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LTC3875

3875fb

For more information www.linear.com/LTC3875

APPLICATIONS INFORMATION

1. SettheTCOMP/ITEMPpinresistanceto23.33k at25°C.

With 30µA flowing out of the TCOMP/ITEMP pin, the

voltage on the TCOMP/ITEMP pin will be 0.7V at room

temperature. Current limit correction will occur for

inductor temperatures greater than 25°C.

2. Calculate the TCOMP/ITEMP pin resistance and the

maximuminductortemperaturewhichistypically100°C.

Use the following equations:

VITEMP100C =

0.7 –1.5

IMAX •DCR (Max)•

100°C– 25°C

(

)•0.4

100

VSENSE(MAX)













= 0.25V

Since VSENSE(MAX) = IMAX • DCR (Max):

RITEMP100C =

VITEMP100C

30µA

= 8.33k

where:

RITEMP100C = TCOMP/ITEMP pin resistance at 100°C.

VITEMP100C = TCOMP/ITEMP pin voltage at 100°C.

VSENSE(MAX) = Maximum current sense threshold at

room temperature.

IMAX = Maximum inductor peak current.

DCR (Max) = Maximum DCR value.

Calculate the values for the NTC network’s parallel and

series resistors, RP and RS. A simple method is to graph

the following RSversusRPequationswithRSonthey-axis

and RP on the x-axis.

RS = RITEMP25C – RNTC25C||RP

RS = RITEMP100C – RNTC100C||RP

Next, find the value of RP that satisfies both equations

which will be the point where the curves intersect. Once

RP is known, solve for RS.

The resistance of the NTC thermistor can be obtained

from the vendor’s data sheet either in the form of graphs,

tabulated data, or formulas. The approximate value for the

NTC thermistor for a given temperature can be calculated

from the following equation:

R =RO •exp B•

T +273

–

TO +273

















where:

R = Resistance at temperature T, which is in degrees C.

RO = Resistance at temperature TO, typically 25°C.

B = B-constant of the thermistor.

Figure5showsatypicalresistancecurvefora100kthermis-

tor and the TCOMP/ITEMP pin network over temperature.

Starting values for the NTC compensation network are:

• NTC RO = 100k

• RS = 3.92k

• RP = 24.3k

But, the final values should be calculated using the above

equations and checked at 25°C and 100°C. After determin-

ing the components for the temperature compensation

network,checktheresultsbyplottingIMAXversusinductor

temperature using the following equations:

IDC(MAX) =

VSENSEMAX(ADJ) –

∆VSENSE

DCR(MAX) at 25°C• 1+ TL(MAX) –25°C

(

)• 0.4

100









where:

10000

1000

100

INDUCTOR TEMPERATURE (°C)

–40

–20

3875 F05

120

100

RITMP

RS = 20k

RP = 43.2k

100k NTC

THERMISTOR RESISTANCE

RO = 100k, TO = 25°C

B = 4334 for 25°C/100°C

Figure 5. Resistance Versus Temperature for

ITEMP Pin Network and the 100k NTC

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