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LTC4013

4013fa

Charge Voltage Temperature Compensation

The LTC4013 can use a thermistor to adjust the battery

charge voltage as a function of temperature. The change

in FB-referred charge voltage from its nominal level is

given by ∆VFB = (VNTC - VINTVCC/2) • 0.21. Voltages

above 3.3V disable the NTC feature. The transfer function,

when combined with a thermistor, is stronger than neces-

sary. It is configured this way so that any thermistor can

be diluted with a low drift resistor to achieve the desired

temperature coefficient. To avoid the 3.3V NTC disable

threshold, the LTC4013 requires that the dilution resis-

tor be placed in parallel with the thermistor rather than

in series. The bias resistor from INTVCC to NTC should

be equal to the parallel combination of the thermistor at

25°C and the dilution resistor.

Figure 5 shows the NTC circuit configuration and Table 3

showssomecommontemperaturecoefficientandassoci-

ated resistor settings using a 10k, ß = 3380K thermistor.

OPERATION

Since the thermistor is used to adjust charge voltage vs

battery temperature, it is ideally placed in thermal contact

with the battery. This is not always practical so the next

best thing is to position the resistor to sense the battery's

ambient temperature.

Charge Termination Timer

The LTC4013 supports timer-based functions wherein

batterychargecyclecontroloccursafteraspecificamount

of time. Timer termination is engaged when a capacitor

(CTMR) is connected from the TMR pin to ground. For a

desired end-of-cycle time (tEOC)CTMRfollowstherelation:

CTMR (µF) = tEOC (Hr) • 0.066

Figure 5. NTC Configuration for Temperature Compensation

Figure 6. Diluted Thermistor Voltage vs Temperature

Figure 7. FB-Referred Charge Voltage vs Temperature

RN1

RNTC

RN2

4013 F05

LTC4013

NTC

INTVCC

SGND

Figure 6 shows the diluted thermistor voltage vs tempera-

ture and Figure 7 shows the resultant charge voltage vs

temperature.

−40 −20

80 100 120

TEMPERATURE (°C)

4013 F06

−5mV/°C

−2.5mV/°C

−40 −20

80 100 120

TEMPERATURE (°C)

1.6

1.8

2.0

2.2

2.4

2.6

4013 F07

FLOAT −5mV/°C

FLOAT −2.5mV/°C

ABSORB −5mV/°C

ABSORB −2.5mV/°C

Table 3. Charge Voltage TC Examples Using ß = 3380 RNTC

TC (WRT VFB)

RN1

RN2

–2.5mV/°C

2.49k

3.32k

–5.0mV/°C

4.99k

10.0k

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