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LTC4100

sn4100 4100is

Adapter Limiting

An important feature of the LTC4100 is the ability to

automatically adjust charging current to a level which

avoids overloading the wall adapter. This allows the prod-

uct to operate at the same time that batteries are being

charged without complex load management algorithms.

Additionally, 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

between 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 0.1

µF is required to eliminate

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

be connected to CLN.

APPLICATIO S I FOR ATIO

input current limit tolerance and use that current to

determine the resistor value.

RCL = 100mV/ILIM

ILIM = Adapter Min Current –

(Adapter Min Current • 7%)

Table 8. Common RCL Resistor Values

ADAPTER

RCL VALUE*

RCL POWER

RATING (A)

(

Ω) 1%

DISSIPATION (W)

RATING (W)

1.5

0.06

0.135

0.25

1.8

0.05

0.162

0.25

0.045

0.18

0.25

2.3

0.039

0.206

0.25

2.5

0.036

0.225

0.5

2.7

0.033

0.241

0.5

0.03

0.27

0.5

* Values shown above are rounded to nearest standard value.

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 (see Table 8).

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 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.

Figure 9. Adapter Current Limiting

100mV

–

4.99k

CLP

CLN

INFET

4100 F09

LTC4100

0.1

µF

RCL*

VIN

CL1

*RCL =

100mV

ADAPTER CURRENT LIMIT

TO LOAD

Setting Input Current Limit

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

minimum wall adapter current rating. Subtract 7% for the

OPERATIO

The AC Present Block (AC_PRESENT)

The DCDIV pin is used to determine AC presence. If the

DCDIV voltage is above the DCDIV comparator threshold

(VACP), then the ACP output pin will be switched to VDD and

the AC_PRESENT bit in the ChargerStatus() function will

be set. If the DCDIV voltage is below the DCDIV compara-

tor threshold minus the DCDIV comparator hysteresis,

then the ACP output pin is switched to GND and the

AC_PRESENT bit in the ChargerStatus() function is cleared.

The ACP output pin is designed to drive 2mA continuously.

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