LINEAR TECHNOLOGY LT3650 Technical data
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 termination 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 0.05
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.
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.
Timer termination, or top-off
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 sup
ply 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
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 condi-
-
tion 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.
Ω external sense resistor. This
Linear Technology Magazine • June 2009
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