Datasheet
23
LTC1759
APPLICATIONS INFORMATION
WUU
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Charge Termination Issues
Batteries with constant current charging and voltage-
based charger termination might experience problems
with reductions of charger current caused by adapter
limiting. It is recommended that input limiting feature be
defeated in such cases. Consult the battery manufacturer
for information on how your battery terminates charging.
Setting Output Current Limit (Refer to Figure 10)
The LTC1759 current DAC and the PWM analog circuitry
must coordinate the setting of the charger current. Failure
to do so will result in incorrect charge currents.
Table 8. Recommended Resistor Values
I
MAX
R
SENSE
R
SENSE
R
S1
= R
S2
R
SET
R
ILIMIT
(A) (Ω) 1% (W) (Ω) 1% (Ω) 1% (Ω) 1%
1.023 0.100 0.25 200 3.83k 0
2.046 0.05 0.25 200 3.83k 10k
4.092 0.025 0.5 200 3.83k 33k
8.184 0.012 1 200 4.02k Open
Warning
DO NOT CHANGE THE VALUE OF R
ILIMIT
DURING OPERA-
TION. The value must remain fixed and track the R
SENSE
and R
SET
values at all times. Changing the current setting
can result in currents that greatly exceed the requested
value and potentially damage the battery or overload the
wall adapter if no input current limiting is provided.
Example Calculations
Setting up the output current to the desired value involves
calculating these values:
1. R
SENSE
: This resistor is the current sense resistor on the
charger output.
2. R
SET
: This resistor sets the current DAC output pro-
gramming current scale.
3. R
ILIMIT
: This resistor programs the full-scale value of
the current DAC (I
MAX
).
The value of R
SENSE
and R
SET
are directly related to each
other based on the values chosen for R
S1
and R
S2
. To
prevent current sense op amp input bias errors, the value
of R
S1
and R
S2
are kept the same, about 200Ω. R
SET
is
used to scale the PROG pin current relative to the R
SENSE
voltage drop to set the maximum current value.
The following example is for a 4A design.
UV
V
DD
SYNC
SDB
CHGEN
V
LIMIT
I
LIMIT
DGND
I
SET
PROG
V
C
COMP1
AGND
RNR
THERM
SDA
SCL
INTB
22
21
8
32
9
10
2
33
34
1
3
35
30
29
31
23
26
36
R
SET
, 3.83k
R4, 1.5k
LTC1759
R7, 1k
RUR
1k
R3
499Ω
R
WEAK
475k
RNR
10k
C9, 0.1µF
C13, 0.33µF
V
DD
V
DD
C11, 1µF
C12, 0.68µF
R
VLIMIT
, 33k
R
CL
, 0.033Ω
R
S1
, 200Ω
R6
68Ω
R
S2
, 200Ω
R1
15.8k
R2
1k
C2
0.47µF
C5, 2.2µF
C6
0.68µF
C8
0.047µF
C4, 0.1µF
C1
1µF
Q1
Q3
D1
Q2
L1
15µH
C16
22µF
SYSTEM
POWER
R
SENSE
0.025Ω
D2 D2
R
ILIMIT
, 33k
7
16
4
5
12
25
24
18
17
28
27
11
6
20
19
14
15
13
DCIN
DCDIV
INFET
V
CC
CLP
CLN
TGATE
BOOSTC
GBIAS
BOOST
SW
BGATE
SPIN
SENSE
BAT1
BAT2
V
SET
PGND
C7
0.015µF
C3
22µF
C14
0.1µF
C15
10µF
35V
Al
AC
ADAPTER
INPUT
SMART
BATTERY
INTB
SMBus
TO
HOST
D1: MBRS130LT3
D2: FMMD7000
L1: SUMIDA CDRH127-150
Q1: Si3457DV
Q2, Q3: Si3456DV
SCL
SDA
1759 F10
+
+
Figure 10. 4A SMBus Smart Battery Charger