DESIGN FEATURES
Fast Rate Li-Ion Battery Charger
by Goran Perica
Introduction
The recent trend in notebook computers has been toward increasing
battery operating time and faster
processor speeds. These two requirements, in conjunction with a need for
faster battery recharging (1–2 hours)
have placed a strain on battery charging circuits and wall adapters. A
typical notebook computer system
configuration is shown in Figure 1.
Wall adapters are typically AC/DC
converters with a 20V output at
3A–4A of load current. When a notebook computer is running, all of the
available current from the wall adapter
may be consumed by the system,
with no power left for charging the
battery. However, as soon as the
system’s power requirements drop
below the wall adapter’s current limit,
the battery charging can resume. In
order to recharge the battery in the
shortest time possible, the recharging should start as soon as there is
any current left over from the system.
The ideal situation is when the sum of
battery charging current and the system current is just below the wall
adapter’s current limit:
IIN_MAX > ISYS + ICHARGER
where IIN_MAX is the wall adapter current limit, ISYS is the system load
current and ICHARGER is the battery
charger current.
To achieve this objective, it is necessary to adjust the battery charger
current so that the sum of the two
currents is just below the maximum
available input current, IIN_MAX. The
INPUT FROM
WALL ADAPTER
LT1505 incorporates a patented battery charger input current limiting
function along with other functions
necessary to provide a complete,
single-chip battery charging circuit
solution.
LT1505 Features
The LT1505 is a constant-current
(CC), constant-voltage (CV) current
mode switching battery charger circuit with the following features:
❏ 0.5% voltage reference
❏ 5% output current regulation
❏ Output voltage is preset for 3 or 4
Li-Ion cells (12.3V, 12.6V, 16.4V
and 16.8V)
❏ Output voltage is programmable
from 1V to 21V
❏ Low VIN-to-VOUT operation
(dropout <0.5V)
❏ Programmable AC wall adapter
current limiting
❏ Programmable peak battery
charging current
❏ Battery drain <10µA in shutdown
❏ 94% efficiency
Circuit Description
The LT1505 is a synchronous buck
converter using N-channel MOSFETs.
The LT1505 operates at 200kHz and
can be synchronized to an external
clock with a frequency higher than
240kHz. The LT1505 IC has an
undervoltage lockout circuit that
detects the presence of an input power
source and enables the battery charging. Once the undervoltage lockout
has been exceeded, the PWM will start
INPUT
CURRENT
SENSE
LT1505
BATTERY
CHARGER
Li-Ion
BATTERY
Figure 1. Typical notebook computer power supply
24
SYSTEM
LOAD
running and the input MOSFET M3 is
turned ON, thus reducing the voltage
drop across its internal body diode
DBODY (see Figure 2).
The LT1505 monitors the current
from the wall adapter and controls
the battery charger current. For
example, if a 3A, 20V wall adapter is
used along with a 12.6V Li-Ion battery pack, the peak battery charging
current, when the system is off, can
be set to:
IBATT MAX = η × IIN_MAX × VIN/VBATT
where IBATT MAX is the maximum battery charging current when the system
is idle, η is the efficiency of battery
charger, VIN is the wall adapter output voltage and VBATT is the battery
charging voltage.
Assuming an efficiency of 90%, the
above example could provide battery
charging current in excess of 4A. The
LT1505 will reduce the battery charging current as soon as the system
current exceeds (IIN_MAX – ICHARGER).
For example, if a 20V, 3A wall adapter
is used and the system draws 2A from
the adapter, the available current for
charging the battery will be ICHARGER =
1A. The resulting battery charging
current IBATT will be:
IBATT = η × ICHARGER × VIN/VBATT
or
IBATT = 0.9 × 1A × 20V/12.6V = 1.428A
The input current from the wall
adapter passes through a current
sense resistor, RS4. One part of the
input current goes to the system load
and the remaining part goes to the
LT1505 battery charger. The voltage
drop across RS4 is monitored by a
current comparator with a 90mV
threshold. Once the threshold of 90mV
is reached, the LT1505 will reduce
the programmed battery charging current so that the peak input current
does not exceed the preset limit. Thus,
the maximum input current (IIN_MAX)
will be:
IIN_MAX = ISYSTEM + ICHARGER = 0.090V/RS4
Linear Technology Magazine • February 1999
DESIGN FEATURES
VIN
(FROM
ADAPTER)
DBODY
TO
SYSTEM POWER
M3
Si4435
RS4
0.025Ω
R7
475Ω
C4
0.1µF
CIN
47µF
35V
C1
1µF
BOOST
VCC
BOOSTC
GBIAS
100k
R5
4.75k
CLN
TGATE
CLP
SW
INFET
L1
10µH
M1
Si4412
4.7Ω
D4
MBRS140
3 CELL
FLAG
VFB
CAP
4.2V
COMP1
4.1V
R1
1k
C7
0.68µF
AGND
RP1
330Ω
SENSE
12.6V
BATTERY
RC1
1k
RS2
200Ω
1%
CP1
1µF
RPROG
4.93k
1%
RS3
200Ω
1%
CC1
0.33µF
PGND
BAT2 BAT
COUT
22µF
25V
×2
R2
22Ω
C8
220pF
PROG
LT1505
SHDN
RS1
0.025Ω
VBAT
M2
Si4412
VC
SYNC
C6
0.1µF
C2
1µF
D2
1N4148
BGATE
UV
R6
4.75k
C3
4.7µF
D3
1N4148
NOTE: DBODY IS THE BODY DIODE OF M3
(619) 661-6835
CIN: SANYO OS-CON
L1: SUMIDA CDRH127 (847) 956-0666
SPIN
1505 F01
Figure 2. 4A Li-Ion battery charger
The battery charger in Figure 2
achieves high efficiency thanks to
synchronous operation and input
power FET. The efficiency is as high
as 94%, as can be seen in Figure 4.
PCB Layout
When laying out the PCB, a multilayer
layout with one of the inner layers as a
solid ground plane is recommended.
IBAT_MAX = (VPROG/RPROG) × (RS2/RS1)
The LT1505 and low power compowhere VPROG is the reference voltage nents associated with it should be kept
of 2.465V. The values in Figure 2 have as close together as possible. Additionbeen selected for a current limit ally, all power components should be
(IBAT_MAX) of 4A. Changing RS1 to kept together and next to LT1505 con0.050Ω will set the IBAT_MAX to 2A.
trol circuitry. The goal is to keep all
Also, the peak battery charging high power switching currents as locurrent (IBAT_MAX) can be programmed calized as possible. Components that
by the host computer. The IBAT_MAX connect to the ground plane should
can be set in increments of 0.25A if have vias placed as close as possible to
RPROG is replaced by a network of the pins connected to the ground plane.
resistors, as shown in Figure 3.
Also, power components should have
larger or multiple vias connecting to
100
95
90
EFFICIENCY (%)
where ISYSTEM is the system load current, ICHARGER is the LT1505 battery
charger current and RS4 is the current sense resistor. With the resistor
value of 0.025Ω in Figure 2, the input
current limit IIN_MAX will be set to
3.6A.
The battery charging current limit
is set by RPROG, RS1 and RS2 and is:
85
80
75
70
65
60
0
1
2
3
OUTPUT CURRENT (A)
4
TA02
Figure 4. Efficiency of 4A, 12.6V1611battery
charger at 20V input
the ground plane. Avoid placing the
power components in such a way that
input and output currents flow by the
LT1505 IC. Also, to keep the component temperature rise low, use as much
copper as possible. The use of polygon
planes for high power nets such as the
o n e s c o n n e c t i n g t o V IN, V CC,
continued on page 35
LT1505
LIMITED
INPUT
POWER
PROG
2A
1A
10k
0.5A
20k
40.2k
4× 2N3904
FROM
µP
0.25A
80.6k
BATTERY
CHARGE
CURRENT
SENSE
RP1
330Ω
CP1
1µF
{
Figure 3. Programming of battery-charge current
Linear Technology Magazine • February 1999
LT1505
BASED
CONVERTER
SYSTEM
LOAD
BATTERY SUPPLIES
ADDITIONAL PEAK POWER
Figure 5. Typical telecom application
25
CONTINUATIONS
range. This is true provided the filter
magnitude response does not change
with varying input signal levels, that
is, the filter gain is linear. The gain
linearity measured at the 100kHz
theoretical center frequency of the
filter is shown in Figure 7. The gain is
perfectly linear for input amplitudes
up to 1.25VRMS (3.5VP-P) so an 84dB
dynamic range can be claimed. The
input signal, however, can reach amplitudes up to 3VRMS (8.4VP-P, 92dB
SNR) with some reduction in gain
linearity.
The LTC1735 and LTC1736 are the
latest members of Linear Technology’s
family of constant frequency, N-channel high efficiency controllers. With
new protection features, improved circuit operation and strong MOSFET
drivers, the LTC1735 is an ideal upgrade to the LTC1435/LTC1435A for
higher current applications. With the
integrated VID control, the LTC1736
is ideal for CPU power applications.
The high performance of these controllers with wide input range, 1%
reference and tight load regulation
makes them ideal for next generation
designs.
LTC1562-2, continued from page 10
References
level is 44µVRMS over a bandwidth of
800kHz or 98dB below the maximum
unclipped output.
1. Hauser, Max. “Universal Continuous-Time Filter Challenges Discrete
Designs.” Linear Technology VIII:1
(February 1998), pp. 1–5 and 32.
2. Sevastopoulos, Nello. “How to Design High Order Filters with Stopband
Notches Using the LTC1562 Quad
Operational Filter, Part 1.” Linear
Technology VIII:2 (May 1998), pp.
28-31.
3. Sevastopoulos, Nello. “How to Design High Order Filters with Stopband
Notches Using the LTC1562 Quad
Operational Filter, Part 2.” in the Design Ideas section of this issue of
Linear Technology.
4. LTC1562 Final Data Sheet.
5. For example: Schwartz, Mischa.
Information Transmission, Modulation, and Noise, fourth edition, pp.
180–192. McGraw-Hill 1990.
band gain can be higher than 0dB or
if internal nodes are allowed to have
gains higher than 0dB. Please contact the LTC Filter Design and
Applications Group for further details.
The low noise behavior of the filter
makes it useful in applications where
the input signal has a wide voltage
LTC1735/LTC1736, continued from page 6
Conclusion
Acknowledgments
Philip Karantzalis and Nello Sevastopoulos of LTC’s Monolithic Filter
Design and Applications Group contributed to the application examples.
LT1505, continued from page 25
SW, VBAT and GND in Figure 2 will
help in spreading the heat and will
reduce the power dissipation in conductors and MOSFETs.
By doing so, the required peak power
from the wall adapter can be much
lower than the peak power required
by the load. The wall adapter has to
supply the average power only.
The LT1505 can also be used in other
system topologies, such as the telecom application shown in Figure 5.
The circuit in Figure 5 uses the battery to supply peak power demands.
Conclusion
The LT1505 is a complete, singlechip battery charger solution for
today’s demanding charging requirements in high performance laptop
applications. The device requires a
small number of external components
and provides all necessary functions
for battery charging and power management. High efficiency and small
size allow for easy integration with
the laptop circuits. Also, by adding a
simple external circuit, charging can
be easily controlled by the host computer, allowing for more sophisticated
charging schemes.
Step-Down Conversion, continued from page 30
cuitry works in the same manner as
in Figure 1. Efficiency and performance are virtually the same as the
LTC1649 solution, but parts count
and system cost are lower.
In a 3.3V to 2.5V application, the
steady-state, no-load duty cycle is
76%. If the input supply drops to
3.135V (3.3V – 5%), the duty cycle
requirement rises to 80% at no load,
and even higher under heavy or
transient load conditions. Both the
LTC1649 and the LTC1430A guarantee a maximum duty cycle of greater
than 90% to provide acceptable load
regulation and transient response.
The standard LTC1430 (not the
LTC1430A) can max out as low as
83%—not high enough for 3.3V to
2.5V circuits. Applications with larger
step-down ratios, such as 3.3V to
2.0V, can use the circuit in Figure 3
successfully with a standar d
LTC1430.
Other Applications
lower cost LTC1430A replacing the
LTC1649. The LTC1430A does not
include the 3.3V to 5V charge pump
and requires a 5V supply to drive the
external MOSFET gates. The current
drawn from the 5V supply depends
on the gate charge of the external
MOSFETs but is typically below 50mA,
regardless of the load current on the
2.5V output. The drains of the Q1/Q2
pair draw the main load current from
the 3.3V supply. The remaining cirLinear Technology Magazine • February 1999
35