DN242 - Li-Ion Charge Termination IC Interfaces with PWM Switchers

Li-Ion Charge Termination IC Interfaces with PWM Switchers
Design Note 242
Fran Hoffart
Rechargeable lithium-ion batteries are rapidly becoming the battery of choice for many battery-powered
products. These products include notebook computers, PDAs, video camcorders, digital cameras, cellular
phones, portable test equipment and others. Compared
to other rechargeable power sources, Li-Ion batteries
have higher energy density for both weight and volume
and provide longer run time between charges.
tery from overtemperature and overcurrent conditions.
In addition, battery packs may also contain back-toback MOSFET switches that disconnect the battery if
an overcurrent condition exists for either charge or
discharge, or if an overvoltage or undervoltage condition exists. Many of these precautions are needed
because lithium-ion cells are easily damaged by both
overcharge and overdischarge conditions.
Charging Li-Ion batteries is a relatively simple process.
Apply a current-limited (at a 1C rate) constant voltage
(±1% tolerance) for approximately three hours, then
stop charging. (C is a battery term used to indicate
the Ampere-hour capacity of a battery). A complete
charge cycle may also include precharge qualification
for battery temperature and precharge qualification for
deeply discharged batteries.
LT®1510 Battery Charger IC
The LT1510 is a high efficiency switching regulator
power IC designed specifically for battery charging
applications. A step-down current mode 200kHz or
500kHz PWM topology is used. Included on the die
is a 2A switch along with programmable current and
voltage control circuitry. Available in a 16-pin SO and
the tiny MSOP surface mount package, the LT1510 is
capable of providing up to 1.5A of charge current in
many situations. Although the LT1510 provides many
charger functions, it lacks a timer and requires precision resistors to program the charge voltage. This is
when the LTC ®1729 should be added.
Battery Pack Protection
Because of the high energy associated with lithium-ion
cells and their sensitivity to abuse, many battery
manufacturers require protective devices inside the
battery pack for both safety and performance reasons.
These devices often consist of poly fuses, thermal
fuses and bimetallic breakers, which protect the bat-
VIN
12V TO 20V
LT1510
1, 4
L1
33μH
CTX33-2
2, 3
D1
MBRS140L
1
C2
0.22μF
3
D3
MMBD914
5
2
4
6
7
8
+
10mV
–
GND
GND
SW
VCC
BOOST
VCC
GND
OVP
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
PROG
VC
SENSE
BAT
GND
GND
GND
GND
16
15
D2
MBRS140L
14
C1 10μF
( )
CHARGE CURRENT = 2.465 (2000)
R1
13
12
11
R2
300Ω
C3
1μF
R3
1k
C5
0.1μF
10
9
R1
4.3k PROGRAM
CHARGE
CURRENT
IBAT
R6
100Ω
C7
1μF
C6
0.1μF
LTC1729-8.4
1
2
3
4
OVP
BAT
VCC
VC
SENSE STATUS
GND
NTC
8
7
6
+
R4
4.7k
C4
22μF
D5
LED1
VBAT
8.4V
+
Li-Ion
BATTERY
THERMAL CONTACT
5
R5 10k
NTC THERMISTOR
DN242 F01
Figure 1. Complete 1.3A Battery Charger for Two Li-Ion Cells
10/00/242_conv
The Charge Cycle
A typical charge cycle of the circuit in Figure 1 is as
follows: with the input voltage applied and no battery connected, the charger output is pulled high by
an internal 200μA current source in the LT1510 and
clamped at 9V by the LTC1729. Connecting a discharged
2-cell battery to the charger will pull the charger output (BAT pin) down to the battery voltage, starting
the charge cycle. For temperature qualification, the
voltage on the NTC (thermistor) pin must be between
0.405V and 2.79V, indicating that the temperature is
between 0°C and 50°C; otherwise the charge cycle is
put on hold until the temperature is within this range.
For deeply discharged batteries with voltages below
5.2V, a preconditioning 16mA trickle charge begins
and continues until the battery voltage exceeds 5.2V.
After the qualification and preconditioning is completed, the constant-current portion of the charge
cycle begins (see curves in Figure 2). As the battery accepts charge, the battery voltage rises and
approaches the programmed voltage of 8.4V, at
which time, the constant-voltage portion of the
charge cycle begins. With the battery voltage held
constant, the charge current will drop exponentially, eventually reaching tens of mA before the
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The open-drain STATUS pin is pulled low when the
battery is installed and the charge current is greater
than 100mA. When the charge current drops below
100mA, a 50μA current source pulls this pin low and,
after the 3-hour timer has timed out, this pin is open
circuit. The 100mA threshold level is programmable
by changing R6.
Board Layout and Testing
Although the LT1510 can provide charge current in
excess of 1A, a good thermal layout of the PC board
is required. Wide copper traces for the ground pins,
feedthrough vias and generous amounts of copper on
both sides of the board are all necessary to minimize
the IC temperature rise.
When testing the charger, use either a battery or a
battery simulator for a load (a conventional electronic
load presents a high impedance, unlike a battery). A
simple battery simulator consists of an adjustable lab
power supply with a load resistor across the output.
Select a resistor that will result in approximately
twice the rated charge current flowing through it.
The power supply can now be used in place of the
battery for testing purposes. A fully discharged to a
fully charged battery can be simulated by varying the
power supply voltage.
For additional information, please consult the LT1510
and LTC1729 data sheets.
TRICKLE
CHARGE
CONSTANT CURRENT
CHARGE
CHARGE CYCLE
COMPLETE
1.5
CHARGE CURRENT (A)
Complete 2-Cell Li-Ion Charger
A complete constant-current/constant-voltage Li-Ion
2-cell charger is shown in Figure 1. The LT1510 provides the charge current and the LTC1729 provides
the charge termination. R1 allows the charge current
to be easily programmed.
3-hour timer expires, thus ending the charge cycle.
CONSTANT VOLTAGE
CHARGE
1.3A
1.0
0.5
100mA
16mA
0
9
8.4V
8
BATTERY VOLTAGE (V)
LTC1729 Li-Ion Charge Termination IC
This 8-pin IC interfaces with the LT1510 (as well as other
LTC charger products, such as the LT1511, LT1769,
LT1505, LT1512 and LT1513) to provide a complete
Li-Ion charger solution. The LTC1729 provides a
precision voltage divider for programming the charge
voltage and includes a preconditioning trickle charge
for deeply discharged cells. Battery temperature is
monitored using a thermistor; a 3-hour timer ends
the charge cycle. Also included is a status output pin
that provides a signal when the charge current drops
below a programmable threshold level, indicating a
near-full-charge condition. This signal can be used
to drive an LED, to provide charge indication to other
circuitry or to terminate the charge when the charge
current drops below the threshold level.
STATUS PIN
(WEAK PULL-DOWN)
7
STATUS PIN
(PULLED LOW)
6
STATUS PIN GOES HIGH
(TIMER STOPS CHARGE CYCLE)
5.2V
5
BATTERY
INSTALLED
4
0
1
2
TIME (HOURS)
3
DN242 F02
Figure 2. Charge Current and Battery Voltage
for a Typical Charge Cycle
For applications help,
call (408) 432-1900
dn242f_conv LT/TP 1000 370K • PRINTED IN THE USA
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