Reduce Component Count and Cut Costs by Disabling the

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Keywords: source-current loop, input-current loop, Li+ battery charger
APPLICATION NOTE 1817
Reduce Component Count and Cut Costs by Disabling
the MAX1737 Source-Current Loop
Dec 16, 2002
Abstract: The MAX1737 source-current loop protects low-power sources from being overloaded, however, this
source-current loop can be disabled when using high-power sources. Disabling the source-current loop reduces
component count and cuts system cost.
The MAX1737 switch-mode lithium-ion (Li+) battery charger includes a source-current loop that regulates the
total current drawn from the power source. Although this control loop is useful in many applications, it is
unnecessary if the power source can provide the maximum required system current plus the maximum required
charger current. Disabling the source-current loop reduces component count and cuts cost.
Figure 1 illustrates the standard implementation of the MAX1737. This typical application of the MAX1737 uses
the components found within the MAX1737EVKIT. In this application, power is derived from a 12V, 1A source,
and the source-current loop ensures that no more than 1A is drawn.
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Figure 1. To limit the power drawn from the source, the standard implementation of the MAX1737 decreases the
battery charge current as the system load increases.
The circuit of Figure 2 is similar to the circuit of Figure 1, but with the source-current loop disabled. Disabling the
source-current loop eliminates seven components. To disable the source-current loop, connect ISETIN to REF
and connect CSSP and CSSN to DCIN. The compensation capacitor at CCS is still recommended.
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Figure 2. Disabling the source-current loop of the MAX1737 reduces cost and size by eliminating seven
components. With the source-current loop disabled, the power source must be able to supply the maximum
battery charging current and the maximum system load current.
Table 1 compares the characteristics of Figure 1 and Figure 2 at different system loads. Download Excel file of
Table 1.
Table 1. Figure 1 and Figure 2 Characteristics
Current Source Energy
Current System Energy
Current Battery Energy Efficiency
Delivered Voltage Delivered Consumed Load Delivered Consumed Voltage Delivered
(%)
by the
(V)
by the
by the Voltage to the
by the
(V)
to the
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Source
(A)
Figure
1
Figure
2
Source
(W)
System
Load (A)
(V)
System Battery (A)
Load (W)
Battery
(W)
0.57
12
6.8
0.0
11.68
0.0
1.01
6.0
6.06
88.6
0.75
12
9.0
0.2
11.65
2.3
1.01
6.0
6.06
93.2
0.95
12
11.4
0.4
11.64
4.7
1.01
6.0
6.06
94.0
1.13
12
13.6
0.6
11.62
7.0
0.99
6.0
5.94
95.2
1.13
12
13.6
0.8
11.61
9.3
0.62
6.0
3.72
95.9
1.08
12
13.0
1.0
11.6
11.6
0.15
6.0
0.90
96.5
0.57
12
6.8
0
11.7
0.0
1.01
6.0
6.06
88.6
0.74
12
8.9
0.2
11.7
2.3
1.01
6.0
6.06
94.6
0.95
12
11.4
0.4
11.68
4.7
1.01
6.0
6.06
94.1
1.14
12
13.7
0.6
11.67
7.0
1.01
6.0
6.06
95.5
1.34
12
16.1
0.8
11.67
9.3
1.01
6.0
6.06
95.7
1.54
12
18.5
1
11.65
11.7
1.01
6.0
6.06
95.8
Thus, if the power source can provide the maximum system current plus the maximum charger current, disabling
the MAX1737 source-current loop provides cost and size benefits by eliminating seven components.
Related Parts
MAX1737
Stand-Alone Switch-Mode Lithium-Ion Battery-Charger
Controller
Free Samples More Information
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