LTC4006 datasheet(13/16 Pages) LINER

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LTC4006

sn4006 4006is

APPLICATIO S I FOR ATIO

Figure 9. Adapter Current Limiting

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 10. 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)

If you are using a thermistor that doesn’t have a 1:7 HIGH/

LOW ratio, or you wish to set the HIGH/LOW limits to

different temperatures, then the more generic network in

Figure 11 should work.

Once the thermistor, RTH, has been selected and the

thermistor value is known at the temperature limits, then

resistors R9 and R9A are given by:

For NTC thermistors:

R9 = 6 RTH(LOW) • RTH(HIGH)/(RTH(LOW) – RTH(HIGH))

R9A = 6 RTH(LOW) • RTH(HIGH)/(RTH(LOW) – 7 • RTH(HIGH))

For PTC thermistors:

R9 = 6 RTH(LOW) • RTH(HIGH)/(RTH(HIGH) – RTH(LOW))

R9A = 6 RTH(LOW)•RTH(HIGH)/(RTH(HIGH)–7• RTH(LOW))

100mV

–

CLP

LTC4006

CLN

4006 F09

15nF

RCL*

CIN

VIN

CL1

AC ADAPTER

INPUT

*RCL =

100mV

ADAPTER CURRENT LIMIT

Example:

VDCIN = 19V, fOSC = 345kHz, QG1 = QG2 = 15nC.

PD = 292mW

Adapter Limiting

An important feature of the LTC4006 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 15nF 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

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%)

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