LINEAR TECHNOLOGY LT3757 Technical data
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
JUNE 2009 VOLUME XIX NUMBER 2
SENSE
LT3757
V
IN
V
IN
10V TO 30V
4.7µF
50V
X5R
48V
1A
0.01Ω
42.2k
GATE
FBX
GND
INTV
CC
SHDN/UVLO
SYNC
RT
SS
V
C
200k
43.2k
0.1µF
8.45k
10nF
10µH
IHLP-5050EZ-01
MBRM360
590k
1%
M1
Si7850
20.0k
1%
4.7µF
10V
X5R
4.7µF
50V
X5R
s2
IN THIS ISSUE…
COVER ARTICLE
Two New Controllers for Boost,
Flyback, SEPIC and Inverting DC/DC
Converters Accept Inputs up to 100V
...........................................................1
Wei Gu
Linear in the News… ...........................2
DESIGN FEATURES
Charge Li-Ion Batteries Directly
from High Voltage Automotive
and Industrial Supplies Using
Standalone Charger in a
3mm × 3mm DFN .................................5
Jay Celani
Power Management IC Combines
USB On-The-Go and USB Charging
in Compact Easy-to-Use Solution .........8
George H. Barbehenn and Sauparna Das
Power Management IC with
Pushbutton Control Generates
Six Voltage Rails from USB or
2 AA Cells Via Low Loss
PowerPath™ Topology .......................12
John Canfield
Improve Hot Swap Performance and
Save Design Time with Hot Swap™
Controller that Integrates
2A MOSFET and Sense Resistor .........16
David Soo
Compact No R
Feature Fast Transient Response
and Regulate to Low V
Wide Ranging V
.........................................................18
Terry J. Groom
™ Controllers
SENSE
IN
OUT
from
Two New Controllers for Boost, Flyback, SEPIC and Inverting DC/DC Converters Accept Inputs up to 100V
Introduction
Two new versatile DC/DC controller
ICs, the LT®3757 and LT3758, are
optimized for boost, flyback, SEPIC
and inverting converter applications.
The LT3757 operates over an input
range of 2.9V to 40V, suitable for applications from single-cell lithium-ion
battery portable electronics up to high
voltage automotive and industrial
power supplies. The LT3758 extends
the input voltage to 100V, providing
flexible, high performance operation
in high voltage, high power telecommunications equipment. Both ICs
exhibit low shutdown quiescent cur -
rent of 1µA, making them an ideal fit
for battery-operated systems.
Both integrate a high voltage, low
dropout linear (LDO) regulator. Thanks
to a novel FBX pin architecture, the
LT3757 and LT3758 can be connected
directly to a divider from either the
positive output or the negative output to ground. They also pack many
popular features such as soft-start,
input undervoltage lockout, adjustable frequency and synchronization
in a small 10-lead MSOP package or
a 3mm × 3mm QFN package.
by Wei Gu
continued on page 3
Space-Saving, Dual Output
DC/DC Converter Yields
Plus/Minus Voltage Outputs
with (Optional) I2C Programming .......22
Mathew Wich
DESIGN IDEAS
....................................................26–36
(complete list on page 26)
New Device Cameos ...........................37
Design Tools ......................................39
Sales Offices .....................................40
L
, Li nea r E xpr ess , L ine ar Te chn olo gy, L T, LTC , L T M, Bo de CAD , B urs t M ode , F ilt erC AD, L T spi ce,
OPTI-LOOP, Over-The-Top, PolyPhase, SwitcherCAD, µModule and the Linear logo are registered trademarks of Linear
Technology Corporation. Adaptive Power, Bat-Track, C-Load, DirectSense, Easy Drive, FilterView, Hot Swap, LTBiCMOS,
LTCMOS, LinearView, Micropower SwitcherCAD, Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No R
Operational Filter, PanelProtect, PowerPath, PowerSOT, SafeSlot, SmartStart, SNEAK-A-BIT, SoftSpan, Stage Shedding,
Super Burst, ThinSOT, TimerBlox, Triple Mode, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Figure 1. A 10V–30V input, 48V at 1A output boost converter
SENSE
,
L LINEAR IN THE NEWS
Linear in the News…
EDN Innovation Award Winners
EDN magazine on March 30 announced the winners of their
annual Innovation Awards. Linear Technology’s LTM®4606
Ultralow EMI, 6A DC/DC µModule® Regulator was selected
as the winner in the Power ICs: Modules category. This
innovative device significantly reduces switching regulator noise by attenuating conducted and radiated energy
at the source. The µModule device is a complete DC/DC
system-in-a-package, including the inductor, controller IC,
MOSFETs, input and output capacitors and the compensation circuitry—all in a surface mount plastic package
in an IC form factor.
The other Innovation Award winner, in the category
Best Contributed Article, was Jim Williams for his article,
“High Voltage, Low-Noise DC/DC Converters,” which you
can read at www.edn.com/jimwilliams.
In addition to these winners, two other Linear Technology products were finalists for Innovation Awards:
q
LTC®6802 Battery Stack Monitor in the Battery ICs
Category
q
LTC3642 50mA Synchronous Step-Down Converter
in the Power ICs Category
Current Source Makes Worldwide Debut
Linear Technology has just introduced an elegant building-block component that promises to simplify many
power designs—the LT3092 2-terminal current source.
The LT3092 has recently been announced worldwide in a
series of articles by Linear Technology CTO Bob Dobkin.
The LT3092 is a new solution to an old problem: how
to create an easy-to-use current source that maintains
regulation in a variety of conditions. In the past, a designer
would have to choose between an imprecise IC solution,
or build a current source from discrete components.
The LT3092 200mA 2-terminal current source solves
the problems of prior approaches, with its wide voltage
range, high AC and DC impedance, good regulation, low
temperature coefficient, and the fact that it requires no
capacitors. The device’s two floating terminals make it
eminently easy to use.
easy, but it is fraught with problems. Although high quality
voltage sources are readily available, the current source
as an IC has, until now, remained elusive.
set of issues, especially if high accuracy and stability over
temperature are required features. A current source must
operate over a wide voltage range, have high DC and AC
impedance when connected in series with unknown reactance, and exhibit good regulation and a low temperature
coefficient. For optimal 2-terminal solutions, no power
supply bypass capacitor should be used since it degrades
AC impedance.
than 1% initial accuracy and a very low temperature coefficient. Output currents can be set from 0.5mA to 200mA,
and current regulation is typically 10ppm per volt. The
LT3092 operates down to 1.5V or up to 40V. This gives an
impedance of 100MΩ at 1mA or 1 MΩ at 100mA. Unlike
almost any other analog integrated circuit, special design
techniques have been used for stable operation without a
supply bypass capacitor, allowing the LT3092 to provide
high AC impedance as well as high DC impedance. Transient and start-up times are about 20µs.
Linear Announces
New Quad PSE Controller for PoE+
Last month, Linear Technology held press meetings in the
US, Europe and Asia to introduce the LTC4266, a 4-port
Power over Ethernet (PoE) controller for Power Sourcing
Equipment (PSE), designed to meet the IEEE 802.3at requirements of 25.5W or proprietary higher power levels.
Next-generation PoE applications call for more power to
support demanding features, while increasing power efficiency in an effort to be more green and reduce costs.
cabling and is fully compliant with the new IEEE 802.3at
PoE+ standard and backward compatible with the prior
IEEE 802.3af PoE standard. To help conserve power, the
LTC4266 delivers the lowest-in-industry heat dissipation
by using low R
eliminating the need for expensive heat sinks and providing a more robust PSE solution.
plications, including next-generation switches, routers,
hubs and midspans. Users will appreciate the extremely
low power dissipation, which simplifies thermal design
when compared to designs that use PSEs with more fragile,
normally higher R
On the surface, current source design appears relatively
The desirable 2-terminal current source brings its own
The LT3092 meets these expectations. It has better
The LTC4266 provides up to 100W over 4-pair Ethernet
MOSFETs and 0.25Ω sense resistors,
DS(ON)
The LTC4266 is suitable for a wide variety of PSE ap-
, MOSFETs.
DS(ON)
L
2
2
Linear Technology Magazine • June 2009
DESIGN FEATURES L
EFFICIENCY (%)
I
LOAD
(mA)
200
98
93
300 400 500 600 700 800 900 1000
94
94
95
95
96
VIN = 12V
V
IN
= 24V
SENSE
LT3757
V
IN
V
IN
4.5V TO 36V
4.7µF
50V
X5R
4.7µF
10V
X5R
10µF
50V
X5R
C1
22µF
50V
V
OUT
–5V
3A
0.01Ω
M1
Si7850
42.2k
GATE
FBX
GND
INTV
CC
SHDN/UVLO
SYNC
RT
SS
V
C
215k
100k
0.1µF
10nF
8.45k
L1
6.8µH
L2
6.8µH
D1
PDS1045
C1: SANYO 50CE22BS
L1, L2: VISHAY IHLP4040DZ-11
105k
1%
20k
1%
100µF
6.3V, X5R
s2
+
EFFICIENCY (%)
I
LOAD
(mA)
0
88
86
72
500 1000 1500 2000 2500 3000
74
76
82
78
80
84
VIN = 5V
V
IN
= 12V
V
IN
= 36V
LT3757/58, continued from page 1
Internal High Voltage LDO
In high voltage applications, the
LT3757 and LT3758 eliminate the
need for an external regulator or a
slow-charge hysteretic start scheme
through the integration of an onboard
linear regulator, allowing simple
start-up and biasing. This regulator
generates INTVCC, the local supply
that runs the IC from the converter
input VIN. The internal LDO can operate the IC continuously, provided
the input voltage and/or MOSFET
gate charge currents are low enough
to avoid excessive power dissipation
in the part.
When the INTVCC pin is driven
externally above its regulated voltage
during operation—from the input,
the output or a third winding—the
internal LDO is automatically turned
off, reducing the power dissipation in
the IC. The LDO also provides internal
current limit function to protect IC
from excessive on-chip power dissipation. The current limit decreases
as VIN increases. If the current limit
is exceeded, the INTVCC voltage falls
and triggers the soft-start.
Sensing Output Voltage
Made Easier
Unlike traditional controllers, which
can only sense positive outputs, the
LT3757 and LT3758 have a novel FBX
pin architecture that simplifies the
design of inverting and non-inverting
converters. The LT3757 and LT3758
each contain two internal error amplifiers; one senses positive outputs
and the other negative. When the
converter starts switching and the
output voltage starts ramping up or
down, depending on the topologies,
one of the error amplifiers seamlessly
takes over the feedback control, while
the other becomes inactive.
The FBX pin can be connected
directly to a divider from either a
positive output or a negative output.
This direct connection saves space and
expense by eliminating the traditional
glue circuitry normally required to
level-shift the feedback signal above
ground in negative converters. The
power supply designer simply decides
the output polarity he needs, the topology he wants to use and the LT3757
or LT3758 does the rest.
Precision UVLO Voltage
and Soft-Start
Input supply UVLO for sequencing
or start-up over-current protection is
easily achieved by driving the UVLO
with a resistor divider from the VIN
supply. The divider output produces
1.25V at the UVLO pin when VIN is at
the desired UVLO rising threshold voltage. The UVLO pin has an adjustable
input hysteresis, which allows the IC
to resist a settable input supply droop
before disabling the converter. During
a UVLO event, the IC is disabled and
VIN quiescent current drops to 1µA
or lower.
Figure 2. Efficiency of the
converter in Figure 1
The SS pin provides access to the
soft-start feature, which reduces the
peak input current and prevents output voltage overshoot during start-up
or recovery from a fault condition. The
SS pin reduces the inrush current by
not only lowering the current limit but
also reducing the switching frequency.
In this way soft-start allows the output
capacitor to charge gradually towards
its final value.
Adjustable/Synchronizable
Switching Frequency
The operating frequency of the LT3757
and LT3758 can be programmed from
100kHz to 1MHz range with a single
resistor from the RT pin to ground, or
synchronized to an external clock via
the SYNC pin.
The adjustable operating frequency
allows it to be set outside certain
frequency bands to fit applications
that are sensitive to spectral noise.
Linear Technology Magazine • June 2009
Figure 3. A 4.5V–36V to –5V at 3A inverting converter
Figure 4. Efficiency of the
converter in Figure 3
3
L DESIGN FEATURES
SENSE
LT3758
V
IN
V
IN
18V TO 72V
C
IN
1µF
× 2
INTV
CC
C
OUT
100µF
× 2
V
OUT
–3.3V
2A
0.04Ω
M1
Si4848
36.5k
GATE
FBX
GND
SHDN/UVLO
SYNC
RT
SS
V
C
t
t
t
105k
8.66k
0.1µF
10k
2.2nF
T1
PA1277NL
BAS516
BAV21W
D1
UPS840
31.6k
10k
4.7nF
4.7µF
10k
51.1Ω
EFFICIENCY (%)
LOAD CURRENT (mA)
100
95
55
200 300 400 500 600 700 800 900 1000
60
70
85
75
80
90
VIN = 18V
V
IN
= 24V
V
IN
= 36V
V
IN
= 48V
V
IN
= 72V
SENSE
LT3758
V
IN
V
IN
18V TO 72V
4.7µF
100V
V
OUT
24V
1A
0.02Ω
M1
FDMS2572
42.2k
GATE
FBX
GND
INTV
CC
SHDN/UVLO
SYNC
RT
SS
V
C
•
•
232k
20k
0.1µF
100pF
2.2µF
100V
4.7nF 4.7nF
30.9k
L1B
L1A
WURTH 744 870 470
D1
PDS3100
280k
1%
20k
1%
C
OUT1
22µF
35V
x2
C
OUT2
3.3µF
25V, X5R
+
Figure 5. A 18V–72V input, 24V/1A output SEPIC converter
Figure 6. Efficiency of the
converter in Figure 5
In space constrained applications,
higher switching frequencies can be
used to reduce the overall solution
size and the output ripple. If power
loss is a concern, switching at a lower
frequency reduces switching losses,
improving efficiency.
Current Mode Control
The LT3757 and LT3758 use a current mode control architecture to
enable a higher supply bandwidth,
thus improving response to line and
load transients. Current mode control
also requires fewer compensation
components than voltage mode control architectures, making it much
easier to compensate over all operating
conditions.
A 10V–30V Input, 48V/1A
Output Boost Converter
Figure 1 shows a 48V, 1A output
converter that takes an input of 10V
to 30V. The LT3757 is configured as
a boost converter for this applications
where the converter output voltage
is higher than the input voltage.
Figure 2 shows the efficiency for this
converter.
A 4.5V–36V Input, –5V/3A
Output Inverting Converter
Figure 3 shows the LT3757 in an inverting converter that operates from a
4.5V to 36V input and delivers 3A to
a –5V load. The negative output can
be either higher or lower in amplitude
than the input. It has output short-
4
circuit protection, which is further
enhanced by the frequency foldback
feature in the LT3757. The 300kHz
operating frequency allows the use of
small inductors. The ceramic capacitor
used for the DC coupling capacitor provides low ESR and high RMS
current capability. The output power
can easily scaled by the choice of the
components around the chip without
modifying the basic design. Figure 4
shows the efficiency for this converter
at different input voltages.
An 18V–72V Input, 24V/1A
Output SEPIC Converter
A SEPIC converter is similar to the
inverting converter in that it can step
up or step down the input, but with
a positive output. It also offers output
Figure 7. 18V–72V input, –3.3V/2A output flyback converter
disconnect and short-circuit protection. Figure 5 illustrates an 18V–72V
input, 24/1A output SEPIC power
supply using LT3758 as the controller.
Figure 6 shows the efficiency for this
converter at different input voltages.
An 18V–72V Input, –3.3V/2A
Output Flyback Converter
Figure 7 shows the LT3758 in a nonisolated flyback converter with an
18V to 72V input voltage range and a
–3.3V / 2A output. It provides robust
output short-circuit protection thanks
to the frequency foldback feature in the
LT3758. The circuit can also be used
for different negative voltages simply
by changing the value of the resistor
divider on the FBX pin.
continued on page 21
Linear Technology Magazine • June 2009
DESIGN FEATURES L
SW
V
IN
V
IN
7.5V TO 32V
(40 MAX)
CLP
RNG/SS
BOOST
SENSE
BAT
NTC
TIMER
CMPSH1-4
CMSH3-40MA
1µF
10µF
6.8µH
0.05Ω
10µF
LT3650-4.2
Li-Ion
CELL
+
SHDN
CHRG
FAULT
GND
Charge Li-Ion Batteries Directly from High Voltage Automotive and Industrial Supplies Using Standalone Charger in a 3mm × 3mm DFN
Introduction
Growth of the portable electronics
market is in no small part due to the
continued evolution of battery capacities. For many portable devices,
rechargeable Li-Ion batteries are the
power source of choice because of their
high energy density, light weight, low
internal resistance, and fast charge
times. Charging these batteries safely
and efficiently, however, requires
a relatively sophisticated charging
system.
One additional problem faced by
battery charger designers is how to deal
with relatively high voltage sources,
such as those found in industrial
and automotive applications. In these
environments, system supply voltages exceed the input ranges of most
charger ICs, so a DC/DC step-down
converter is required to provide a local
low voltage supply for the charger IC.
The LT3650 standalone monolithic
switching battery charger does not
need this additional DC/DC converter.
It directly accepts input voltages up to
40V and provides charge currents as
high as 2A. It also includes a wealth
of advanced features that assure safe
battery charging and expand its applicability.
The LT3650 includes features that
minimize the overall solution size,
requiring only a few external components to complete a charger circuit. A
fast 1MHz switching frequency allows
the use of small inductors, and the IC
is housed inside a tiny 3mm × 3mm
DFN12-pin package. The IC has builtin reverse current protection, which
blocks current flow from the battery
back to the input supply if that supply
is disabled or discharged to ground,
so a single-cell LT3650 charger does
not require an external blocking diode
on the input supply.
A Charger Designed for
Lithium-Ion Batteries
A Li-Ion battery requires constantcurrent/constant-voltage (CC/CV)
charging system. A Li-Ion battery
is initially charged with a constant
current, generally between 0.5C and
1C, where C is the battery capacity
in ampere-hours. As it is charged,
the battery voltage increases until
it approaches the full-charge float
voltage. The charger then transitions
into constant voltage operation as
the charge current is slowly reduced.
The LT3650-4.1 and LT3650-4.2 are
designed to charge single-cell Li-Ion
by Jay Celani
batteries to float voltages of 4.1V and
4.2V, respectively. The LT3650-8.2
and LT3650-8.4 are designed to charge
2-cell battery stacks to float voltages
of 8.2V and 8.4V.
Once the charge current falls below
one tenth of the maximum constant
charge current, or 0.1C, the battery
is considered charged and the charging cycle is terminated. The charger
must be completely disabled after
terminating charging, since indefinite
trickle charging of Li-Ion cells, even at
miniscule currents, can cause battery
damage and compromise battery stability. A charger can top-off a battery
by continuing to operate as the current falls lower than the 0.1C charge
current threshold to make full use of
battery capacity, but in such cases a
backup timer is used to disable the
charger after a controlled period of
time. Most Li-Ion batteries charge fully
in three hours.
The LT3650 addresses all of the
charging requirements for a Li-Ion
battery. The IC provides a CC/CV
charging characteristic, transitioning
automatically as the requirements of
the battery change during a charging
cycle. During constant-current operation, the maximum charge current
Figure 1. An LT3650 standalone battery
charger is small and efficient.
Linear Technology Magazine • June 2009
Figure 2. A single-cell 2A Li-Ion battery charger configured for C/10 charge termination
5
L DESIGN FEATURES
I
BAT
(A)
0
EFFICIENCY (%)
80
90
100
70
60
0.5
1
1.5
2
VIN = 12V
VIN = 20V
V
BAT
(V)
CHARGE CURRENT (A)
1.0
1.2
1.4
1.6
1.8
2.0
0.6
0.8
0
0.2
0.4
3.0 3.22.6 2.8
3.4
3.8 4.0 4.23.6
FAULT
CHG
V
IN
10k
10k
LT3650
Figure 3. Battery charge current vs BAT pin
voltage for the charger shown in Figure 2
provided to the battery is programmable via a sense resistor, up to a
maximum of 2A. Maximum charge
current can also be adjusted using the
RNG/SS pin. The charger transitions
to constant-voltage mode operation as
the battery approaches the full-charge
float voltage. Power is transferred
through an internal NPN switch element, driven by a boosted drive to
maximize efficiency. A precision SHDN
pin threshold allows incorporation
of accurate UVLO functions using a
simple resistor divider.
Charge Cycle Termination
and Automatic Restart
A LT3650 charger can be configured
to terminate a battery charge cycle
using one of two methods: it can use
low charge current (C/10) detection,
enabled by connecting the TIMER
pin to ground, or terminate based on
the onboard safety timer, enabled by
connecting a capacitor to the TIMER
pin. After termination, a new charge
cycle automatically restarts should
the battery voltage fall to 97.5% of the
float voltage.
is selected, the LT3650 terminates
a charging cycle when the output
current has dropped to 1/10 of the
6
When C/10 ter mination mode
Figure 5. Visual charger status is
easily implemented using LEDs
A Basic Charger
Figure 2 shows a basic 2A single-cell
Li-Ion battery charger that operates
from a 7.5V to 32V input. Charging is
suspended if the input supply voltage
exceeds 32V, but the IC can withstand
input voltages as high as 40V without
damage. The 2A maximum charge
current corresponds to 100mV across
the 0.05Ω external sense resistor. This
basic design does not take advantage
of the status pins, battery temperature
Figure 4. Power conversion efficiency vs
charger output current (I
charger shown in Figure 2
) for the battery
BAT
programmed maximum. In a 2A
charger, for example, the charge cycle
terminates when the battery charge
current falls to 200mA.
Timer termination, or top-of f
charging, is enabled when a capacitor is connected to the TIMER pin.
The value of the capacitor sets the
safety timer duration—0.68µF corresponds to a 3-hour cycle time. When
timer termination is implemented,
the charger continues to operate in
constant-voltage mode when charge
currents fall below C/10, allowing additional low current charging to occur
until the timer cycle has elapsed, thus
maximizing use of the battery capacity.
During top-off charging, the CHRG
and FAULT status pins communicate
“charge complete.” At the end of the
timer cycle, the LT3650 terminates
the charging cycle.
After charge cycle termination, the
LT3650 enters standby mode where
the IC draws 85µA from the input supply and less than 1µA from the battery.
Both the CHRG and FAULT pins are
high impedance during standby mode.
Should the battery voltage drop to
97.5% of the float voltage, the LT3650
automatically restarts and initializes
a new charging cycle.
Table 1. Status pin state and corresponding operating states
CHRG FAULT Charger Status
High Impedance High Impedance Standby/Shutdown/Top-off
Low High Impedance CV/CC Charging (>C/10)
High Impedance Low Bad Battery Detected
Low Low Temperature Fault
monitoring, or a safety timer features.
The battery charging cycle terminates
when the battery voltage approaches
4.2V and the charge current falls to
200mA. A new charge cycle is automatically initiated when the battery
voltage falls to 4.1V.
Safety Features:
Preconditioning,
Bad Battery Detection,
and Temperature Monitor
Li-Ion batteries can sustain irreversible damage when deeply discharged,
so care must be taken when charging
such a battery. A gentle preconditioning charge current is recommended to
activate any safety circuitry in a battery
pack and to re-energize deeply discharged cells, followed by a full charge
cycle. If a battery has sustained damage from excessive discharge, however,
the battery should not be recharged.
Deeply discharged cells can form
copper shunts that create resistive
shorts, and charging such a damaged
battery could cause an unsafe condition due to excessive heat generation.
Should a deeply discharged battery be
encountered, a battery charger must
be intelligent enough to determine
whether or not the battery has sustained deep-discharge damage, and
avoid initiating a full charge cycle on
such a damaged battery.
Linear Technology Magazine • June 2009
DESIGN FEATURES L
CLP
SYSTEM LOAD
INPUT SUPPLY
V
IN
R
CLP
LT3650
SW
V
IN
CLP
RUN/SS
BOOST
SENSE
BAT
NTC
TIMER
1N914
CMSH3-40MA
SYSTEM LOAD
1µF
6.8µH
0.057
LT3650-X
GND
10µF
Li-Ion
CELL
10k
INPUT SUPPLY
12V TO 32V
(40V MAX)
BZX384-C9V1
(9.1V)
10µF
10µF
0.05Ω
SHDN
CHRG
FAULT
10k
36k
3k
10k
0.68µF
+
0.1µF
The LT3650 employs an automatic
precondition mode, which gracefully
initiates a charging cycle into a deeply
discharged battery. If the battery voltage is below the precondition threshold
of 70% of the float voltage, the maximum charge current is reduced to 15%
of the programmed maximum (0.15C)
until the battery voltage rises past the
precondition threshold.
If the battery does not respond
to the precondition current and the
battery voltage does not rise past the
temperature, and suspends charging
should the temperature fall outside of
the safe charging range.
precondition threshold, a full-current
charge cycle does not initiate.
If the safety timer is used for termination, the LT3650 also enables
deep-discharge damage detection
and incorporates a “bad battery”
detection fault. Should the battery
voltage remain below the precondition threshold for 1/8 of the charge
cycle time (typically 22.5 minutes), the
charger suspends the charging cycle
and signals a “bad battery” fault on
the status pins. The LT3650 maintains this fault state indefinitely, but
automatically resets itself and starts
a new charging cycle if the damaged
battery is removed and another battery
is connected.
Li-Ion batteries have a relatively
narrow temperature range where they
can be safely charged. The LT3650
has a provision for monitoring battery
Figure 7. A single cell Li-Ion 2A battery charger with 3 hour safety timer termination, LED status
indicators, temperature sensing, low input voltage charge current foldback, and input supply
current limit
Linear Technology Magazine • June 2009
is enabled by connecting a 10k (B =
3380) NTC thermistor from the IC’s
NTC pin to ground. This thermistor
must be in close proximity to the battery, and is generally housed in the
battery case. This function suspends a
charging cycle if the temperature of the
thermistor is greater than 40°C or less
than 0°C. Hysteresis corresponding to
5°C on both thresholds prevents mode
glitching. Both the CHRG and FAULT
status output pins are pulled low during a temperature fault, signaling that
the charging cycle is suspended. If the
safety timer is used for termination,
the timer is paused for the duration
of a temperature fault, so a battery
receives a full-duration charging cycle,
even if that cycle is interrupted by
the battery being out of the allowed
temperature range.
Figure 6. R
supply current limit
Under/overtemperature protection
sets the input
CLP
Status Indicator Pins
The status of a LT3650 charger is communicated via the state of two pins:
CHRG and FAULT. These status pins
are open-collector pull-down, reporting the operational and fault status of
the battery charger. CC/CV charging
is indicated while charge currents are
greater than 1/10 the programmed
maximum charge current. The status
pins also communicate bad battery
and battery temperature fault states.
Table 1 shows a fault-state matrix for
these two pins.
The status outputs can be used as
digital status signals in processorcontrolled systems, and/or connected
to pull current through an LED for
visual status display. The status pins
can sink currents up to 10mA and can
handle voltages as high as 40V, so a
visual display can be implemented by
simply connecting an LED and series
resistor to VIN.
Maximum Charging Current
Programming and Adjustment
Maximum charge current is set using an external sense resistor placed
between the BAT and SENSE pins of
the LT3650. Maximum charge current
corresponds to 100mV across this resistor. The LT3650 supports maximum
charge currents up to 2A, corresponding to a 0.05Ω sense resistor.
The LT3650 includes two control
pins that allow reduction of the programmed maximum charge current.
The RNG/SS pin voltage directly affects the maximum charge current
such that the maximum voltage allowed across the sense resistor is 1/10
the voltage on RNG/SS for RNG/SS
< 1V. This pin sources a constant
50µA, so the voltage on the pin can
be programmed by simply connecting
a resistor from the pin to ground. A
capacitor tied to this pin generates a
voltage ramp at start-up, creating a
soft-start function. The pin voltage can
be forced externally for direct control
over charge current.
The IC includes a PowerPath™
control feature, activated via the CLP
pin, which acts to reduce battery
charge current should the load on a
continued on page 38
7
16
20
39
+
+
–
+
–
ENABLE
V
IN3
SW3
FB3
GND
37
I
LIM0
30
CHRG
1
CLPROG
3
NTCBIAS
4
NTC
6
OVSENS
V
C
5
OVGATE
38
I
LIM1
11
ENOTG
10
EN1
22
EN2
19
EN3
12
DV
CC
14
SDA
13
SCL
1A 2.25MHz
BUCK
REGULATOR
17
24
25
ENABLE
V
IN2
SW2
FB2
400mA 2.25MHz
BUCK
REGULATOR
23
8
7
ENABLE
V
IN1
29
PROG
32
BAT
15mV
0.3V
3.6V
IDEAL
1.18V
OR 1.15V
+
–
5.1V
SW1
FB1
21
RST3
400mA 2.25MHz
BUCK
REGULATOR
2.25MHz
BIDIRECTIONAL
PowerPath
SWITCHING
REGULATOR
9
D/A
D/A
D/A
4
4
4
I2C PORT
I
LIM
DECODE
LOGIC
CC/CV
CHARGER
3.3V LDO
CHARGE
STATUS
OVP
27
26
28
WALL
DETECT
V
C
CONTROL
31
IDGATE
33
V
OUT
SW
ACPR
WALL
+
–
+
–
+
–
BATTERY
TEMPERATURE
MONITOR
SUSPEND LDO
500µA/2.5mA
36
LDO3V3
2
35
V
BUS
34
V
BUS
L DESIGN FEATURES
Power Management IC Combines USB On-The-Go and USB Charging in Compact Easy-to-Use Solution
by George H. Barbehenn and Sauparna Das
Introduction
The USB interface was originally
designed so that the device providing
power (an “A” device) would act as the
host and the device receiving power (a
“B” device) was the peripheral. The A
plug of the USB cable would always
connect to the host device and the B
plug would connect to the peripheral.
The USB On-The-Go (OTG) standard,
however, removes that restriction, so
that the B device can now become a
host and the A device can act as a
peripheral.
In the USB specification, standard
hosts and hubs are limited to providing
500mA to each downstream device,
but if a device is designated as a USB
charger, it can supply up to 1.5A. USB
chargers come in two flavors. A “dedicated charger” is a charger that is not
capable of data communication with
the attached B device. A ”host/hub
charger” is a charger that is capable
of data communications with attached
B devices.
When USB OTG functionality is
combined with a USB battery charger
in an end-user product, power can
flow in both directions, with relatively
complicated logic and handshaking
steering the flow. To implement a
robust solution, an integrated USB
battery charger and power manager
is a necessity. This article shows how
to use the LTC3576 USB power management IC to easily combine USB
On-The-Go functionality and battery
charger capability into a single portable product.
Overview of the LTC3576
The LTC3576 provides the power
resources needed to implement a portable device with USB OTG and USB
battery charger detection capabilities
(see block diagram in Figure 1). The
USB input block contains a bidirec-
8
Figure 1. The LTC3576 combines USB charging and USB On-The-Go by using bidirectional DC/DC
conversion from V
BUS
to V
OUT
Linear Technology Magazine • June 2009
DESIGN FEATURES L
< 6.5µF
ENOTG
MINI/MICRO A/B
MINI/MICRO A PLUG
OTG
COMPATIBLE
DEVICE SUCH
AS LTC3576
OTG
COMPATIBLE
BUSS
TRANSCEIVER
A DEVICE
B DEVICE
MINI/MICRO A/B
MINI/MICRO A PLUG
ENOTG
OTG
COMPATIBLE
DEVICE SUCH
AS LTC3576
OTG
COMPATIBLE
BUSS
TRANSCEIVER
VBUS
D+
D–
ID
GND
VBUS
D+
D–
ID
GND
3.3V
FOR LOW SPEED
ONLY
FOR FULL/HIGH SPEED
ONLY
3.3V
< 6.5µF
ATTACH PHASE
PHYSICAL CONNECTION
OF DEVICES
CONNECT PHASE
DETECT VOLTAGE LEVELS ON
D+/D– TO DETERMINE DATA
SPEED AND POWER LEVELS
ENUMERATION PHASE
SOFTWARE
HANDSHAKE
Figure 2. USB On-The-Go system diagram
Figure 3. USB sequence of events at start-up
tional switching regulator between
V
BUS
from the USB input, this regulator operates as a step-down converter. Using
the Bat-Track™ charging technique,
the switching regulator sets the voltage at V
very efficient charging solution. When
operating as an OTG A device, the
regulator acts as a step-up converter
by taking power from V
5V on V
The LTC3576 also has overvoltage
protection and can be used with an
external HV Buck regulator to provide
V
OUT
switching regulator can take power
from the HV buck regulator to supply
power to the USB connection.
In addition, the LTC3576 provides
two 400mA and one 1A step-down
1.5kΩ to 3.3V on D– during Connect for
1.5kΩ to 3.3V on D+ during Connect for
Full/High Speed, measure voltage on D–
Linear Technology Magazine • June 2009
and V
. When power is coming
OUT
OUT
BUS
to V
.
+ 0.3V, providing a
BAT
OUT
to produce
. In OTG mode, the bidirectional
Table 1. Load power signaling during Attach and Connect
Voltage on D–
with VDAT_SRC on D+ during Attach
Low Speed, measure voltage on D+
switching regulators for generating
three independent voltage rails for the
portable device. The LTC3576 allows
all three step-down switching regulator
output voltages to be enabled/disabled
and adjusted over a 2:1 range via I2C.
All three step-down regulators feature
pulse-skipping mode, Burst Mode
®
operation and LDO mode, which can
also be adjusted on-the-fly via I
2
C.
Mode Detection
The USB specification allows for a
number of different modes of operation
for products supporting both the USB
OTG specification1 and the battery
charger specification2. Figure 2 shows
a typical OTG system and Figure 3
shows the sequence of events that
occur when the USB cable is plugged
in. The product can be a B device
I
BUS
Host/Hub
< 500mA
0V 0.5V–0.7V 0.5V–0.7V
— > 2V < 0.8V
— > 2V < 0.8V
Dedicated Charger
I
BUS
< 1.5A
and can draw up to 100mA, 500mA,
900mA or 1.5A, depending on the type
of A device powering V
, as shown
BUS
in the Table 1.
When an OTG device has a micro/
mini-A plug connected to its micro/
mini-AB connector, the OTG device
becomes the A device and starts off as
the host. The OTG A device supplies
power to V
, as any other host A
BUS
device would, when requested by an
attached peripheral or OTG B Device.
As an A device, the LTC3576 can supply up to 500mA
The USB OTG specification provides
two means for a B device to signal to
the A device that it wants power. The
B device may drive the V
line above
BUS
2.1V, momentarily, or it may signal
by driving the D+ or D– signal lines.
The D+/D– signaling method could be
Host/Hub Charger
I
< (LS,FS < 1.5A/HS < 0.9A)
BUS
9
L DESIGN FEATURES
R5
7.68k
C
C1
1500pF
V
PROCESSOR
M3
UNLESS NOTED, RESISTORS: OHMS, 0402 1% 1/16 WATT
* THREE 1Ω, 5% RESISTORS IN PARALLEL
CAPACTORS: µF, 0402, 10% 25V
D2, D3: 1N4148
L1: 1098AS-2R0M
L2, L3: 1098AS-4R7M
L4: LPS4018-3R3MLC
M2, M3, M6: NDS0610
M4, M5, M7: 2N7002L
Q1, Q2, Q3: MMBT3904LT1
DV
CC
4.35V TO 5.5V
NON-OPERATING FAULT TOLERANCE
TO 30V CONTINUOUS
47k
10k
100k 100k
VBUS
D–
D+
IO
GNDSHGND
IDPUEN
FSPUEN
VBATVEN
VBATV
1ACHARGEEN
BAT
J2
DF3-3P-2DSA
GND
NTC-EXT
V
PROCESSOR
M2
V
PROCESSOR
V
PROCESSOR
V
PROCESSOR
V
PROCESSOR
3.6V AT
400mA
V
BUS
47k 15k 15k
V
BAT
M6
M7
47k
2.00k 2.00k
10k
10k
M4
BATTERY CHARGER HANDSHAKE
M5
U2B
LTC202
Q1
Q3
44.2k
D2
D3
100k
100k
100k
LEAKAGE
CURRENT
MUST BE
<400nA
6.2k
PROGV
CLPROGV
SCL
SDA
DV
CC
RST3
CHRG
VDAT_SRCEN
IDAT_SINKEN
D-V
VBUSV
D–
D+
HUBEN
IDV
DV
CC
I
DAT_SINK
V
DAT_SRC
4.7k
4.7k
100k
1.5k
3.01k
4.7µF
50V
22µF
6.3V
1µF
10V
22µF
6.3V
10µF
6.3V
0.1µF
16V
3.3V
U2A
LTC202
“V” SUF FIX
INDICATES
A/D INP UT
VC
WALL
ACPR
LD03V3
OVSENS
OVGATE
ILM0
ILM1
V
BUS
ENOTG
EN1
EN2
EN3
V
OUT
V
OUT
V
BAT
SW
IDGATE
BAT
V
IN1
SW1
FB1
CHRG
RST3
DVCC
SDA
SCL
CLPROG
PROG
NTCBIAS
NTC
M1
Si2306BDS
M8
Si2333DS
100µF
6.3V
0.337*
U1
LTC3576EUFE
L4
3.3µH
LEAKAGE CURRENT MUST BE < 50nA
0.1µF
16V
L3
4.7µH
2.2µF
6.3V
1.02M
324k
12pF
50V 5%
3.3V AT
400mA
10µF
6.3V
V
IN2
SW2
FB2
L2
4.7µH
2.2µF
6.3V
1.02M
R23
324k
18pF
50V 5%
1.8V
AT1A
22µF
6.3V
V
IN1
SW3
FB3
GND
L1
2.0µH
2.2µF
6.3V
402k
324k
27pF
50V 5%
Q2
J1
USBMICRO-AB
µC
10
Figure 4. Portable system with OTG and battery charger support
Linear Technology Magazine • June 2009
DESIGN FEATURES L
V
IH
V
(D+ or D–)
V
BUS
V
IL
5V
2.1V
0V
100ms
7.5ms
4.9s
B DEVICE SIGNALING
A DEVICE DELIVERING V
USB
detected by an OTG compatible USB
module on the system microcontroller
(µC ). The V
signaling method could
BUS
be detected via an A/D on the µC.
The LTC3576 bidirectional switching
regulator is then enabled as a step-up
converter (OTG mode) by setting the
appropriate bit in the control registers
via I2C.
Implementing a System
for USB OTG and
Battery Charging
Figure 4 shows an application for a portable device that supports both USB
battery charging and USB OTG.
When IDPUEN is low, the ID pin is
pulled up via R5, and if IDV is > 3V
then it is configured to be a B device.
If IDV is < 0.5V then it is configured
to be an A device. The components
enclosed in the box labeled “battery
charger handshake” enable communication of the power capabilities
depending on whether the portable
device is configured as an A device or
a B device. During the Attach phase,
if the portable device is a B device, it
can apply V
DAT_SRC
D+ line, load the D– line with I
(50µA~150µA), and measure the resultant voltage on D– via D–V. If the
voltage is 0, the A device is a Host/Hub,
if the voltage is V
device is a USB Charger.
During the Connect phase, FSPUEN
is pulled low to apply 3.3V to D+,
indicating a full/high speed device.
At the same time the voltage on the
D– line is read again via D–V. If it is
less than 0.8V, then the A device is a
Host/Hub Charger. If the voltage on
D–V is above 2V, then the A device is
a Dedicated Charger.
Linear Technology Magazine • June 2009
(0.5V~0.7V) to the
DAT_SRC
Figure 5. Session Request Protocol timing reference
then the A
DAT_SINK
For OTG functionality, if the portable device is configured as an A
device, then it must drive V
V
, which in this case is powered
OUT
BUS
from
from the battery. Since the LTC3576
is capable of supplying 500mA as an A
device, the µC asserts HUBEN to indi-
cate it is a Host/Hub. The bidirectional
switching regulator in the LTC3576
is enabled by setting the appropriate
bit in the control registers via the I2C
port. If the B device drawing current
from the V
line goes idle, then the
BUS
OTG A device may turn off the V
voltage to conserve the battery. When
the B device needs the V
voltage
BUS
to be present at some later time, it
can request that the A device again
drive V
by turning the bidirectional
BUS
switching regulator back on. It can do
this by signaling on the D+ or D– lines
or by driving the V
line to > 2.1V
BUS
(see Figure 5).
The Host A device only needs to
respond to one of two SRP signaling
methods. However, since not all USB
engines respond to the D+/D– signaling, the V
line is sensed to check if
BUS
it is higher than 2.1V via the VBUSV
A/D input.
When the portable device’s µC detects that the B device is requesting
power on V
, either by sensing the
BUS
D+/D– signaling or by sensing that
V
has been driven higher than 2.1V,
BUS
it should again turn on the OTG stepup converter in the LTC3576.
The PROG (PROGV) and BAT
(VBATV) pins allow a Coulomb counter
to be implemented in the µC. Reading the BAT voltage requires that the
sensing divider be enabled by setting
VBATVEN low. This ensures that the
sense divider network does not dis-
1
charge the battery when the battery
voltage isn’t being measured.
The default battery charge current has been set to 500mA, but can
be increased to 1A by asserting the
1AchargeEN signal. This turns on M7,
halving the PROG resistance and increasing the charge current. The input
current limit will need to be set to 10X
mode (1A) using the I2C port.
The optional network of C14 and
R27/R28/R29 suppresses ripple on
the BAT pin (and consequently on the
V
BUS
pin) if there is no battery present.
BUS
This ripple can be in the tens of mV.
While this will not damage anything,
it may be desirable to suppress this
signal.
The CLPROG (CLPROGV) and
CHRG signals are often useful for
housekeeping tasks in the µC.
The LTC3576 has an overvoltage
protection function that controls M1,
and protects the system from excessive
voltages on the USB (J1) connector.
Because the A/D is configured to monitor V
, it must also be protected by
BUS
D1 from excessive voltages.
The LDO3V3 regulator is configured
to power the µC in low power mode
(<20mA). When the µC needs to leave
low power mode it first enables Buck
Regulator 2, which will provide up to
400mA.
Conclusion
The LTC3576 is a versatile PMIC
consisting of a bidirectional power
manager, overvoltage protection, three
step-down switching regulators and
a controller for an external high voltage step-down switching regulator.
In conjunction with a few support
components, the LTC3576 allows the
implementation of a complete power
management system for portable devices that support both USB OTG and
USB battery charging.
Bibliography
1
”On-The-Go Supplement to the USB Specification”,
Revision 1.3
2
“Battery Charging Specification”, Revision 1.0
3
www.usb,org/developers/docs
L
11
L DESIGN FEATURES
LTC3101
BUCK-BOOST
LDO
ON/OFF
AC
ADAPTER
USB
or
HOT SWAP OUT
3.xV AT 300 to 800mA
1.xV AT 350mA
1.xV AT 350mA
1.8V AT 50mA
3.xV AT 100mA
x.xV AT 200mA
TRACKING OUT
LI-ION
USB
BAT
Power Management IC with Pushbutton Control Generates Six Voltage Rails from USB or 2 AA Cells Via Low Loss PowerPath Topology
Introduction
As the complexity of portable electronic
devices continues to increase, the demands placed on power supplies, and
their designers, expand dramatically.
Not only must typical power systems
accommodate multiple input sources,
with voltages as low as 1.8V for two
AA cells, but they must also provide
an increasing number of independent
output rails to support a wide range
of requirements—for memory, microprocessors, backlights, audio and RF
components. To further complicate
matters, expanding feature sets add up
to increased power dissipation, making
it important to optimize overall power
system efficiency. This is particularly
challenging given that the constant
drive to minimize the required board
area and profile height of the power
system is at direct odds with improving efficiency.
The LTC3101 addresses all of these
challenges with a single-IC power
management solution that allows a
designer to easily maximize overall
power system efficiency while minimizing space requirements. The LTC3101
can generate six power rails by integrating three synchronous switching
converters, two protected switched
Figure 2. Complete portable power
solution with a 16mm × 19mm footprint
12
by John Canfield
Figure 1. Six output rails, a low loss PowerPath and integrated pushbutton control
power outputs, and an LDO. Its integrated low loss PowerPath™ topology
allows each switching converter to
run directly from either of two input
power sources.
Two 350mA, high efficiency low
voltage rails, typically used to power
processors and memory, are generated
by synchronous buck converters. Each
converter is able to operate down to an
input voltage of 1.8V thereby enabling
single stage conversion from any input
power source.
A single inductor buck-boost
converter generates a high efficiency
intermediate output rail, typically at
3V or 3.3V, and is able to operate from
either input power source and with
input voltages that are above, below,
or even equal to the output regulation
voltage. The buck-boost converter
can supply a 300mA load at 3.3V for
battery voltages down to 1.8V and an
800mA load for input voltages of 3.0V
and greater.
Two always-alive outputs—MAX,
which tracks the higher voltage input
power source and LDO, a fixed 1.8V
output—provide power to critical
functions that must remain powered
under all conditions. An integrated
pushbutton controller with programmable µP reset generator provides
complete ON/OFF control using only
a minimal number of external components while independent enables allow
total power-up sequencing flexibility.
This complete portable power solution
is packaged in a single low profile 24lead 4mm × 4mm QFN package and
the entire power supply, including
all external components, occupies a
PCB area of less than 3cm2 as shown
in Figure 2.
Zero Loss PowerPath
Topology Maximizes
Efficiency
Although rechargeable Li-Ion and
Li-Polymer batteries are the leading
chemistries for powering portable
devices due to their high energy density and long cycle life, many portable
devices continue to be powered by
alkaline and NiMH cells. This allows
indefinite periods of use away from a
Linear Technology Magazine • June 2009
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