LTC4156IUFD View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC4156IUFD

Dual-Input Power Manager/3.5A LiFePO4 Battery Charger with I2C Control and USB OTG

Linear Technology 

LTC4156

OPERATION

thermistor current, and its associated battery drain by a

factor of 2000 from its DC value. A typical network using

a 10k thermistor causes 115nA of battery drain. A 100k

thermistor would reduce this drain to 11.5nA.

To improve measurement resolution over the temperature

range of interest, the full-scale range of the analog-to-

digital converter is restricted to the range 0.113 to 0.895

NTCBIAS. The NTC ADC result can be interpreted as follows:

αT

≡

rT

r25

=

kSPAN • NTCVAL + kOFFSET

1− kSPAN • NTCVAL – kOFFSET

where NTCVAL is the decimal representation of the

NTCVAL[6:0] status report in the range [0-127], ADC con-

stant kSPAN = 0.006162, ADC constant kOFFSET = 0.1127,

rT is the resistance of the thermistor at temperature T,

and αT is the resistance ratio of the thermistor at the two

temperatures T and 25°C.

Thermistor manufacturer data sheets will either provide a

temperature lookup table relating aT to T, or will supply a

curve fit parameter b which can be used with the follow-

ing equations to determine the thermistor temperature:

T

=

b

ln (αΤ )

+

b

T0

T=

b

ln





kSPAN • NTCVAL + kOFFSET

1− kSPAN • NTCVAL – kOFFSET





+

b

T0

where:

T = Temperature result expressed in Kelvin

T0 = Thermistor model nominal temperature, expressed

in Kelvin. Typically 298.15K (25°C + 273.15°C)

b = Thermistor model material constant, expressed in

Kelvin.

In addition to thermistor value reporting, the LTC4156

automatically pauses battery charging if the thermistor

reading falls outside of limits corresponding to the

range 0°C to 60°C for a Vishay curve 2 thermistor.

The NTC_TOO_COLD and NTC_HOT_FAULT conditions

are encoded in the I2C status report NTCSTAT[1:0].

CHARGER_STATUS[2:0] will also report temperature

warnings and faults when the battery charger is enabled.

See Table 13 and Table 16. Optionally, a charger status

interrupt request may be generated when the thermistor

reading enters or exits this temperature range. If the

temperature reading is above a limit corresponding to

60°C for a Vishay curve 2 thermistor, an optional NTC_

HOT_FAULT interrupt may also be generated.

The NTC_TOO_COLD temperature indication is triggered

when NTCVAL rises to decimal result 102. This corre-

sponds to a aCOLD,WARNING = 2.86 and 0°C for a Vishay

curve 2 thermistor. The low temperature indication is

cleared when the NTCVAL falls to decimal result 98. This

corresponds to aCOLD,RESET = 2.53 and 2°C for a Vishay

curve 2 thermistor.

The NTC_HOT_FAULT temperature indication is triggered

when NTCVAL falls to decimal result 19. This corre-

sponds to aCRITICAL,FAULT = 0.298 and 60°C for a Vishay

curve 2 thermistor. The critically hot temperature indica-

tion is cleared when NTCVAL rises to decimal result 23.

This corresponds to aCRITICAL,RESET = 0.341 and 55.5°C

for a Vishay curve 2 thermistor.

It is possible to modify the thermistor bias network to adjust

either one or both of the above temperature thresholds. See

Alternate NTC Thermistors and Biasing in the Applications

Information section for implementation details.

4156f

33

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