Dual Smart Battery Charger Simplifies Battery Backup
for Servers – Design Note 342
Mark Gurries
Introduction
Smart Batteries are an increasingly popular choice for
more than just traditional compact consumer electronic
devices. For example, Smart Batteries are being used
as battery backup for products such as blade servers,
where knowing battery status is very important.
LTC®1760 Dual Smart Battery Charger
Figure 1 shows a typical dual battery charger. This
circuit can charge batteries with up to 4A and switch
continuously down to zero load currents. This circuit
takes advantage of ceramic capacitors’ space saving
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and PowerPath is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
*U.S. Patent number 6650174 **U.S. Patent number 5723970
PowerPathTM MUX
VIN
6V TO 28V
C8, 1μF
RCL
0.03Ω
R1
4.99k
R4
12.7k
R5
1.21k
R7
49.9k
C3
0.012μF
C11
1800pF
VDDS
SMBALERT
RPU
SCL
RPU
SDA
VDDS
C1
0.1μF
R10, 100Ω
C1
0.1μF
RVLIMIT
10k
C9, 0.1μF
36
CLP
41
DCIN
3
BAT1
2
BAT2
LTC1760
VPLUS
GDCI
GDCO
GB1I
GB1O
GB2I
GB2O
SCP
16
SCN
DCDIV
37
LOPWR
COMP1
47
CSN
GCH2
48
CSP
SCH2
46
ITH
GCH1
45
ISET
SCH1
13
SW
V
40 SET
BOOST
V
24 CC
TGATE
VSS
BGATE
PGND
25
TH2A
VCC2
29
TH2B
SMBALERT
18
SCL2
SCL
22
SDA2
SDA
20
TH1A
V
33 DDS
TH1B
V
32 LIMIT
SCL1
ILIMIT
26
SDA1
MODE
BAT2
D2
R11
1k
Q6
Q7
Q2
Q5
Q8
RSC
0.02Ω
LOAD
R2
280k
C7
0.1μF
R9
3.3k C12
1000pF
C5
0.15μF
CB1, 0.1μF
TH
SCL
SDA
BAT1
TH
SCL
SDA
CIN
20μF
L1
10μH
RSENSE
0.025Ω
C4, 0.22μF
QBG
D1
R6
100Ω
D1: MBR130T3
D2: IN4148 TYPE
Q1, Q2, Q5, Q6, Q7, Q8: Si4925DY
Q3, Q4, Q9, Q10, QTG, QBG: FDS6912A
COUT
20μF
CHARGE
MUX
Q4
Q9
Q3
Q10
DN342 F01
Figure 1. 4A Dual Battery System
10/04/342_conv
BAT2
R1B, 54.9k
QTG
C6
4.7μF
R2A, 1.13k SAFETY 2
R1A, 1.13k SAFETY 1
D4
C13
0.1μF
CL
20μF
R3
49.9k
R2B, 54.9k
BAT1
D3
I5
I6
CB2
0.47μF
1
7
6
9
8
11
10
5
4
12
34
35
14
15
42
43
44
39
38
28
27
17
21
30
31
19
23
Q1
features without producing any audible noise. The high
300kHz switching frequency allows the use of small
low cost 10μH inductors.
battery drain—drain battery 1, then battery 2. New
server applications are also using batteries and demand
drain currents beyond the capability of a single battery.
The LTC1760 complies with the Smart Battery System
Manager (SBSM) specification V1.1. It has a very wide
input and charge output voltage range of 6V to 28V.
Current and voltage accuracies of 0.2% of the reported
values provide precision charge capability. Low dropout
is achieved with 99% maximum duty cycle while maintaining efficiency greater than 95%. The LTC1760 also
offers many unique features, including a special current
limit and voltage limit system that prevents SMBus data
corruption errors from generating false charge values,
which could harm the battery. An SMBus accelerator
increases data rates in high capacitance traces while
preventing bus noise from corrupting data*.
The LTC1760 addresses this need by allowing the safe
parallel discharge of two batteries. Parallel discharge
offers more than just increased current capability. It
reduces I2R losses and improves voltage regulation
under extremely high load conditions, both of which
can improve total discharge time over a sequential solution. Figure 2 compares discharge times for equivalent
parallel and sequential solutions. In high current, rapid
discharge applications, quick recharging of the batteries is a priority. Again the LTC1760 goes beyond the
simple sequential solution and offers parallel charging,
which, depending on the battery chemistry, can result
in significant charge time reductions over a sequential
solution, as shown in Figure 3.
Other features include: an AC present signal with precision 3%-accurate user adjustable trip points; a safety
signal circuit that rejects false thermistor tripping due
to ground bounce caused by the sudden presence of
high charge currents, and an ultrafast overvoltage
comparator circuit that prevents voltage overshoots
when the battery is suddenly removed or disconnects
itself during charge. Last but not least is an input current
limit sensing circuit that limits charge current to prevent
wall adapter overload as the system power increases**.
12.0
BAT1
VOLTAGE
11.0
3500
3000
2500
2000
1500
1000
500
0
3500
3000
2500
2000
1500
1000
500
0
BAT1
CURRENT
BAT2
CURRENT
SEQUENTIAL
BAT1
CURRENT
BATTERY VOLTAGE (V)
BATTERY CURRENT (mA)
LTC1760 Power Management
Dual battery systems are traditionally used to simply
extend system battery run time by allowing a sequential
Safely managing the charge and discharge states of multiple batteries and the DC input power source presents
a significant power management issue that historically
has involved a host processor running custom written application software. The LTC1760 simplifies this
task by operating in a standalone Level 3 Bus Master
mode. It autonomously controls simultaneous battery
charging and discharging, full dual battery conditioning
support and ideal diode PowerPath switching between
two batteries and a wall adapter without requiring any
host processor.
BAT2
VOLTAGE
10.0
9.0
BAT2
VOLTAGE
8.0
11.0
BAT1
VOLTAGE
9.0
DUAL
8.0
0
100
MINUTES
0
50
100
150
200
TIME (MINUTES)
250
300
BATTERY TYPE: 10.8V Li-Ion (MOLTECH NI2020)
REQUESTED CURRENT = 3A
REQUESTED VOLTAGE = 12.3V
MAX CHARGER CURRENT = 4.1A
DN342 F02
SEQUENTIAL
12.0
10.0
BAT2
CURRENT
DUAL
20
40
11
MINUTES
60 80 100 120 140 160 180
TIME (MINUTES)
BATTERY TYPE: 10.8V Li-Ion (MOLTECH NI2020)
LOAD CURRENT = 3A
DN342 F03
Figure 3. Dual Battery vs Sequential Battery
Discharge Time
Figure 2. Dual Battery vs Sequential Battery Charge Time
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn342f_conv LT/TP 1004 344K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004