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LTC4008

4008fa

APPLICATIO S I FOR ATIO

control is used, with closed-loop feedback ensuring that

adapter load current remains within limits. Amplifier CL1

in Figure 7 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 5k

Ω and 15nF is required to eliminate

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

should be connected to CLP.

Note that the ICL pin will be asserted when the voltage

across RCL is 93mV, before the adapter limit regulation

threshold.

Table 5. 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 5).

Designing the Thermistor Network

There are several networks that will yield the desired

function of voltage vs temperature needed for proper

operation of the thermistor. The simplest of these is the

voltage divider shown in Figure 8. Unfortunately, since the

HIGH/LOW comparator thresholds are fixed internally,

there is only one thermistor type that can be used in this

network; the thermistor must have a HIGH/LOW resis-

tance ratio of 1:7. If this happy circumstance is true for

you, then simply set R9 = RTH(LOW)

–

CLP

15nF

RCL*

VIN

LTC4008

CLN

100mV

4008 F07

CIN

SYSTEM

LOAD

CL1

*RCL =

100mV

ADAPTER CURRENT LIMIT

Figure 7. Adapter Current Limiting

Setting Input Current Limit

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

minimum wall adapter current rating. Subtract 7% 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 • 7%)

LTC4008

NTC

RTH

4008 F08

Figure 8. Voltage Divider Thermistor Network

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