LINER LTC3455 Dual dc/dc converter with usb power manager and li-ion battery charger Datasheet

LTC3455/LTC3455-1
Dual DC/DC Converter
with USB Power Manager
and Li-Ion Battery Charger
DESCRIPTION
FEATURES
n
n
n
n
n
n
n
n
n
n
Seamless Transition between Input Power Sources:
Li-Ion Battery, USB, and 5V Wall Adapter
Accurate USB Current Limiting (500mA/100mA)
Two High Efficiency DC/DC Converters: Up to 96%
Thermal Regulation Maximizes Battery Charge
Rate without Risk of Overheating
Full-Featured Li-Ion Battery Charger with 4.2V Float
Voltage for LTC3455 and 4.1V for LTC3455-1
4.1V Float Voltage (LTC3455-1) Improves Battery Life
and High Temperature Safety Margin
Hot Swap™ Output for SDIO and Memory Cards
Pin-Selectable Burst Mode® Operation
Output Disconnect: All Outputs Discharged to Ground
During Shutdown
Available in a 4mm × 4mm × 0.75mm 24-Pin
QFN Package
The LTC®3455/LTC3455-1 are complete power management 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
power for the device is drawn from the appropriate external
power source. All outputs are discharged to ground during
shutdown to provide complete output disconnect. These
devices are available in a 4mm × 4mm 24-pin exposedpad QFN package.
APPLICATIONS
n
n
n
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.
Handheld Computers
Digital Cameras
MP3 Players
TYPICAL APPLICATION
USB 5V
1Ω
5.6V
4.7μF
USB
CONTROLLER
USB
MODE
SUSPEND
HSON
USBHP
VMAX
WALL 5V
Efficiency
RST
100
1M
LTC3455/LTC3455-1
1k
WALLFB
0.1μF
ON
TIMER
PROG
2.49k
VBAT
HSI
4.7μH
SW2
10pF
4.7μF
+
3.3V, HS
1μF
HSO
1.24k
1.8V
3.3V
0.5A
10μF
80.6k
AO
VBAT
10pF
AI
100k
100
75
70
10
POWER LOSS FOR
BOTH OUTPUTS
65
60
55
VBAT = 3.6V
1
10
100
LOAD CURRENT (mA)
1
1000
1.8V
0.4A
10μF
FB1
GND
SWITCHER 1
VOUT1 = 1.8V
85
80
3455 TA01b
4.7μH
SW1
2.49M
806k
249k
FB2
1M
LBO
90
1.8V
ON/OFF
POWER LOSS (mW)
CHRG
1M
EFFICIENCY (%)
3.32k
1000
SWITCHER 2
VOUT2 = 3.3V
95
4.7μF
SINGLE
CELL Li-ION
3.3V TO 4.2V
μC
PBSTAT
10μF
1Ω
ON2
PWRON
80.6k
3455 TA01a
3455fc
1
LTC3455/LTC3455-1
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
ON2
RST
MODE
PWRON
PBSTAT
ON
TOP VIEW
24 23 22 21 20 19
FB1 1
18 FB2
PROG 2
17 AO
TIMER 3
16 AI
25
CHRG 4
15 HSON
USBHP 5
14 HSO
SUSPEND 6
VBAT
SW2
9 10 11 12
WALLFB
8
VMAX
7
USB
13 HSI
SW1
Transient (t < 1ms and Duty Cycle < 1%):
VMAX USB Voltages .................................. –0.3V to 7V
Steady State:
VBAT, VMAX, USB Voltages ........................ –0.3V to 6V
SW1, SW2 Voltages ................... –0.3V to (VMAX + 0.3V)
TIMER Voltage ........................... –0.3V to (VMAX + 0.3V)
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
UF PACKAGE
24-LEAD (4mm s 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 37°C/W, θJC = 4.3°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3455EUF#PBF
LTC3455EUF#TRPBF
3455
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
LTC3455EUF-1#PBF
LTC3455EUF-1#TRPBF
34551
24-Lead (4mm × 4mm) Plastic QFN
–40°C to 85°C
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
MAX
Battery Undervoltage Lockout Voltage
VBAT Rising
2.9
3.0
3.2
Battery Undervoltage Lockout Hysteresis
VBAT Pin Quiescent Current (Note 3)
Burst Mode, Battery Powered
PWM Mode, Battery Powered
USB Powered
Wall Powered
Shutdown
450
VON2 = VMODE = 1V, Not Switching
VON2 = 1V, VMODE = 0V, Not Switching
VUSB = 5V, Charger Off
VWALL = 1.5V, VMAX = 4.5V, Charger Off
VPWRON = 0V, VMAX = 0V
UNITS
V
mV
110
500
10
10
2
160
800
20
20
4
μA
μA
μA
μA
μA
ON Pin Threshold
0.8
1.1
V
PWRON Pin Threshold
0.8
1.0
V
ON2 Pin Threshold
0.8
1.0
V
0.8
1.0
V
1.23
1.26
V
MODE Pin Threshold
WALLFB Pin Threshold Voltage
WALLFB Pin Hysteresis
WALLFB Rising
l
1.20
60
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
ON Pin Pullup Current
VON = 1V
2.5
μA
PWRON Pin Pulldown Current
VPWRON = 1V
2.5
μA
ON2 Pin Pulldown Current
VON2 = 1V
2.5
μA
MODE Pin Pullup Current
VMODE = 1V
WALLFB Pin Input Bias Current
VWALLFB = 1.35V
PBSTAT Pin Low Voltage
MIN
TYP
MAX
2.5
l
UNITS
μA
±1
±30
nA
VON = 0V, IPBSTAT = 100μA
VON = 0V, IPBSTAT = 1mA
0.02
0.20
0.10
0.35
V
V
RST Pin Low Voltage
IRST = 100μA
IRST = 1mA
0.02
0.20
0.10
0.35
V
V
RST Pulse Duration
After FB1 and FB2 in Regulation
200
ms
0.15
Ω
2.5
4.0
A
0.4
0.9
A
0.784
0.805
0.826
–8
0
8
mV
±1
±25
nA
Battery-VMAX PMOS
VMAX PMOS Switch On-Resistance
VMAX Switch Current Limit
VMAX Switch Current Limit at Startup
With VMAX Rising, VMAX = 3V, VBAT = 3.6V
Gain Block
l
AI Pin Threshold Voltage
AI Pin/FB2 Pin Voltage Difference
VFB2 – VAI
AI Pin Input Bias Current
VAI = 0.85V
AO Pin Sink Current
VAI = 0.6V, VAO = 1.5V
AO Pin Voltage
VAI = 0.6V, IAO = 1mA
l
1.0
V
1.8
2.5
mA
0.8
1.2
V
0.805
0.826
Switching Regulators
l
FB1, FB2 Voltage
FB1, FB2 Voltage Line Regulation
0.784
VMAX = 3V to 5V
0.01
V
%/V
FB1, FB2 Voltage Burst Mode Hysteresis
VMODE = 2V
FB1, FB2 Pin Input Bias Current
VFB1 = VFB2 = 0.85V
8
Switching Frequency
Both Switchers
PMOS Switch On-Resistance
Both Switchers
0.35
Ω
NMOS Switch On-Resistance
Both Switchers
0.45
Ω
PMOS Switch Current Limit
Switcher 1
Switcher 2
450
700
600
900
850
1300
From Low to High
3.75
3.90
4.10
l
1.2
mV
±1
±25
nA
1.5
1.8
MHz
mA
mA
USB Power Manager
USB Undervoltage Lockout Voltage
USB Undervoltage Lockout Hysteresis
150
USB Minimum Voltage to Charge Battery
USB PMOS Switch On-Resistance
VUSB = 5V
USB Current Limit
VUSB = 5V, VUSBHP = 2V
VUSB = 5V, VUSBHP = 0V
USB Suspend Mode Bias Current
VUSB = 5V, VSUSPEND = 2V
l
l
440
60
V
mV
4.0
V
0.5
Ω
475
80
500
100
mA
mA
4
20
μA
SUSPEND Pin Threshold
0.8
1.1
V
USBHP Pin Threshold
0.8
1.1
V
SUSPEND Pin Pulldown Current
VSUSPEND = 0.5V
2.5
μA
USBHP Pin Pulldown Current
VUSBHP = 0.5V
2.5
μA
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
VHSI = 3.3V
Hot Swap PMOS Switch Current Limit
VHSI = 3.3V, VHSO = 2.5V
120
0.9
Ω
160
mA
HSON Pin Threshold
0.8
HSON Pin Pulldown Current
2.5
1.1
V
μA
Battery Charger
Regulated Charger VBAT Voltage
0°C ≤ TA ≤ 85°C (LTC3455)
0°C ≤ TA ≤ 85°C (LTC3455-1)
Charger Current Limit (USB Powered)
RPROG = 2.49kΩ, VUSBHP = 2V, VUSB = 5V, 0°C ≤ TA ≤ 85°C
RPROG = 2.49kΩ, VUSBHP = 0V, VUSB = 5V, 0°C ≤ TA ≤ 85°C
4.158
4.058
4.200
4.1
4.242
4.142
425
400
50
470
90
575
V
V
mA
mA
Charger Current Limit (Wall Powered)
RPROG = 2.49kΩ, VMAX = 4.5V, 0°C ≤ TA ≤ 85°C
500
Recharge Battery Voltage Threshold
VBAT(REGULATED) – VRECHARGE
150
mV
Trickle Charge Trip Threshold
Battery Voltage Rising
2.85
V
Trickle Charge Trip Hysteresis
mA
60
mV
65
mA
2
μA
1.23
V
Trickle Charge Current
RPROG = 2.49kΩ, VBAT = 2V
PROG Pin Current
Internal Pull-Up Current, No RPROG
PROG Pin Voltage
RPROG = 2.49kΩ
CHRG Pin Output Low Voltage
ICHRG = 5mA
0.75
V
Timer Accuracy
CTIMER = 0.1μF
Junction Temperature in Constant
Temperature Mode
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.
Note 2: The LTC3455/LTC3455-1 are guaranteed to meet specified
performance from 0°C to 85°C and is designed, characterized and
±10
%
105
°C
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 VBAT pin when neither USB
or wall power is present, and from the VMAX pin when either USB or Wall
power is present.
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Quiescent Current
PWM Mode Quiescent Current
120
60
40
20
ONLY SWITCHER 1 ENABLED
400
300
200
100
VBAT = 3.6V
NOT SWITCHING
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
VBAT = 3.6V
NOT SWITCHING
100
125
3455 G01
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3455 G02
QUIESCENT CURRENT (μA)
80
500
QUIESCENT CURRENT (μA)
ONLY SWITCHER 1 ENABLED
5
BOTH SWITCHERS ENABLED
BOTH SWITCHERS ENABLED
100
QUIESCENT CURRENT (μA)
Shutdown Quiescent Current
600
VBAT = 3.6V
4
3
2
1
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3455 G03
3455fc
4
LTC3455/LTC3455-1
TYPICAL PERFORMANCE CHARACTERISTICS
Feedback Pins (FB1, FB2)
and AI Pin Voltage
Switching Regulator Oscillator
Frequency
SWITCHING FREQUENCY (MHz)
AI
805
800
FB2
FB1
795
790
785
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
SWITCHER 2
800
1.5
FOR BOTH SWITCHERS
1.0
0.5
0
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
100
3455 G04
USB Pin Current Limit
VMAX CURRENT LIMIT (A)
200
100
USBHP = 0V
3.5
3.0
2.5
2.0
1.5
STARTUP
100
0
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
3455 G07
Battery Undervoltage Lockout
50
100
0
–50 –25
125
3.75
3.75
USB UVLO (V)
3.25
RISING
1.24
FALLING
3.25
3.00
2.75
2.75
125
3455 G10
2.50
–50 –25
RISING
1.22
1.20
1.18
1.16
FALLING
1.14
1.12
FALLING
100
125
WALLFB Trip Voltage
3.50
3.00
100
1.26
RISING
3.50
50
25
75
0
TEMPERATURE (°C)
3455 G09
USB Undervoltage Lockout
4.00
50
25
75
0
TEMPERATURE (°C)
100
3455 G08
4.00
2.50
–50 –25
150
VHSI = 3.3V
VHSO = 2.5V
WALLFB TRIP VOLTAGE (V)
50
25
75
0
TEMPERATURE (°C)
NORMAL OPERATION
0.5
VUSB = 5V
125
HSO Pin Current Limit
4.0
1.0
100
200
4.5
300
50
25
75
0
TEMPERATURE (oC)
3455 G06
VMAX Pin Current Limit
USBHP = 2V
USB PIN CURRENT (mA)
0
–50 –25
125
5.0
0
–50 –25
400
3455 G05
500
400
SWITCHER 1
600
200
HSO PIN CURRENT LIMIT (mA)
VOLTAGE (mV)
810
BATTERY UVLO (V)
Switching Regulator Current Limit
1000
2.0
CURRENT LIMIT (mA)
815
50
25
75
0
TEMPERATURE (°C)
100
125
3455 G11
1.10
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3455 G12
3455fc
5
LTC3455/LTC3455-1
TYPICAL PERFORMANCE CHARACTERISTICS
Battery Charger Regulation
Voltage
Battery Charger Recharge
Threshold
4.20
3.0
TRICKLE CHARGE THRESHOLD (V)
4.30
Battery Charger Trickle-Charge
Threshold
4.15
4.25
LTC3455
VRECHARGE (V)
VBAT (V)
4.20
4.15
LTC3455-1
4.10
4.10
LTC3455
4.05
4.00
LTC3455-1
3.95
4.05
4.00
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
3.90
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
3455 G13
2.6
THERMAL CONTROL
LOOP IN OPERATION
300
200
100
400
300
200
100
7.5
5.0
2.5
VUSBHP = 0V
50
25
75
0
TEMPERATURE (°C)
10.0
100
0
–50 –25
125
50
25
75
0
TEMPERATURE (°C)
0.7
100
125
3455 G18
RDS(ON) for Switching Regulator
Power Switches
VBAT = 3.6V
VMAX = 4.5V
1.25 RPROG = 2.49
TA = 25°C
125
VBAT = 4.2V
CHARGER OFF
3455 G17
1.50
100
12.5
THERMAL CONTROL
LOOP IN OPERATION
0
–50 –25
125
15.0
VUSBHP = 2V
PROG Pin Voltage
vs Charge Current
RDS(ON) for VMAX, USB, and HSO
PMOS Switches
1.4
VBAT = 3.6V
VHSI = 3.3V
VUSB = 5V
1.2 V
BAT = 3.6V
0.6
HSO
0.75
0.50
0.25
500
3455 G19
0.5
NMOS
0.4
PMOS
1.0
RDS(ON)
RDS(ON)
1.00
400
300
100
200
CHARGE CURRENT (mA)
50
25
75
0
TEMPERATURE (°C)
Battery Current When
USB- or Wall-Powered
VBAT = 3.6V
VUSB = 5V
500 RPROG = 2.49k
3455 G16
VPROG (V)
2.7
3455 G15
BATTERY CURRENT (μA)
BATTERY CHARGE CURRENT (mA)
BATTERY CHARGE CURRENT (mA)
500
0
FALLING
2.5
–50 –25
125
600
100 VBAT = 3.6V
VMAX = 4.5V
RPROG = 2.49k
0
50
–50 –25
25
75
0
TEMPERATURE (°C)
2.8
Charge Current When
USB-Powered
600
400
RISING
3455 G14
Charge Current When
Wall-Powered
0
100
2.9
0.3
0.8
0.6
0.2
0.4
0.1
0.2
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
3455 G20
0
–50 –25
USB
VMAX
50
25
75
0
TEMPERATURE (°C)
100
125
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.
PROG (Pin 2): Charge Current Program and Charge Current Monitor Pin. Connect a resistor, RPROG, from this pin
to ground to program battery charge current.
IBAT = 1000 • 1.23V / RPROG
In all modes the voltage on the PROG pin can be used to
measure charge current. PROG has a weak pull-up current
source to turn the charger off if the pin is left open.
TIMER (Pin 3): Timer Capacitor Pin. Connect a capacitor,
CTIMER, between this pin and ground to set the charge
cycle termination time. The timer starts when USB or wall
power is first present. The timer period is:
TTIMER (hours) = CTIMER • (3 hours) / (0.1μF)
Tie TIMER to ground to disable just the internal timer
function. Tie TIMER to VMAX to use the charger in a constant-current-only mode (which disables the timer, voltage
amplifier and trickle charge function).
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.
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).
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.
SW1 (Pin 7): Switch Pin for Switcher 1. Minimize the length
of the metal trace connected to this pin. Place the inductor
for Switcher 1 as close to this pin as possible.
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).
VBAT (Pin 9): Battery Input Pin. Bypass this pin with a
capacitor as close to the device as possible.
VMAX (Pin 10): Max Voltage Pin. This pin is used to
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
wall power are available, an internal PMOS switch connects
this pin to the VBAT pin. When either USB or wall power
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.
WALLFB (Pin 11): Wall Power Detection Pin. This pin is the
input to a comparator used to signal the presence of a 5V
wall adapter. A resistor divider taken from the wall adapter
input is connected to this pin to tell the LTC3455/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
is connected to the VMAX pin through a Schottky diode.
Tie WALLFB to ground if a wall adapter is not used.
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.
HSI (Pin 13): Hot Swap Input Pin. This pin is connected
to the HSO pin through a current-limited PMOS switch.
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.
HSON (Pin 15): Hot Swap Enable Pin. This pin turns on
the PMOS that connects the HSI and HSO pins.
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.
AO (Pin 17): Gain Block Output Pin. This pin is an opendrain 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.
3455fc
7
LTC3455/LTC3455-1
PIN FUNCTIONS
FB2 (Pin 18): Feedback Pin for Switcher 2. Set the output
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.
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
to allow Burst Mode operation for the LTC3455/LTC3455-1.
Burst Mode operation will provide superior efficiency when
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).
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 momentary-on switch. Tie PWRON to ground if not used.
PBSTAT (Pin 23): Push-Button Status Pin. This pin is an
open drain output that indicates the state of the ON pin
(which is usually connected to a momentary-on push-button) to the microcontroller. This pin follows the state of the
ON pin (PBSTAT goes low when ON is pulled low).
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.
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.
SIMPLIFIED BLOCK DIAGRAM
VMAX IS CONNECTED TO THE BEST
AVAILABLE INPUT POWER SOURCE
(WALL ADAPTER, USB OR BATTERY)
USB POWER
3.9V TO 5.3V
Li-Ion BATTERY
3.3V TO 4.2V
USE FOR LDO
OR LOW BATTERY
INDICATOR
USB POWER
MANAGER
5V WALL ADAPTER
VMAX
USE TO POWER OTHER
DC/DCs AND LDOs
BATTERY
PMOS SWITCH
SWITCHER 1
VOUT1
1.8V TYPICAL
BATTERY
CHARGER
SWITCHER 2
VOUT2
3.3V TYPICAL
GAIN BLOCK
HOT SWAP
HOT SWAP OUTPUT
3.3V TYPICAL
3455 SBD
3455fc
8
LTC3455/LTC3455-1
BLOCK DIAGRAM
WALL 5V
3.9V
–
3.32k
USB POWER MANAGER
USB
USB
5V
EXTPWR
1
+
+
8
1000
–
BATTERY CHARGER
5.6V
–
1
REF
BATTERY PMOS SWITCH
USB
CONTROLLER
USBHP
PROG
0.1μF
2.49k
GND
VBAT
VBAT
3.3V to 4.2V
5
4R
4
CHARGE
CONTROL
3
VMAX
VMAX
1.23V
10μF
2.41R
+
CHRG
TIMER
R
–
+
+
–
1k
VMAX
10
6
4.7μF
1.23V
1000
+
SUSPEND
WALLFB
1.24k
1Ω
4.7μF
11
1Ω
R
2
25
SWITCHER 1
PWM
DRIVER
9
7
SW1
4.7μH
1.8V
2.43M
AI
4.7μF
100k
16
–
806k
1
10μF
FB1
80.6k
AO
LBO
ENABLE
–
17
+
0.8V
806k
+
1.8V
ON/OFF
ON
PBSTAT
0.8V
SWITCHER 2
24
VBAT
+
3.0V
–
UVLO
PWM
DRIVER
12
SW2
4.7μH
3.3V
23
806k
249k
EXTPWR
1.8V
–
18
10μF
FB2
80.6k
μC
PWRON
ON2
RST
MODE
HSON
22
ENABLE
+
0.8V
19
20
200ms RESET PULSE
21
BURST MODE ENABLE
15
HOT SWAP
13
HSI
ENABLE
14
3455 BD01
HSO
3.3V, HS
1μF
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 stepdown switching regulators, a full-featured battery charger,
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
switch connects the VMAX pin to the VBAT pin to provide
power to both switching regulators (and any other devices
powered from VMAX). When external power is applied, the
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.
Maximum charge current and charge time are programmed
using an external resistor and capacitor, respectively. The
USB power manager provides accurate current limiting for
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.
APPLICATIONS INFORMATION
Undervoltage Lockout (UVLO)
If no external power is present, the LTC3455/LTC3455-1 will
start only if the battery voltage is above 3.0V. This prevents
starting up with a battery that is too close to deep discharge.
Once started, the battery must drop below 2.6V before the
LTC3455/LTC3455-1 will shut off. This hysteresis is set
intentionally large to prevent the LTC3455/LTC3455-1 from
turning off at an inappropriate time, like during the read- 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
voltage too low and damaging it. An accurate, user-settable
low-battery threshold can be implemented using the gain
block (see the “Gain Block” section for details) which gives
the microcontroller complete control over the timing of a
shutdown due to a low-battery condition.
If external power is present and the battery voltage is less
than 3.0V, the VMAX pin voltage must be greater than 3.9V
for the LTC3455/LTC3455-1 to start, and once started, the
VMAX pin must stay above 3.1V for the device to continue
running.
Selecting the Input Power Source
The priority for supplying power to both DC/DC converters, all internal circuitry, and the VMAX pin is: Wall adapter,
USB, battery.
Whenever the WALLFB pin is above 1.23V, system power
is drawn from the wall adapter via the VMAX pin, and the
battery charger is active. The 5V wall adapter output is
connected to the VMAX pin through a Schottky diode, and
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 VMAX pin
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 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
and the VMAX voltage begins to decrease. If VMAX continues
to decrease, eventually the battery will provide the additional
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.
3455fc
10
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Operation When No Battery Is Present
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
battery means that there is no reservoir if the system needs
more power than the USB port can supply. This is especially 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.
The LTC3455/LTC3455-1 are also a good choice for systems
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.
Concerns When Wall Adapter Powered
Always choose a wall adapter that can provide power for all
load and battery charging requirements. Choosing a wall
adapter with a power rating that is too small will result in
very long charge times and erratic system operation. If the
total current needed (load and battery charging) exceeds
what the adapter can provide, the voltage on the VMAX pin
will begin to droop. If it droops close enough to the battery voltage (the VBAT pin), the charge current decreases
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
rest. When wall powered, this operation is intended only
for surviving fault conditions and should not be a normal
mode of operation.
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.
To further complicate matters, a USB port is not the ideal
power source. Each device can draw a maximum of 500mA
(in high power mode), but the voltage provided to the portable 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
presents problems when trying to fully charge a single-cell
Lithium-Ion battery when the USB voltage may itself be
below or near the float voltage.
The LTC3455/LTC3455-1 are specifically designed to alleviate these problems and make the most of the power
the USB port has to offer. See the sections entitled ”Large
Transient Loads when USB powered” and ”Special Charger
Features when USB powered” for more detailed discussions
of the LTC3455/LTC3455-1’s special USB features.
3455fc
11
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
USB High Power/Low Power/Suspend Modes
There are three basic modes for the USB power manager:
high power, low power, and suspend. High power mode
allows the LTC3455/LTC3455-1 to draw up to 500mA from
the USB port, and is selected by pulling the USBHP pin high.
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.
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
to the USB bus. While the LTC3455/LTC3455-1 will operate
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.
Startup Issues in USB Low Power Mode When No
Battery Is Present
For applications that must operate in USB low power
mode when no battery is present, careful attention must
be given to how the VMAX pin and the output of the two
switching regulators are loaded, especially during startup.
Keep in mind that when the LTC3455/LTC3455-1 are USB
powered and the battery is not present, absence of the
battery means that there is no reservoir if the system needs
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
VMAX pin or the switching regulator outputs are loaded too
heavily, the LTC3455/LTC3455-1 will be unable to regulate
the outputs (due to insufficient input power), and an internal protection circuit will turn off the part after 200ms.
This protection feature is discussed in detail in the section
entitled “Low or Bad Battery Protection (200ms Timeout)”.
Once this protection circuit is tripped, USB power must
be removed and reapplied to restart the part.
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 VMAX pin by delaying
the turn on of all devices that are powered from VMAX
until after USB high power mode is available.
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.
For some applications, USB high power mode should
be selected during startup (at least briefly) to allow the
LTC3455/LTC3455-1 to turn on properly. Startup in USB high
power mode is typically not a problem, as more than 2W
of power is available from the USB port in this mode.
3455fc
12
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Handling Large Transient Loads when USB Powered
Many portable devices have nominal loads that can easily
be supported by the USB supply, but they have brief transient 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
to the VMAX pin) need more total power than the USB bus
can supply, the battery charger turns off completely and
the USB power controller becomes a 500mA (or 100mA)
current source and the VMAX voltage begins to decrease.
At this point, the capacitance connected to the VMAX pin
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
VMAX pin drops just below the VBAT voltage, the battery
will provide the additional current needed. This operation
allows the LTC3455/LTC3455-1 to withstand load transients
that briefly demand more power than can be provided by
the USB bus.
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
turns off the charger but does not dip the VMAX voltage.
The bottom photo shows a prolonged transient condition
that turns off the charger and completely dips the VMAX
voltage to the point where the battery must provide current. For both cases, normal operation resumes as soon
as the transient passes.
Extra capacitance can be connected to the VMAX pin to
act as a reservoir to help support large transient currents. 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
capacitor connected to VMAX to act as a larger reservoir.
Up to 50μF of ceramic capacitance may be connected to
the VMAX pin without difficulty. More than 50μF requires
using a capacitor with some ESR 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.
VMAX
2V/DIV
IMAX
500mA/DIV
IUSB
500mA/DIV
IBAT
500mA/DIV
100μs/DIV
3455 F01a
USB Maximum Current Condition
VMAX
2V/DIV
IMAX
500mA/DIV
IUSB
500mA/DIV
IBAT
500mA/DIV
100μs/DIV
3455 F01b
USB Heavy Over-Current Condition
Figure 1. Handling Load Transients when USB Powered
3455fc
13
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Using the VMAX Pin to Power Other Devices
Startup and Shutdown when Battery-Powered
The VMAX pin can be used to provide power for other
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 VMAX will always draw power from the best available
input power source.
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 using 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.
The internal PMOS connecting VMAX to the battery is current
limited to 900mA at startup (to minimize in-rush current)
and to 4A once VMAX has risen close to the battery voltage.
Because of the reduced startup current limit, the turn-on
of other devices powered from VMAX should always be
delayed to minimize the currrent initially needed from the
VMAX pin. The best choice is to enable these devices from
either switcher output, since the turn-on of both switchers is always delayed until the VMAX pin has reached the
VBAT pin voltage. The VMAX pin is discharged to ground
when the LTC3455/LTC3455-1 are shut down, so that
any device supplied by VMAX will have its input grounded
during shutdown. This ensures output disconnect for all
supply voltages within the system.
When the momentary-on switch is first pressed, shorting the ON pin to ground, PBSTAT goes low and the
LTC3455/LTC3455-1 first bring up the VMAX pin, then
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-button is released. When the push-button is pressed
again to turn off the device, the PBSTAT pin is pulled low
to notify the microcontroller that the push-button has been
pressed. The microcontroller prepares for shutdown then
pulls the PWRON signal low. When the push-button is released, the ON pin goes high and the LTC3455/LTC3455-1
turn off. The ON and PWRON pins enable Switcher 1 (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
both switchers is always delayed until the VMAX pin has
reached the VBAT pin voltage.
LTC3455/LTC3455-1
PBSTAT
ON
μC
23
24
PUSH
BUTTON
PWRON
ON2
22
SWITCHER 1
ENABLED
19
SWITCHER 2
ENABLED
3455 F02
Figure 2. Momentary Push-Button Operation
3455fc
14
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
LTC3455/LTC3455-1
ON2 19
PBSTAT 23
ON 24
SWITCHER 2
ENABLED
PWRON 22
+
VBAT 9
SWITCHER 1
ENABLED
3V
–
+
WALLFB 11
1.23V
CHARGER
ENABLED
–
+
USB 8
3.9V
USB POWER
CONTROLLER
ENABLED
–
SUSPEND 6
3455 F03
Figure 3. Turn-On Logic Diagram
Startup and Shutdown When USB or Wall Powered
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
PWRON pins. This provides maximum battery run-time by
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.
the output sequencing when both switchers are enabled
at startup with the ON2 pin tied to VMAX. The turn-on of
both switchers is always delayed until the VMAX pin has
reached the VBAT pin voltage.
Reset Signal (RST)
A 200ms reset signal (the RST pin is pulled low) is provided
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
RST signal is also pulled low whenever the entire LTC3455/
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
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
the VMAX pin. This will enable Switcher 2 after the output
of 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. Figure 4 shows
PWRON/ON2
2V/DIV
VMAX
2V/DIV
VOUT1 (1.8V)
2V/DIV
VOUT2 (3.3V)
2V/DIV
100μs/DIV
3455 F04
Figure 4. Sequencing for Switcher 1 and 2 Outputs
3455fc
15
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Low or Bad Battery Protection (200ms Timeout)
The 200ms reset timer is also used to prevent starting
the LTC3455/LTC3455-1 when there is insufficient external 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
the PWRON pin is held high (the VMAX pin will remain on
as long as ON is low or PWRON is high). This automatic
shutdown feature prevents possible damage to a defective or overdischarged Li-Ion battery. If ON2 is tied to
VMAX so that Switcher 2 is also turned on at startup, then
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.
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 Semiconductor (www.onsemi.com), have reverse leakage currents under 1μA at room temperature, and have forward
drops of around 500mV at their maximum rated current
(1A or 2A).
VMAX
VMAX
10
LTC3455/
LTC3455-1
WALL 5V
ILEAKAGE
3.32K
WALLFB
11
1.24K
3455 F05
Figure 5. Schottky Leakage Current Path
Schottky Diode Selection/WALLFB Resistor Selection
When a 5V wall adapter is used, power is provided to the
VMAX pin through a Schottky diode. The most important
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 picking 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 turnon, 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.
Switching Regulator General Information
The LTC3455/LTC3455-1 contain two 1.5MHz constantfrequency 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),
and Switcher 2 provides up to 500mA at 3V/3.3V (to power
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.
3455fc
16
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Switching Regulator Inductor Selection
Switching Regulator Output Capacitor Selection
Many different sizes and shapes of inductors are available 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 in mind that most inductors that are very thin or have
a very small volume typically have much higher core and
DCR losses, and will not give the best efficiency.
Low ESR (equivalent series resistance) ceramic capacitors
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 temperature ranges than other ceramic types. A 10μF output
capacitor is sufficient for most applications. Table 2 shows
a list of several ceramic capacitor manufacturers. Consult
each manufacturer for detailed information on their entire
selection of ceramic capacitors. Many manufacturers now
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
times larger than the maximum load current to ensure that
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 compromise in current rating, DCR and physical size. Consult
each manufacturer for detailed information on their entire
selection of inductors.
Table 1. Recommended Inductors
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
TDK
(888) 835-6646
www.tdk.com
VBAT Pin Capacitor Selection
Inductor
Type
L
(μH)
Max
IDC
(A)
Max
DCR
(Ω)
Height
(mm)
DB318C
4.7
10
0.86
0.58
0.1
0.18
1.8
1.8
Toko
(847)297-0070
www.toko.com
CLS4D09
4.7
10
0.75
0.5
0.19
0.37
1
1
Sumida
(847)956-0666
www.sumida.com
CDRH3D16
4.7
10
0.9
0.55
0.11
0.21
1.8
1.8
Sumida
SD12
4.7
10
1.29
0.82
0.12
0.28
1.2
1.2
Cooper
(561)752-5000
www.cooperet.com
ELT5KT
4.7
10
1
0.68
0.2
0.36
1.2
1.2
Panasonic
(408)945-5660
www.panasonic.com
Manufacturer
For the VBAT pin, a 4.7μF to 10μF ceramic capacitor is the
best choice. Only X5R or X7R type ceramic capacitors
should be used.
VMAX Pin Capacitor Selection
For the VMAX pin, a 10μF ceramic capacitor is the best
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 connected to VMAX to act as a reservoir when the LTC3455/
LTC3455-1 are USB powered. Up to 50μF of ceramic
capacitance may be connected to the VMAX 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.
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), but only if other bulk capacitance is present. The USB
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 capacitors 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.
Protecting the USB Pin and Wall Adapter Input from
Overvoltage Transients
Caution must be exercised when using ceramic capacitors
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 Application Note 88, entitled “Ceramic Input Capacitors Can Cause
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. To bypass the USB pin and the wall adapter input,
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.
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
4.7μF ONLY
2V/DIV
4.7μF + 1Ω
2V/DIV
20μs/DIV
3455 F06
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.
Even with the additional 1Ω resistor, bad design techniques
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 minimize 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 effective 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. For the most robust solution, 6V transorbs or zener
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.
Always use an oscilloscope to check the voltage waveforms at the USB and VMAX pins during USB and wall
adapter hot-plug events to ensure that overvoltage
transients have been adequately removed.
3455fc
18
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Programming Switching Regulator Output Voltage
The output voltage for each switching regulator is programmed using a resistor divider from the output connected
to the feedback pins (FB1 and FB2):
⎛ R2 ⎞
VOUT = 0.8 V • ⎜ 1 + ⎟
⎝ R1⎠
Typical values for R1 are in the range of 80k to 400k.
VOUT
R2
FB1, FB2
1, 18
LTC3455/
LTC3455-1
25
GND
R1
3455 F07
Tie the MODE pin to VMAX to always allow automatic Burst
Mode operation. Even when the MODE pin is high, the
LTC3455/LTC3455-1 will only enter Burst Mode when the
load current is low. For many noise-sensitive systems, 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
enable or disable Burst Mode operation at any time, offering
both low-noise and low-power operation when they are
needed the most. Burst Mode is disabled initially at startup
(for the first 200ms) and also whenever external power is
available, even if the MODE pin is pulled high.
Figure 9 shows the switching waveforms for switcher 1
(both PWM mode and Burst Mode Operation) with VIN =
3.6V, VOUT1 = 1.8V, and IOUT1 = 25mA.
Figure 7. Setting the Output Voltage
Burst Mode
Burst Mode Operation
For highest efficiencies at light loads, both DC/DC converters are capable of operating in Burst Mode. In this mode,
energy is delivered to the outputs in shorts bursts, which
minimizes switching losses and quiescent-current losses.
Output voltage ripple is slightly higher in this mode, but
efficiency is greatly improved. As shown in Figure 8, the
efficiency at low load currents increases significantly when
Burst Mode operation is used.
VSW1
2V/DIV
VOUT1
50mV/DIV
AC-COUPLED
IL1
100mA/DIV
5μs/DIV
3455 F09a
PWM Mode
100
90
Burst Mode
3.3V
VSW1
2V/DIV
EFFICIENCY (%)
80
1.8V
Burst
Mode
70
60
50
VOUT1
10mV/DIV
AC-COUPLED
3.3V
PWM Mode
1.8V
PWM Mode
40
IL1
100mA/DIV
1μs/DIV
3455 F09b
30
VBAT = 3.6V
20
1
10
100
LOAD CURRENT (mA)
Figure 9. Burst Mode and PWM Mode Waveforms
1000
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. Figure 10 shows the battery current during startup.
A soft-start cycle occurs whenever each switcher first
turns on, or after a fault condition has occurred (thermal
shutdown or UVLO).
In-Rush Current Limiting
When the LTC3455/LTC3455-1 are battery-powered, an
internal 0.15Ω PMOS switch connects the battery (VBAT
pin) to the VMAX pin to provide power for both switchers
and other internal circuitry. This PMOS switch is turned
off in shutdown, and the VMAX pin discharges to ground,
providing output disconnect for all outputs. At startup,
this PMOS must first charge up any capacitance present
on the VMAX pin to the battery voltage. To minimize the inrush current needed from the battery, the PMOS switch is
current-limited to 900mA and both switchers are disabled
while the VMAX voltage is ramping up. Once VMAX reaches
the battery voltage, the PMOS current-limit increases to 4A
and both switchers are allowed to turn on. Figure 10 shows
the startup battery current for the LTC3455/LTC3455-1,
which stays well-controlled while VMAX is ramping up and
while both switchers outputs are rising.
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
VMAX
2V/DIV
VOUT1 (1.8V)
2V/DIV
VOUT2 (3.3V)
2V/DIV
external power is available. For some applications, it may be
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 RPROG
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 constantcurrent, 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.
Charge and Recharge Cycles
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 battery 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 current 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.
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 temporarily, or the SUSPEND pin is temporarily pulled high (if the
LTC3455/LTC3455-1 are under USB power).
IBAT
500mA/DIV
100μs/DIV
3455 F10
Figure 10. In-Rush Current at Start-Up
3455fc
20
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Programming Charge Current
The maximum charge current is programmed using one
external resistor connected between the PROG pin and
GND (use the closest 1% resistor value):
RPROG = 1000 • 1.23V / IBAT
If only USB power is used (no wall adapter), select the
RPROG value to be 2.49kΩ (or larger) to set the maximum
charge current at 500mA. If a wall adapter is also used,
ICHARGE can be programmed up to 1A (with a 1.24kΩ RPROG
value), and the USB power manager will automatically
throttle back the charge current to below 500mA when
under USB power.
Monitoring Charge Current
The voltage on the PROG pin is an accurate indication of the
battery charge current under all charging conditions.
IBAT = 1000 • VPROG / RPROG
Capacitance on the PROG pin should be minimized to
ensure loop stability when in constant-current mode. Do
not place a capacitor directly from the PROG pin to ground.
Adding an external R-C network (see Figure 11) allows the
monitoring of average, rather than instantaneous, battery
charge current. Average charge current is typically of more
interest to the user, especially when the LTC3455/LTC3455-1
are USB powered, as the battery charge current varies
significantly with normal load transients.
LTC3455/
LTC3455-1
PROG
GND
25
CHARGE
CURRENT
MONITOR
CIRCUITRY
10k
2
RPROG
CFILTER
3455 F11
Figure 11. Monitoring Average Charge Current
Programming the Battery Charger Timer
An external capacitor on the TIMER pin sets the total charge
time. When this timer elapses the charge cycle terminates
and the CHRG pin assumes a high impedance state. The
total charge time is programmed as:
TTIMER (hours) = CTIMER • (3 hours) / (0.1μF)
For most applications, a two to three hour timer will provide
sufficient time to completely recharge the battery. But for
some applications with larger capacity batteries, four to
five hours of charging may be needed. A potential problem
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. This type of charging is undesirable for some battery
applications, and can be avoided by choosing a shorter
timer period (but not less than 1 hour). At the end of a
charge cycle, the LTC3455 will measure the battery voltage
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 battery 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).
Trickle Charge and Defective Battery Detection
If the battery voltage is below 2.85V at the beginning of the
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
quarter of the programmed total charge time, the battery is
assumed to be defective, the charge cycle is terminated, and
the CHRG pin goes to a high impedance state. This fault is
cleared if any of the following occurs: The battery voltage
rises above 2.85V, external power is removed and reapplied,
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.
3455fc
21
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
Battery Charger Thermal Limit
CHARGE AND USB CURRENT (mA)
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 excessive 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
that the charger will automatically reduce the current under
worst-case conditions.
500
IUSB
400
300
IBAT
200
100
0 USB HIGH POWER MODE
VUSB = 5V
VBAT = 3.6V
–100
0
500
200
300
400
100
TOTAL SYSTEM CURRENT (mA)
600
3455 F12
Figure 12. Charge Current vs Total System Current
CHRG Status Output
Special Charger Features while USB Powered
The LTC3455/LTC3455-1 have several special features that
help make the most of the power available from the USB
power supply. The internal USB power controller automatically throttles back the battery charge current to help keep
the total system current under the strict 500mA/100mA USB
limit. The graph in Figure 12 shows how charge current,
IBAT, decreases as the current needed for the rest of the
system increases (both switchers and all other external
devices pull current from the VMAX pin). The total USB
current, IUSB, always stays below 500mA.
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.
500
CHARGE CURRENT (mA)
The CHRG 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.
This NMOS device is capable of driving an external LED.
This pin does not provide any C/10 information.
USB HIGH POWER MODE
VBAT = 3.6V
400
IBAT
300
200
100
0
3.75
4.00
4.25
4.50 4.75
VUSB (V)
5.00
5.25
3455 F13
Figure 13. Charge Current vs USB Voltage
Because the charge current can vary dramatically when the
LTC3455/LTC3455-1 are USB powered, battery charging
can take considerably longer using the USB supply (as
compared to a wall adapter).
Constant-Current-Only Charger/Disabling the
Charger Timer
To use the charger in a constant-current-only mode, connect the TIMER pin to VMAX to disable the timer, voltage
amplifier, and trickle charge function. To disable only the
timer function and leave all others intact, connect the TIMER
3455fc
22
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
pin to GND. Since the charge cycle is terminated only by
the charge timer, external charge termination is required
when using either of these methods. Use an external NMOS
to float the PROG pin and disable charging.
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/backup battery to charge up fully to the available
USB or wall adapter voltage.
Hot Swap Output
A current limited Hot Swap output is provided for powering 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.
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
applications needing a third output (possibly a low current
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.
LOW-BATTERY
DECTECTOR
1.8V
LDO
3.3V
1M
100k
17
LBO
VBAT
2.49M
17
AO
LTC3455/
LTC3455-1
2.5V
AI
10μF
AO
100pF LTC3455/
LTC3455-1
169k
16
806k
16
AI
80.6k
3455 F14
Figure 14. Low-Battery Detector and LDO Using the Gain Block
The gain block is alive whenever switcher 1 is enabled, 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
turned on. This is not a problem for most applications since
the LTC3455/LTC3455-1 usually power the microcontroller
and all other intelligence in the system.
PCB Layout Considerations
As with all DC/DC converters, careful attention must be paid
to the printed circuit board (PCB) layout and component
placement. The VBAT capacitor, VMAX capacitor, and both
inductors must all be placed as close as possible to the
LTC3455/LTC3455-1. These components, along with both
DC/DC converter output capacitors, should be placed on the
same side of the circuit board as the LTC3455/LTC3455-1,
with their connections made on that top layer. Place a local,
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.
3455fc
23
LTC3455/LTC3455-1
APPLICATIONS INFORMATION
1
L1
L2
VOUT1
VOUT2
C1
C2
C6
C5
C7
C4
GND
GND
D1
GND
USB VBAT
VMAX
5V WALL
ADAPTER
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
Although designed primarily for Li-Ion powered portable
applications, the LTC3455/LTC3455-1 are also good choices
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
shown, the low-battery indicator (AI and AO pins) triggers when the VMAX pin voltage drops to 4V, notifying
the microcontroller of an impending dropout condition.
The 1MΩ resistor connected between the AI and AO pins
provides 150mV of hysteresis (the dropout indicator
stays low until the VMAX pin rises back above 4.15V). A
4700μF backup capacitor connected to the VBAT pin briefly
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 VBAT pin has several advantages.
It provides a large energy reservoir that is isolated from
both the USB pin (the USB specification limits capacitance
on the USB supply pin to 10μF or less) and the VMAX pin
(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
8
USB 5V
1Ω
C6
4.7μF
6
5.6V
USB
CONTROLLER
5
10
C5
10μF
VMAX, the battery charger operates in constant-current
mode (the voltage-loop and timer function are disabled),
so the 4700μF capacitor is always fully charged to the
available USB voltage.
USB
MODE
SUSPEND
HSON
USBHP
ON2
PWRON
RST
VMAX
PBSTAT
21
15
19
μC
22
20
23
1M
1M
1.8V
LTC3455/LTC3455-1
4
11
VMAX
3
2
CHRG
WALLFB
C4
4700μF
HSO
ON/OFF
24
14
3.3V, HS
C3
1μF
TIMER
PROG
2.49k
9
ON
VBAT
HSI
SW2
FB2
1.8V
13
12
18
L2, 4.7μH
10pF
82.5k
80.6k
17
AO
SW1
1M
16
20k
249k
C2
10μF
10k
DROPOUT
VMAX
3.3V
0.4A
AI
FB1
GND
7
1
L1, 4.7μH
10pF
25
1.8V
0.2A
100k
C1
10μF
80.6k
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
USB 5V
6
1Ω
C6
4.7μF
USB
5.6V
CONTROLLER
5
10
WALL 5V
USB
MODE
SUSPEND
HSON
USBHP
ON2
PWRON
RST
VMAX
PBSTAT
21
15
19
μC
22
20
23
C5
D1
10μF
1Ω
C7
4.7μF
3.32k
1M
4
11
CHRG
WALLFB
C8, 0.1μF
3
1.24k
2
9
+
1.8V
C4
4.7μF
ON
HSO
ON/OFF
24
14
3.3V, HS
C3
1μF
TIMER
2k
PROG
2.49k
SINGLE
CELL Li-ION
3.3V TO 4.2V
1M
LTC3455/LTC3455-1
1k
AO
VBAT
HSI
SW2
AI
FB2
17
13
12
16
L2, 4.7μH
100pF
VMAX
M1
FDN304P
OR
Si2305DS
249k
3.3V
1.2A
C2
2x10μF
2.49k
18
80.6k
SW1
C1 TO C8: X5R OR X7R CERAMIC
L1, L2: TOKO DB318C
D1: ON SEMI MBRM110E
ALL RESISTORS 1%
FB1
GND
7
L1, 4.7μH
1
25
10pF
100k
1.8V
0.4A
C1
10μF
80.6k
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
With an internal current limit of 900mA, Switcher 2 typically
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
normally provide all of the output current, and the LDO will
turn on briefly to provide the higher load currents.
When the load current exceeds what Switcher 2 can provide,
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.
VOUT2 (3.3V)
100mV/DIV
AC-COUPLED
IOUT2
0.5A/DIV
0.5A TO 2A STEP
M1 GATE
2V/DIV
500μs/DIV
3455 F18
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
2.45 ± 0.05
3.10 ± 0.05 (4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
4.00 ± 0.10
(4 SIDES)
BOTTOM VIEW—EXPOSED PAD
0.75 ± 0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.20 TYP OR
0.35 × 45° CHAMFER
23 24
0.40 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF24) QFN 0105
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
27
LTC3455/LTC3455-1
TYPICAL APPLICATION
8
USB 5V
6
1Ω
C6
4.7μF
5.6V
USB
CONTROLLER
5
D1
1Ω
C7
4.7μF
SUSPEND
HSON
VMAX
PBSTAT
19
μC
22
20
23
1M
1M
1.8V
LTC3455/LTC3455-1
1k
CHRG
11
WALLFB
C8, 0.1μF
3
1.24k
ON
HSO
ON/OFF
24
14
3.3V, HS
C3
1μF
TIMER
2
PROG
2.49k
9
+
PWRON
RST
4
SINGLE
CELL Li-ION
3.3V TO 4.2V
ON2
21
15
C5
10μF
3.32k
REMOVE THESE
COMPONENTS IF
WALL ADAPTER
IS NOT USED
MODE
USBHP
10
WALL 5V
USB
VBAT
C4
4.7μF
HSI
SW2
FB2
1.8V
13
12
18
L2, 4.7μH
10pF
3.3V
0.5A
C2
10μF
80.6k
1M
LBO
249k
17
AO
VBAT
SW1
7
2.49M
16
AI
806k
FB1
GND
1
25
L1, 4.7μH
10pF
100k
1.8V
0.4A
C1
10μF
80.6k
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
LT1616
500mA (IOUT), 1.4MHz, High Efficiency Step-Down
DC/DC Converter
90% Efficiency, VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 1.9mA,
ISD <1μA, ThinSOT
LTC1879
1.2A (IOUT), 550kHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IQ = 15μA,
ISD <1μA, TSSOP16
LTC3405/LTC3405A
300mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 20μA,
ISD <1μA, ThinSOT
LTC3406/LTC3406B
600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA,
ISD <1μA, ThinSOT
LTC3407
Dual 600mA (IOUT), 1.5MHz, Synchronous Step-Down
DC/DC Converter
96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA,
ISD <1μA, MS10E
LTC3412
2.5A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA,
ISD <1μA, TSSOP16E
LTC3414
4A (IOUT), 4MHz, Synchronous Step-Down
DC/DC Converter
95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64μA,
ISD <1μA, TSSOP16E
LTC3440/LTC3441
600mA/1A (IOUT), 2MHz/1MHz, Synchronous Buck-Boost
DC/DC Converter
95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 2.5V, IQ = 25μA/50μA,
ISD <1μA, MS/DFN
3455fc
28 Linear Technology Corporation
LT 0708 REV C • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006