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LTC1760

sn1760 1760is

Setting Input Current Limit

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

minimum wall adapter current rating. Subtract 5% for the

input current limit tolerance and use that current to deter-

mine the resistor value.

RCL = 100mV/ILIM

ILIM = Adapter Min Current

– (Adapter Min Current • 5%)

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 1).

Table 1. 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

0.030

0.27

0.5

*Values shown above are rounded to nearest standard value.

Extending System to More than 2 Batteries

The LTC1760 can be extended to manage systems with

more than 3 sources of power. Contact Linear Technology

Applications Engineering for more information.

Charge Termination Issues

Batteries with constant-current charging and voltage-

based charger termination might experience problems

From here we can simplify.

IBAT1/CBAT1 = dV/dt = IBAT2/CBAT2

IBAT2 = IBAT1 CBAT2/CBAT1

At this point you can see that the current divides as the ratio

of the two batteries capacity ratings. The sum of the

current into both batteries is the same as the current being

supply by the charger. This is independent of the mode of

the charger (CC or CV).

ICHRG = IBAT1 + IBAT2

From here we solve for the actual current for each battery.

IBAT2 = ICHRG CBAT2/(CBAT1 + CBAT2)

IBAT1 = ICHRG CBAT1/(CBAT1 + CBAT2)

Please note that the actual observed current sharing will

vary from manufactures claimed capacity ratings since it

is actual physical capacity rating at the time of charge.

Capacity rating will change with age and use and hence the

current sharing ratios can change over time.

Adapter Limiting

An important feature of the LTC1760 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 DCIN pins. When this voltage ex-

ceeds 100mV, the amplifier will override programmed

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

lowpass filter formed by 5k

Ω and 0.1µF is required to

eliminate switching noise. If the current limit is not used,

CLP should be connected to DCIN.

Figure 9.

100mV

–

Ω

CLP

DCIN

1760 F09

0.1

µF

RCL*

CIN

VIN

CL1

AC ADAPTER

INPUT

*RCL =

100mV

ADAPTER CURRENT LIMIT

APPLICATIO S I FOR ATIO

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