LTC4100_09 datasheet(21/30 Pages) LINER

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LTC4100

4100fb

APPLICATIONS INFORMATION

Adapter Limiting

An important feature of the LTC4100 is the ability to auto-

matically adjust charging current to a level which avoids

overloading the wall adapter. This allows the product to

operate at the same time that batteries are being charged

without complex load management algorithms. Addition-

ally, batteries will automatically be charged at the maximum

possible rate of which the adapter is capable.

This feature is created by sensing total adapter output

current and adjusting charging current downward if a

preset adapter current limit is exceeded. True analog

control is used, with closed loop feedback ensuring that

adapter load current remains within limits. Ampliﬁer CL1 in

Figure 9 senses the voltage across RCL, connected be-

tween the CLP and CLN pins. When this voltage exceeds

100mV, the ampliﬁer will override programmed charging

current to limit adapter current to 100mV/RCL. A lowpass

ﬁlter formed by 4.99k and 0.1μF is required to eliminate

switching noise. If the current limit is not used, CLP should

be connected to CLN.

Setting Input Current Limit

To set the input current limit, you need to know the mini-

mum wall adapter current rating. Subtract 7% for the input

current limit tolerance and use that current to determine

the resistor value.

RCL = 100mV/ILIM

ILIM = Adapter Min Current

– (Adapter Min Current • 7%)

As is often the case, the wall adapter will usually have at

least a +10% current limit margin and many times one

can simply set the adapter current limit value to the actual

adapter rating (Figure 9).

Charge Termination Issues

Batteries with constant current charging and voltage-based

charger termination might experience problems with re-

ductions of charger current caused by adapter limiting. It

is recommended that input limiting feature be defeated in

such cases. Consult the battery manufacturer for informa-

tion on how your battery terminates charging.

Setting Output Current Limit (Refer to Figure 1)

The LTC4100 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. Common RCL Resistor Values

ADAPTER RATING (A) –7% ADAPTER RATING (A)

RCL VALUE* (Ω) 1%

RCL LIMIT (A) RCL POWER DISSIPATION (W) RCL POWER RATING (W)

1.5

1.40

0.068

1.47

0.15

0.25

1.8

1.67

0.062

1.61

0.16

0.25

2.0

1.86

0.051

1.96

0.20

0.25

2.3

2.14

0.047

2.13

0.21

0.25

2.5

2.33

0.043

2.33

0.23

0.50

2.7

2.51

0.039

2.56

0.26

0.50

3.0

2.79

0.036

2.79

0.28

0.50

3.3

3.07

0.033

3.07

0.31

0.50

3.6

3.35

0.030

3.35

0.33

0.50

4.0

3.72

0.027

3.72

0.37

0.50

* Rounded to nearest 5% standard step value. Many non standard values are popular.

100mV

–

4.99k

CLP

CLN

INFET

4100 F09

LTC4100

0.1μF

RCL*

VIN

CL1

*RCL =

100mV

ADAPTER CURRENT LIMIT

TO LOAD

Figure 9. Adaptor Current Limiting

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