DN111 - LT1510 High Efficiency Lithium-Ion Battery Charger

High Efficiency Lithium-Ion Battery Charger
Design Note 111
Chiawei Liao
Lithium-Ion Battery Charger
The circuit in Figure 1 uses the 16-lead LT1510 to charge
lithium-ion batteries at a constant 1.3A until battery
voltage reaches 8.4V set by R3 and R4. The charger
will then automatically go into a constant voltage mode
with current decreasing toward near zero over time as
the battery reaches full charge. This is the normal regimen for lithium-ion charging, with the charger holding
the battery at “float” voltage indefinitely. In this case,
no external sensing of full charge is needed. Figure 2
shows typical charging characteristics.
The battery DC charging current is programmed by a
resistor RPROG ( or a DAC output current) at the PROG pin.
High DC accuracy is achieved with averaging capacitor
CPROG. The basic formula for full charging current is:
IBAT = (IPROG)(2000) = (2.465/RPROG)(2000)
= (2.465/3.83k)(2000) = 1.3A
Approximately 0.25mA flows out of the BAT pin at
all times when adapter power is applied. Therefore,
09/95/111_conv
SW
VCC
+
0.22μF
BOOST
L1
33μH
D2
1N914
PROG
VC
0.1μF
+
–
47k
1μF CPROG
300Ω
LT1510
GND
CIN
10μF
VIN
11V TO
25V
RPROG
3.83k
1k
OVP
BAT
SENSE
+
CB
22μF
TANT
+
Q3
VN2222
4.2V
+
R3
12k
0.25%
4.2V
R4
4.99k
0.25%
COMPLETE LITHIUM-ION CHARGER, NO TERMINATION REQUIRED
CIN: TOKIN 25V CERAMIC SURFACE MOUNT 1E106ZY5U-C205
L1: COILTRONICS CTX33-2
DN111 F01
Figure 1. Charging Lithium-Ion Batteries
(Efficiency at 1.3A = 86%)
1400
8.6
BATTERY VOLTAGE
8.4
1200
BATTERY CURRENT (mA)
The LT1510 can charge batteries ranging from 1V to
20V. A blocking diode is not required between the chip
and the battery because the chip goes into sleep mode
and drains only 3μA when the wall adaptor is unplugged.
Soft start and shutdown features are also provided.
D3
1N5819
D1
1N5819
BATTERY VOLTAGE (V)
The LT®1510 current mode PWM battery charger is
the simplest, most efficient solution for fast charging
modern rechargeable batteries including lithium-ion
(Li-Ion), nickel-metal-hydride (NiMH) and nickelcadmium (NiCd) that require constant current and/
or constant voltage charging. The internal switch is
capable of delivering 1.5A DC current (2A peak current).
The onboard current sense resistor (0.1Ω) makes the
charge current programming very simple. One resistor (or a programming current from a DAC) is used to
set the charging current to within 5% accuracy. With
0.5% reference voltage accuracy, the LT1510 16-lead
S package meets the critical constant voltage charging
requirement for lithium cells.
1000
8.2
800
8.0
BATTERY CURRENT
7.8
600
7.6
400
7.4
200
7.2
0
0
25
50
100
75
TIME (MINUTES)
125
DN111 • F02
Figure 2. Battery Charging Characteristics
to ensure a regulated output even when the battery
is removed, the voltage divider current should be set
at 0.5mA. Q3 is used to eliminate this current drain
when adapter power is off, with a 47k resistor to pull
its gate low.
With divider current set as 0.5mA, R4 = 2.465/0.5mA
= 4.93k, let R4 = 4.99k:
VBAT
8.4
R3 = R4 2.465
– 1 = 4.99k
– 1 = 12k
2.465
VIN has to be at least 3V higher than battery voltage
and between 8.5V to 25V.
Lithium-ion batteries typically require float voltage
accuracy of 1% to 2%. The LT1510 OVP voltage has
0.5% accuracy at 25°C and 1% over full temperature.
This may suggest that very accurate (0.1%) resistors
are needed for R3 and R4. Actually, in float mode the
charging currents have tapered off to a low value and
the LT1510 will rarely heat up past 50°C, so 0.25% resistors will provide the required level of overall accuracy.
Thermal Calculations
Although the battery charger achieves efficiency of approximately 86% at 1.3A, a thermal calculation should
be done to ensure that junction temperature will not
exceed 125°C. Power dissipation in the IC is caused by
bias and driver current, switch resistance, switch transition losses and the current sense resistor. The 16-lead
SO, with a thermal resistance of 50°C/W, can provide a
full 1.5A charging current in many situations. Figure 3
shows the efficiency for charging currents up to 1.5A.
100
VCC = 16V
VBAT = 8.4V
EFFICIENCY INCLUDES
LOSS IN DIODE D3
98
96
EFFICIENCY (%)
94
92
90
88
86
84
82
80
0.1
0.3
0.5
0.7 0.9
IBAT (A)
1.1
1.3
1.5
DN111 • F03
Figure 3. Efficiency of Figure 1 Circuit
PBIAS = (3.5mA)(VCC) + (1.5mA)(VBAT)
(V )2(7.5mA + 0.012 • IBAT)
+ BAT
VCC
(I )(V )2
PDRIVE = BAT BAT
50 (VCC)
Example: VIN = 16V, VBAT = 8.4V, IBAT = 1.3A
PBIAS = (3.5mA)(15.6) + (1.5mA)(8.4)
(8.4)2(7.5mA + 0.012 • 1.3)
= 0.10W
+
15.6
PDRIVE =
50 (15.6)
Total power in the IC is 0.1 + 0.12 + 0.36 + 0.30 = 0.88W
Temperature rise in the IC will be: (50°C/W)(0.88W) = 44°C
Some battery manufacturers recommend termination
of constant voltage float mode 30 to 90 minutes after
charging current has dropped below a specified level
(typically 50mA to 100mA). Check with the manufacturers for details. The circuit in Figure 4 will detect
when charging current has dropped below 75mA. This
logic signal is used to initiate a timeout period, after
which the LT1510 can be shut down by pulling the VC
pin low with an open collector or drain. Some external
means may be used to detect the need for additional
charging or the charger may be turned on periodically
to complete a short float voltage cycle. The current trip
level is determined by the battery voltage, R1 through
R3, and the internal LT1510 sense resistor (≈ 0.18Ω
pin-to-pin). D2 generates hysteresis in the trip level to
avoid multiple comparator transitions. R2 and R3 are
chosen to total about 1M to minimize battery loading.
D2 is assumed to be off during high current charging
when the comparator output is high. To ensure this, the
ratio of R2 to R3 is chosen to make the center node
voltage less than the logic supply. R4 is somewhat
arbitrary and does not affect trip point. R1 is adjusted
to set the trip level:
(I
)(R2 + R3)(0.18<)
R1 = TRIP
VBAT
(75mA)(560k + 430k)(0.18)
=
= 1.6k
8.4V
INTERNAL
SENSE
RESISTOR
0.18Ω
BAT
ADAPTER
OUTPUT
SENSE
R1*
1.6k
)2
RSW = Switch on resistance ≈ 0.35<
TOL = Effective switch overlap time ≈ 10ns
VCC = VIN – 0.4V
(1.3)(8.4)2
PSENSE = (0.18)(1.3)2 = 0.30W
LT1510
(I )2(RSW)(VBAT)
PSWITCH = BAT
+ (TOL)(VCC)(IBAT)
VCC
PSENSE = (0.18<)(IBAT
(1.3)2(0.35)(8.4)
15.6
+ (10– 8)(15.6)(1.3)(200kHz) = 0.36W
PSWITCH =
D1
1N4148
C1
0.1μF
3
*TRIP CURRENT =
R1 (VBAT)
(R2 + R3) (0.18Ω)
2
R2
560k
D2
1N4148
–
+
3.3V OR
5V
8
7
LT1011
R4
470k
NEGATIVE EDGE
TO TIMER
4
1
R3
430k
DN111 • F04
Figure 4. Current Comparator for Initiating
Float Timeout
= 0.12W
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn111f_conv LT/GP 0995 190K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1995
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