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

APPLICATIONS INFORMATION

where:

e = Natural logarithm base, approximately 2.71828

T = Temperature of interest, expressed in Kelvin

T0 = Thermistor model nominal temperature, expressed

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

b = Model material constant, expressed in Kelvin. This

model is a curve fit at T0 and a second temperature. b is

close to 4000K for most thermistors. Higher b results in

increased temperature sensitivity at the expense of reduced

linearity over a wide temperature range

Simple Alternate Thermistor Bias Network

By simply adjusting the bias resistor, RBIAS, and omitting the

optional RTEMP_RANGE, one of the temperature thresholds

may be adjusted. The other temperature comparator

threshold will be determined by the choice of the first

temperature threshold and the NTCVAL thresholds fixed

in the LTC4156. Increasing RBIAS above r25 shifts both

temperature thresholds colder while simultaneously

slightly compressing the temperature span between

thresholds. Likewise, decreasing RBIAS below r25 shifts

both temperature thresholds warmer while simultaneously

slightly expanding the temperature span between

thresholds. To program a single temperature threshold,

obtain the value of either aTOO_COLD or aHOT_FAULT for

the chosen temperature threshold using one of the afore-

mentioned methods and substitute into the appropriate

following equation to calculate the value aBIAS and then

RBIAS.

aBIAS = 0.34917 • aTOO_COLD

aBIAS = 3.35249 • aHOT_FAULT

RBIAS = aBIAS • r25

With aBIAS for the newly programmed temperature

threshold now determined, the other dependent tempera-

ture threshold may be found by substituting aBIAS into

the remaining equation.

The following equation may be used to convert any other

NTC ADC result (NTCVAL) by substituting the κSPAN and

κOFFSET values found in the Electrical Characteristics table.

αT

=







k

1−

SPAN •

k SPAN

NTCVAL + kOFFSET

• NTCVAL – kOFFSET



 αBIAS



aT may then be used to determine the temperature using

either the lookup table provided by the thermistor manu-

facturer or the curve fit model:

T

=

ln

b

(αT )+

b

T0

Advanced Alternate Thermistor Bias Network

If an adjustment to RBIAS does not yield an acceptable span

between temperature thresholds, a second low drift bias

resistor may be added to the bias network between the

NTC pin and the top of the thermistor. This resistor has

the net effect of diluting the high thermal sensitivity of the

thermistor with low drift resistance. The result is reduced

thermal gain and a wider temperature span between the

preprogrammed voltage thresholds of the LTC4156. Using

this additional resistor, both of the temperature comparator

thresholds may be adjusted. After determining the aT

values for the two temperature thresholds of interest, the

following equations may be used to determine aBIAS and

aTEMP_RANGE.

aTEMP_RANGE = 0.11626 • aTOO_COLD – 1.11626

• aHOT_FAULT

RTEMP_RANGE = aTEMP_RANGE • r25

aBIAS = 0.38976 • (aTOO_COLD – aHOT_FAULT)

RBIAS = aBIAS • r25

It is possible to obtain a negative result for RTEMP_RANGE

which is not physically realizable using the previous equa-

tions. A negative result indicates that the two chosen tem-

perature thresholds are too close in temperature and require

more thermal sensitivity than the thermistor can provide.

4156f

41

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