal 200ms timer
starts. If only Switcher 1 is enabled (ON2 is low) and its
output does not reach 90% of its final value within 200ms,
Switcher 1 is shut down even if the ON pin is held low or if
V
MAX
the PWRON pin is held high (the V
pin will remain on
10
MAX
V
MAX
as long as ON is low or PWRON is high). This automatic
shutdown feature prevents possible damage to a defec-
tive or overdischarged Li-Ion battery. If ON2 is tied to
LTC3455/
WALL 5V
I
LEAKAGE
LTC3455-1
3.32K
11
V
so that Switcher 2 is also turned on at startup, then
WALLFB
MAX
both outputs must reach 90% of their final values within
200ms. Once the output(s) are in regulation, the timer is
reset for a full 200ms.
1.24K
3455 F05
Figure 5. Schottky Leakage Current Path
Schottky Diode Selection/WALLFB Resistor Selection
Switching Regulator General Information
When a 5V wall adapter is used, power is provided to the
The LTC3455/LTC3455-1 contain two 1.5MHz constant-
frequency current mode switching regulators that operate
with efficiencies up to 96%. Switcher 1 provides up to
400mA at 1.5V/1.8V (to power a microcontroller core),
andSwitcher2providesupto500mAat3V/3.3V(topower
microcontroller I/O, memory and other logic circuitry).
Both converters support 100% duty cycle operation (low
dropout mode) when the input voltage drops very close
to the output voltage, and both are capable of operating
in Burst Mode operation for highest efficiencies at light
loads (Burst Mode operation is pin selectable). Switcher 2
has independent ON/OFF control, but operates only when
Switcher 1 is also enabled and in regulation. If both are
enabled at power-up, Switcher 2 is allowed to turn on only
after Switcher 1 has reached 90% of its final value. This
power-up delay ensures proper supply sequencing and
reduces the peak battery current at startup. If the output
of Switcher 1 drops to below 85% of its programmed
output voltage, Switcher 2 will turn off. This ensures that
any problems with the core supply will shut down the rest
of the system.
V
pin through a Schottky diode. The most important
MAX
specification in picking this diode is its reverse leakage
current. When the LTC3455/LTC3455-1 are turned on but
wall power is not present, the Schottky will leak current to
ground through the WALLFB resistor divider (see Figure
5). This leakage current should be minimized (by pick-
ing an appropriate low-leakage Schottky diode) as it can
dramatically reduce Burst Mode efficiency at light loads.
In addition, a high leakage current can also false trip the
WALLFB pin and turn on the LTC3455/LTC3455-1 even if
wall power is not available. To help prevent this false turn-
on, use the WALLFB resistor values shown in Figure 5.
The diode forward voltage drop should be around 500mV
or less at its maximum rated current to allow charging
even when the wall adapter voltage is lower than normal.
Some manufacturers have recently introduced Schottky
diodes optimized for a very low forward drop, but their
reverse leakage currents can be more than 100μA at
room temperature, and over 1mA at high temperatures.
These diodes are not recommended for use with the
LTC3455/LTC3455-1, but if they are used operation at
high temperature should be checked thoroughly to avoid
problems due to excessive diode leakage current.
3455fc
16
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Switching Regulator Inductor Selection
Switching Regulator Output Capacitor Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler. To maximize
efficiency, choose an inductor with a low DC resistance.
Keep inmind thatmostinductors thatarevery thin orhave
a very small volume typically have much higher core and
DCR losses, and will not give the best efficiency.
LowESR(equivalentseriesresistance)ceramiccapacitors
should be used at both switching regulator outputs. Only
X5R or X7R ceramic capacitors should be used because
they retain their capacitance over wider voltage and tem-
perature ranges than other ceramic types. A 10μF output
capacitorissufficientformostapplications.Table2shows
a list of several ceramic capacitor manufacturers. Consult
each manufacturer for detailed information on their entire
selectionofceramiccapacitors.Manymanufacturersnow
offer very thin (<1mm tall) ceramic capacitors ideal for
use in height-restricted designs.
Choose an inductor with a DC current rating at least 1.5
timeslargerthanthemaximumloadcurrenttoensurethat
the inductor does not saturate during normal operation.
Table 1 shows several inductors that work well with the
LTC3455/LTC3455-1. These inductors offer a good com-
promise in current rating, DCR and physical size. Consult
each manufacturer for detailed information on their entire
selection of inductors.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden
AVX
(408) 573-4150
(803) 448-9411
(714) 852-2001
(888) 835-6646
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.tdk.com
Murata
TDK
Table 1. Recommended Inductors
V
Pin Capacitor Selection
BAT
Max
IDC
(A)
Max
Inductor
Type
L
(μH)
DCR Height
For the V pin, a 4.7μF to 10μF ceramic capacitor is the
BAT
(Ω)
(mm)
Manufacturer
best choice. Only X5R or X7R type ceramic capacitors
DB318C
4.7
10
0.86
0.58
0.1
0.18
1.8
1.8
Toko
(847)297-0070
www.toko.com
should be used.
V
Pin Capacitor Selection
CLS4D09
4.7
10
0.75
0.5
0.19
0.37
1
1
Sumida
(847)956-0666
www.sumida.com
MAX
For the V
pin, a 10μF ceramic capacitor is the best
MAX
choice. Only X5R or X7R type ceramic capacitors should
be used. Do not use less than 10μF on this pin. For some
designs it may be desirable to use a larger capacitor con-
CDRH3D16
SD12
4.7
10
0.9
0.11
0.21
1.8
1.8
Sumida
0.55
4.7
10
1.29
0.82
0.12
0.28
1.2
1.2
Cooper
(561)752-5000
www.cooperet.com
nected to V
to act as a reservoir when the LTC3455/
MAX
LTC3455-1 are USB powered. Up to 50μF of ceramic
capacitance may be connected to the V pin without
difficulty. More than 50μF requires using a capacitor with
some ESR (like a Tantalum or OS-CON) or adding some
resistance in series with some of the ceramic capacitance.
This is necessary to ensure loop stability in the battery
charger loop when under USB power.
ELT5KT
4.7
10
1
0.68
0.2
0.36
1.2
1.2
Panasonic
(408)945-5660
MAX
www.panasonic.com
3455fc
17
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
USB Pin and Wall Adapter Capacitor Selection
The USB and wall adapter inputs should be bypassed with
a 4.7μF to 10μF capacitor. For some applications, the wall
input can be bypassed locally with a lower value (down to
1μF),butonlyifotherbulkcapacitanceispresent.TheUSB
pin should always have at least 4.7μF. Ceramic capacitors
(only type X5R or X7R) are typically the best choice due to
their small size and good surge current ratings, but care
must be taken when they are used. When ceramic capaci-
tors are used for input bypassing, a 1Ω series resistor
must be added to prevent overvoltage ringing that often
occurs when these inputs are hot-plugged. A tantalum,
OS-CON, or electrolytic capacitor can be used in place of
the ceramic and resistor, as their higher ESR reduces the
Q, thus reducing the voltage ringing.
4.7μF ONLY
2V/DIV
4.7μF + 1Ω
2V/DIV
3455 F06
20μs/DIV
Figure 6. Waveforms Resulting from Hot-Plugging a
5V Input Supply
1Ω series resistor) is used to locally bypass the input.
This trace shows excessive ringing when the 5V cable is
inserted, with the overvoltage spike reaching 10V; more
than enough to damage the LTC3455/LTC3455-1. For the
bottom trace, a 1Ω resistor is added in series with the
4.7μF capacitor to locally bypass the 5V input. This trace
shows the clean response resulting from the addition of
the 1Ω resistor.
Protecting the USB Pin and Wall Adapter Input from
Overvoltage Transients
Cautionmustbeexercisedwhenusingceramiccapacitors
to bypass the USB pin or the wall adapter inputs. High
voltage transients can be generated when the USB or wall
adapter is hot plugged. When power is supplied via the
USB bus or wall adapter, the cable inductance along with
the self resonant and high Q characteristics of ceramic
capacitors can cause substantial ringing which can easily
exceed the maximum voltage pin ratings and damage the
LTC3455/LTC3455-1. Refer to Linear Technology Applica-
tionNote88,entitled“CeramicInputCapacitorsCanCause
Overvoltage Transients” for a detailed discussion of this
problem. The long cable lengths of most wall adapters
and USB cables makes them especially susceptible to this
problem.TobypasstheUSBpinandthewalladapterinput,
add a 1Ω resistor in series with a ceramic capacitor to
lower the effective Q of the network and greatly reduce the
ringing. A tantalum, OS-CON, or electrolytic capacitor can
be used in place of the ceramic and resistor, as their higher
ESR reduces the Q, thus reducing the voltage ringing.
Evenwiththeadditional1Ωresistor,baddesigntechniques
and poor board layout can often make the overvoltage
problem even worse. System designers often add extra
inductance in series with input lines in an attempt to mini-
mize the noise fed back to those inputs by the application.
In reality, adding these extra inductances only makes the
overvoltage transients worse. Since cable inductance is
one of the fundamental causes of the excessive ringing,
adding a series ferrite bead or inductor increases the ef-
fective cable inductance, making the problem even worse.
For this reason, do not add additional inductance (ferrite
beads or inductors) in series with the USB or wall adapter
inputs.Forthemostrobustsolution,6Vtransorbsorzener
diodes may also be added to further protect the USB and
wall adapter inputs. Two possible protection devices are
the SM2T from STMicroelectronics and the EDZ series
devices from ROHM.
The oscilloscope photograph in Figure 6 shows how
serious the overvoltage transient can be for the USB
and wall adapter inputs. For both traces, a 5V supply is
hot-plugged using a three foot long cable. For the top
trace, only a 4.7μF capacitor (without the recommended
Always use an oscilloscope to check the voltage wave-
forms at the USB and V
adapter hot-plug events to ensure that overvoltage
transients have been adequately removed.
pins during USB and wall
MAX
3455fc
18
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Programming Switching Regulator Output Voltage
Tie the MODE pin to V
to always allow automatic Burst
MAX
Mode operation. Even when the MODE pin is high, the
LTC3455/LTC3455-1 will only enter Burst Mode when the
loadcurrentislow.Formanynoise-sensitivesystems,Burst
Mode operation might be undesirable at certain times (i.e.
during a transmit or receive cycle of a wireless device),
but highly desirable at others (i.e. when the device is in
low-power standby mode). The MODE pin can be used to
enableordisableBurstModeoperationatanytime,offering
both low-noise and low-power operation when they are
neededthemost. BurstModeisdisabledinitiallyatstartup
(for the first 200ms) and also whenever external power is
available, even if the MODE pin is pulled high.
The output voltage for each switching regulator is pro-
grammedusingaresistordividerfromtheoutputconnected
to the feedback pins (FB1 and FB2):
R2
R1
⎛
⎝
⎞
⎟
⎠
VOUT = 0.8V • 1+
⎜
Typical values for R1 are in the range of 80k to 400k.
V
OUT
R2
1, 18
FB1, FB2
LTC3455/
R1
Figure 9 shows the switching waveforms for switcher 1
LTC3455-1
25
GND
(both PWM mode and Burst Mode Operation) with V =
IN
3455 F07
3.6V, V
= 1.8V, and I
= 25mA.
OUT1
OUT1
Figure 7. Setting the Output Voltage
Burst Mode
Burst Mode Operation
V
Forhighestefficienciesatlightloads,bothDC/DCconvert-
ers are capable of operating in Burst Mode. In this mode,
energy is delivered to the outputs in shorts bursts, which
minimizesswitchinglossesandquiescent-currentlosses.
Output voltage ripple is slightly higher in this mode, but
efficiency is greatly improved. As shown in Figure 8, the
efficiencyatlowloadcurrentsincreasessignificantlywhen
Burst Mode operation is used.
SW1
2V/DIV
V
OUT1
50mV/DIV
AC-COUPLED
I
L1
100mA/DIV
3455 F09a
5μs/DIV
PWM Mode
100
Burst Mode
3.3V
90
V
SW1
2V/DIV
80
V
OUT1
1.8V
3.3V
70
10mV/DIV
Burst
PWM Mode
AC-COUPLED
Mode
60
50
1.8V
PWM Mode
I
L1
100mA/DIV
40
3455 F09b
1μs/DIV
30
V
= 3.6V
Figure 9. Burst Mode and PWM Mode Waveforms
BAT
20
1
10
100
1000
LOAD CURRENT (mA)
3455 F08
Figure 8. PWM and Burst Mode Efficiency
3455fc
19
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Soft-start is accomplished by gradually increasing the
peak inductor current for each switcher. This allows each
output to rise slowly, helping minimize the battery in-rush
current.Figure10showsthebatterycurrentduringstartup.
A soft-start cycle occurs whenever each switcher first
turns on, or after a fault condition has occurred (thermal
shutdown or UVLO).
externalpowerisavailable.Forsomeapplications,itmaybe
undesirable for the charger to become active immediately
when external power is applied. For such applications,
an NMOS switch can be used to disconnect the R
PROG
resistor and allow the PROG pin to float high, turning off
the charger. In this manner, charging occurs only when
allowed by the microcontroller.
The LTC3455/LTC3455-1 battery chargers are constant-
current, constant-voltage chargers. In constant-current
mode, the maximum charge current is set by a single
external resistor. When the battery approaches the final
float voltage, the charge current begins to decrease as the
charger switches to constant-voltage mode. The charge
cycle is terminated only by the charge timer.
In-Rush Current Limiting
When the LTC3455/LTC3455-1 are battery-powered, an
internal 0.15Ω PMOS switch connects the battery (V
BAT
pin) to the V
pin to provide power for both switchers
MAX
and other internal circuitry. This PMOS switch is turned
off in shutdown, and the V pin discharges to ground,
MAX
providing output disconnect for all outputs. At startup,
this PMOS must first charge up any capacitance present
Charge and Recharge Cycles
on the V
pin to the battery voltage. To minimize the in-
MAX
When external power is first applied, a new charge cycle
is always initiated. The battery will continue charging
until the programmed charge time is reached. If the bat-
tery voltage is below 4.05V at the end of this cycle, the
LTC3455/LTC3455-1 will start a new charge cycle. This
action will continue until the battery voltage exceeds the
4.05V threshold. This operation is typically seen only
when charging from USB power. Because the charge cur-
rent can vary dramatically when the LTC3455/LTC3455-1
are USB powered, it takes considerably longer to charge
a battery using the USB supply (as compared to a wall
adapter). If the timer capacitor is chosen correctly, the
battery should be fully charged on one cycle when wall
power is available.
rush current needed from the battery, the PMOS switch is
current-limited to 900mA and both switchers are disabled
while the V
voltage is ramping up. Once V
reaches
MAX
MAX
thebatteryvoltage,thePMOScurrent-limitincreasesto4A
andbothswitchersareallowedtoturnon.Figure10shows
the startup battery current for the LTC3455/LTC3455-1,
which stays well-controlled while V
while both switchers outputs are rising.
is ramping up and
MAX
Battery Charger General Information
The battery charger and Switcher 1 will always be enabled
whenever USB or wall power is present (as sensed by the
USB and WALLFB pins). This ensures that the battery can
be charged and that the microcontroller is alive whenever
If the battery is above the 4.05V threshold when a charge
cycle has expired, charging will stop. At this point, a
recharge cycle is initiated if any of the following occurs:
The battery voltage drops below 4.05V, external power is
removed and reapplied, the PROG pin is floated temporar-
ily, or the SUSPEND pin is temporarily pulled high (if the
LTC3455/LTC3455-1 are under USB power).
V
MAX
2V/DIV
V
V
(1.8V)
OUT1
2V/DIV
(3.3V)
2V/DIV
OUT2
I
BAT
500mA/DIV
3455 F10
100μs/DIV
Figure 10. In-Rush Current at Start-Up
3455fc
20
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Programming Charge Current
and the CHRG pin assumes a high impedance state. The
total charge time is programmed as:
The maximum charge current is programmed using one
external resistor connected between the PROG pin and
GND (use the closest 1% resistor value):
T
(hours) = C
• (3 hours) / (0.1μF)
TIMER
TIMER
Formostapplications,atwotothreehourtimerwillprovide
sufficient time to completely recharge the battery. But for
some applications with larger capacity batteries, four to
fivehoursofchargingmaybeneeded. Apotentialproblem
arises with setting such long timer periods (longer than 3
hours): If the battery is just below the recharge threshold
(meaning it is almost fully charged) it will still be charged
for the total timer period when external power is applied.
This means that the battery will be continually charged
at a very, very low charge current for the full four to five
hours, even if the battery reaches the float voltage right
away.Thistypeofchargingisundesirableforsomebattery
applications, and can be avoided by choosing a shorter
timer period (but not less than 1 hour). At the end of a
chargecycle,theLTC3455willmeasurethebatteryvoltage
to see if it is above the 4.05V recharge threshold. If it is
not above 4.05V, a new charge cycle will begin, repeating
until the battery voltage is above 4.05V. Even if the bat-
tery charges to just above the 4.05V threshold using this
shorter timer method, more than 90% charge capacity
should easily be reached (Note: The LTC3455-1 recharge
threshold is 3.95V instead of 4.05V).
R
= 1000 • 1.23V / I
BAT
PROG
If only USB power is used (no wall adapter), select the
value to be 2.49kΩ (or larger) to set the maximum
R
PROG
charge current at 500mA. If a wall adapter is also used,
canbeprogrammedupto1A(witha1.24kΩR
I
CHARGE
PROG
value), and the USB power manager will automatically
throttle back the charge current to below 500mA when
under USB power.
Monitoring Charge Current
ThevoltageonthePROGpinisanaccurateindicationofthe
battery charge current under all charging conditions.
I
= 1000 • V
/ R
PROG PROG
BAT
Capacitance on the PROG pin should be minimized to
ensure loop stability when in constant-current mode. Do
notplaceacapacitordirectlyfromthePROGpintoground.
Adding an external R-C network (see Figure 11) allows the
monitoring of average, rather than instantaneous, battery
chargecurrent.Averagechargecurrentistypicallyofmore
interesttotheuser,especiallywhentheLTC3455/LTC3455-1
are USB powered, as the battery charge current varies
significantly with normal load transients.
Trickle Charge and Defective Battery Detection
Ifthebatteryvoltageisbelow2.85Vatthebeginningofthe
charge cycle, the charger goes into trickle charge mode,
reducing the charge current to 10% of its programmed
full-scale value. If the low battery voltage remains for one
quarteroftheprogrammedtotalchargetime,thebatteryis
assumedtobedefective,thechargecycleisterminated,and
the CHRG pin goes to a high impedance state. This fault is
cleared if any of the following occurs: The battery voltage
risesabove2.85V,externalpowerisremovedandreapplied,
the PROG pin is floated temporarily, or the SUSPEND pin
is temporarily pulled high (if the LTC3455/LTC3455-1 are
under USB power). The device will still operate normally
from USB or wall power even if the charger has turned
off due to a trickle-charge timeout.
LTC3455/
LTC3455-1
CHARGE
10k
2
CURRENT
MONITOR
CIRCUITRY
PROG
R
C
PROG
FILTER
GND
25
3455 F11
Figure 11. Monitoring Average Charge Current
Programming the Battery Charger Timer
AnexternalcapacitorontheTIMERpinsetsthetotalcharge
time. When this timer elapses the charge cycle terminates
3455fc
21
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Battery Charger Thermal Limit
500
400
300
200
100
0
I
USB
An internal thermal limit reduces the charge current if
the die temperature attempts to rise above approximately
105°C. This protects the LTC3455/LTC3455-1 from ex-
cessive temperature, and allows the user to push the
limits of the power handling capability of a given circuit
board without risk of damaging the LTC3455/LTC3455-1.
Another benefit of the thermal limit is that charge current
can be set according to typical, not worst-case, ambient
temperatures for a given application with the assurance
thatthechargerwillautomaticallyreducethecurrentunder
worst-case conditions.
I
BAT
USB HIGH POWER MODE
V
V
= 5V
USB
BAT
= 3.6V
–100
0
200
300
400
500
600
100
TOTAL SYSTEM CURRENT (mA)
3455 F12
Figure 12. Charge Current vs Total System Current
CHRG Status Output
500
USB HIGH POWER MODE
BAT
The CHRG pin is pulled low with an internal N-channel
MOSFET whenever the battery charger is enabled, and is
forcedintoahighimpedancestatewheneveritisdisabled.
This NMOS device is capable of driving an external LED.
This pin does not provide any C/10 information.
V
= 3.6V
I
BAT
400
300
200
Special Charger Features while USB Powered
100
0
TheLTC3455/LTC3455-1haveseveralspecialfeaturesthat
help make the most of the power available from the USB
powersupply.TheinternalUSBpowercontrollerautomati-
cally throttles back the battery charge current to help keep
thetotalsystemcurrentunderthestrict500mA/100mAUSB
limit. The graph in Figure 12 shows how charge current,
3.75
4.25 4.50 4.75
(V)
5.00 5.25
4.00
V
USB
3455 F13
Figure 13. Charge Current vs USB Voltage
I
, decreases as the current needed for the rest of the
Becausethechargecurrentcanvarydramaticallywhenthe
LTC3455/LTC3455-1 are USB powered, battery charging
can take considerably longer using the USB supply (as
compared to a wall adapter).
BAT
system increases (both switchers and all other external
devices pull current from the V
current, I , always stays below 500mA.
pin). The total USB
MAX
USB
As the USB voltage drops below 4.5V, the charge current
gradually reduces (and eventually shuts off around 4V).
This helps prevent “chattering” and stability problems
when using long, resistive USB cables. Figure 13 shows
this reduction in charge current.
Constant-Current-Only Charger/Disabling the
Charger Timer
To use the charger in a constant-current-only mode, con-
nect the TIMER pin to V
to disable the timer, voltage
MAX
amplifier, and trickle charge function. To disable only the
timerfunctionandleaveallothersintact,connecttheTIMER
3455fc
22
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
LOW-BATTERY
DECTECTOR
pin to GND. Since the charge cycle is terminated only by
the charge timer, external charge termination is required
whenusingeitherofthesemethods.UseanexternalNMOS
to float the PROG pin and disable charging.
LDO
1.8V
3.3V
1M
100k
17
16
17
AO
AO
LBO
LTC3455/
LTC3455-1
LTC3455/
LTC3455-1
Constant-current-only mode is a good choice for systems
that are always powered by a USB supply or wall adapter,
and the charger can be used to charge a super-cap or
backup battery. Disabling the voltage amplifier allows the
super-cap/backupbatterytochargeupfullytotheavailable
USB or wall adapter voltage.
V
2.5V
10μF
100pF
16
BAT
2.49M
806k
169k
AI
AI
80.6k
3455 F14
Figure 14. Low-Battery Detector and LDO Using the Gain Block
Hot Swap Output
Thegainblockisalivewheneverswitcher1isenabled, and
is turned off during shutdown to minimize battery drain.
This means that the low-battery detector will not report
a low-battery condition until the LTC3455/LTC3455-1 are
turnedon.Thisisnotaproblemformostapplicationssince
theLTC3455/LTC3455-1usuallypowerthemicrocontroller
and all other intelligence in the system.
A current limited Hot Swap output is provided for power-
ing memory cards or other external devices that can be
hot-plugged into the system. Typically connected to the
3.3V supply, this output provides isolation to prevent the
external device from disturbing the 3.3V supply when
inserted. The Hot Swap output can only operate when
the LTC3455/LTC3455-1 are on, and is enabled using the
HSON pin. If this hot-plugging protection is not needed,
this output can be used as a load switch for other devices
within the system. The HSO pin is discharged to ground
when the LTC3455/LTC3455-1 are shut down.
PCB Layout Considerations
AswithallDC/DCconverters,carefulattentionmustbepaid
to the printed circuit board (PCB) layout and component
placement. The V
capacitor, V
capacitor, and both
BAT
MAX
inductors must all be placed as close as possible to the
LTC3455/LTC3455-1. These components, along with both
DC/DCconverteroutputcapacitors,shouldbeplacedonthe
same side of the circuit board as the LTC3455/LTC3455-1,
withtheirconnectionsmadeonthattoplayer.Placealocal,
unbroken ground plane below these components that is
tied to the Exposed Pad of the LTC3455/LTC3455-1. The
Exposed Pad (pin 25) must be soldered to the PCB (to
system ground) for proper operation. Figure 15 shows
the recommended placement for the power sections of
the LTC3455/LTC3455-1.
Gain Block
The LTC3455/LTC3455-1 contain a gain block (pins AI
and AO) that can be used as either a low-battery indicator,
or as an LDO with the addition of an external PNP. Both
circuits are shown in Figure 14. The LDO is convenient for
applicationsneedingathirdoutput(possiblyalowcurrent
2.5V or a quiet 3V supply). The AO pin can sink around
1mA, which typically limits the LDO current to 100mA
or less (due to the current gain of the PNP). An external
PMOS can be used for the LDO, but a much larger output
capacitor is needed to ensure stability at light loads.
3455fc
23
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
1
L1
L2
V
V
OUT2
OUT1
C1
C2
C6
C5
C7
C4
GND
GND
D1
GND
USB V
V
5V WALL
ADAPTER
BAT
MAX
VIAS TO LOCAL GROUND PLANE.
Figure 15. Recommended Board Layout and Component Placement for Power Sections of LTC3455/LTC3455-1
(Refer to Schematic on Back Page)
Standalone USB Power Supply
with Temporary Backup Power
shown, the low-battery indicator (AI and AO pins) trig-
gers when the V pin voltage drops to 4V, notifying
MAX
the microcontroller of an impending dropout condition.
The 1MΩ resistor connected between the AI and AO pins
provides 150mV of hysteresis (the dropout indicator
Although designed primarily for Li-Ion powered portable
applications,theLTC3455/LTC3455-1arealsogoodchoices
for systems that are always powered by a USB supply
or wall adapter. The battery charger can then be used
to charge up a large capacitor or backup battery, which
briefly provides power to the system after the external
power has been removed. This gives the microcontroller
enough time to follow proper shutdown procedures when
the main power source is abruptly removed. Figure 14
shows a standalone power supply for USB high power
applications (500mA maximum USB current) using the
LTC3455/LTC3455-1. The total system power should be
kept below 1.8W to ensure clean operation even under
worst-case USB conditions. With the resistor values
stays low until the V
pin rises back above 4.15V). A
MAX
4700μF backup capacitorconnected to theV pinbriefly
BAT
provides power to the system after the USB supply has
been removed, and also helps support transient loads
that slightly exceed the USB current limit. Connecting this
large capacitance to the V pin has several advantages.
BAT
It provides a large energy reservoir that is isolated from
boththeUSBpin(theUSBspecificationlimitscapacitance
on the USB supply pin to 10μF or less) and the V
pin
MAX
(using a very large capacitance on this pin will delay the
system turn-on), and it prevents large inrush currents by
3455fc
24
LTC3455/LTC3455-1
TYPICAL APPLICATIONS
using the battery charger to slowly charge this capacitor
(normally using such a large capacitor would result in
very large inrush currents). With the TIMER pin tied to
V
, the battery charger operates in constant-current
MAX
mode (the voltage-loop and timer function are disabled),
so the 4700μF capacitor is always fully charged to the
available USB voltage.
8
21
USB 5V
1Ω
C6
4.7μF
MODE
USB
6
15
HSON
SUSPEND
USB
19
5.6V
ON2
5
CONTROLLER
μC
USBHP
22
PWRON
20
RST
10
23
V
MAX
PBSTAT
C5
10μF
1M
1M
ON/OFF
1.8V
LTC3455/LTC3455-1
4
24
14
CHRG
ON
11
WALLFB
3.3V, HS
HSO
C3
1μF
3
2
TIMER
PROG
V
MAX
13
12
HSI
2.49k
L2, 4.7μH
10pF
3.3V
0.4A
9
SW2
V
BAT
C4
4700μF
249k
18
7
C2
10μF
FB2
1.8V
10k
80.6k
17
16
DROPOUT
AO
AI
L1, 4.7μH
10pF
1.8V
0.2A
V
SW1
MAX
1M
82.5k
100k
1
C1
10μF
FB1
25
80.6k
20k
GND
3455 F16
C1, C2, C3, C5, C6: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
ALL RESISTORS 1%
Figure 16. Standalone USB Power Supply with Temporary Backup Power
3455fc
25
LTC3455/LTC3455-1
TYPICAL APPLICATIONS
8
6
21
15
19
22
20
23
USB 5V
MODE
HSON
ON2
USB
1Ω
SUSPEND
USB
5.6V
C6
4.7μF
5
CONTROLLER
μC
USBHP
PWRON
RST
10
WALL 5V
V
MAX
PBSTAT
C5
10μF
D1
1Ω
1M
1M
ON/OFF
C7
4.7μF
1.8V
LTC3455/LTC3455-1
3.32k
1k
24
14
4
CHRG
ON
11
WALLFB
3.3V, HS
C3
1μF
2k
HSO
C8, 0.1μF
3
2
1.24k
V
MAX
TIMER
PROG
M1
17
13
FDN304P
OR
Si2305DS
2.49k
AO
9
V
HSI
BAT
L2, 4.7μH
100pF
C4
12
16
18
4.7μF
3.3V
1.2A
SW2
SINGLE
CELL Li-ION
3.3V TO 4.2V
+
249k
2.49k
80.6k
C2
2x10μF
AI
FB2
L1, 4.7μH
10pF
7
1.8V
0.4A
SW1
100k
1
C1
10μF
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
FB1
25
80.6k
GND
3455 F17
Figure 17. LTC3455/LTC3455-1 Application with 3.3V Output Current Increased to 1.2A
Increasing 3.3V Output Current to 1.2A
WhentheloadcurrentexceedswhatSwitcher2canprovide,
the 3.3V output droops slightly and the LDO provides the
additional current needed. Figure 18 shows the transient
response when the 3.3V output current is stepped from
0.5A to 1.2A. More output capacitance can be added to
improve the 3.3V transient response during these high
current load steps.
Withaninternalcurrentlimitof900mA,Switcher2typically
provides a 3.3V, 600mA output. While this output current
is sufficient for many portable devices, some applications
need a 3.3V supply capable of providing more than 1A.
Figure 17 shows how to implement a higher current 3.3V
output using the LTC3455/LTC3455-1. By adding one tiny
SOT23 PMOS and using the AI/AO amplifier as an LDO,
the 3.3V output now provides 1.2A of output current.
Switcher 2 is programmed for an output voltage of 3.3V,
and the LDO is programmed for an output voltage of 3.2V
(3% lower). As long as the load current is low enough for
Switcher 2 to provide, the LDO is turned off completely.
This circuit is ideal for applications that need the higher
3.3V output current for only a brief time. Switcher 2 will
normallyprovidealloftheoutputcurrent, andtheLDOwill
turn on briefly to provide the higher load currents.
V
(3.3V)
OUT2
100mV/DIV
AC-COUPLED
I
OUT2
0.5A/DIV
0.5A TO 2A STEP
M1 GATE
2V/DIV
3455 F18
500μs/DIV
Figure 18. Load Current Step (0.5A to 1.2A) for 3.3V Output
3455fc
26
LTC3455/LTC3455-1
PACKAGE DESCRIPTION
UF Package
24-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1697)
0.70 0.05
4.50 0.05
3.10 0.05
2.45 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
PIN 1 NOTCH
R = 0.20 TYP OR
R = 0.115
0.75 0.05
0.35 × 45° CHAMFER
4.00 0.10
(4 SIDES)
TYP
23 24
PIN 1
TOP MARK
(NOTE 6)
0.40 0.10
1
2
2.45 0.10
(4-SIDES)
(UF24) QFN 0105
0.25 0.05
0.50 BSC
0.200 REF
0.00 – 0.05
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3455fc
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LTC3455/LTC3455-1
TYPICAL APPLICATION
8
6
21
15
19
22
20
23
USB 5V
MODE
HSON
ON2
USB
1Ω
SUSPEND
USB
C6
5.6V
5
CONTROLLER
μC
4.7μF
USBHP
PWRON
RST
10
WALL 5V
1Ω
V
MAX
PBSTAT
C5
10μF
D1
1M
1M
ON/OFF
C7
4.7μF
1.8V
LTC3455/LTC3455-1
3.32k
1k
4
24
14
CHRG
ON
11
REMOVE THESE
COMPONENTS IF
WALL ADAPTER
IS NOT USED
WALLFB
3.3V, HS
C3
1μF
HSO
C8, 0.1μF
3
2
1.24k
TIMER
PROG
13
12
HSI
2.49k
L2, 4.7μH
10pF
9
3.3V
0.5A
SW2
V
BAT
C4
4.7μF
249k
18
7
C2
10μF
SINGLE
CELL Li-ION
3.3V TO 4.2V
FB2
+
1.8V
1M
80.6k
17
16
LBO
AO
AI
L1, 4.7μH
10pF
V
1.8V
0.4A
SW1
BAT
2.49M
100k
1
C1
10μF
FB1
25
80.6k
806k
GND
3455 TA03
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
90% Efficiency, V : 3.6V to 25V, V
LT1616
500mA (I ), 1.4MHz, High Efficiency Step-Down
DC/DC Converter
= 1.25V, I = 1.9mA,
Q
OUT
IN
OUT(MIN)
OUT(MIN)
I
<1μA, ThinSOT
SD
LTC1879
1.2A (I ), 550kHz, Synchronous Step-Down
95% Efficiency, V : 2.7V to 10V, V
= 0.8V, I = 15μA,
Q
OUT
IN
DC/DC Converter
I
<1μA, TSSOP16
SD
LTC3405/LTC3405A
LTC3406/LTC3406B
LTC3407
300mA (I ), 1.5MHz, Synchronous Step-Down
95% Efficiency, V : 2.7V to 6V, V
= 0.8V, I = 20μA,
OUT
IN
OUT(MIN) Q
DC/DC Converter
I
<1μA, ThinSOT
SD
600mA (I ), 1.5MHz, Synchronous Step-Down
96% Efficiency, V : 2.5V to 5.5V, V
= 0.6V, I = 20μA,
Q
OUT
IN
OUT(MIN)
OUT(MIN)
OUT(MIN)
DC/DC Converter
I
<1μA, ThinSOT
SD
Dual 600mA (I ), 1.5MHz, Synchronous Step-Down
96% Efficiency, V : 2.5V to 5.5V, V
= 0.6V, I = 40μA,
Q
OUT
IN
DC/DC Converter
I
<1μA, MS10E
SD
LTC3412
2.5A (I ), 4MHz, Synchronous Step-Down
95% Efficiency, V : 2.5V to 5.5V, V
= 0.8V, I = 60μA,
Q
OUT
IN
DC/DC Converter
I
<1μA, TSSOP16E
SD
LTC3414
4A (I ), 4MHz, Synchronous Step-Down
95% Efficiency, V : 2.25V to 5.5V, V
= 0.8V, I = 64μA,
OUT(MIN) Q
OUT
IN
DC/DC Converter
I
<1μA, TSSOP16E
SD
LTC3440/LTC3441
600mA/1A (I ), 2MHz/1MHz, Synchronous Buck-Boost 95% Efficiency, V : 2.5V to 5.5V, V
= 2.5V, I = 25μA/50μA,
OUT
IN
OUT(MIN)
Q
DC/DC Converter
I
<1μA, MS/DFN
SD
3455fc
LT 0708 REV C • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
28
●
●
© LINEAR TECHNOLOGY CORPORATION 2006
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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