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APPLICATIONS INFORMATION
1. Set the TCOMP/ITEMP pin resistance to 23.33k at 25°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
maximum inductor temperature which is typically 100°C.
Use the following equations:
V
=
ITEMP100C
I
•DCR (Max)•
MAX
0.7 – 1.5
= 0.25V
Since V
= I
SENSE(MAX)
MAX
V
ITEMP100C
R
=
ITEMP100C
30µA
where:
R
= TCOMP/ITEMP pin resistance at 100°C.
ITEMP100C
V
= TCOMP/ITEMP pin voltage at 100°C.
ITEMP100C
V
= Maximum current sense threshold at
SENSE(MAX)
room temperature.
I
= Maximum inductor peak current.
MAX
DCR (Max) = Maximum DCR value.
Calculate the values for the NTC network's parallel and
series resistors, R
and R
P
the following R
versus R
S
P
and R
on the x-axis.
P
R
= R
– R
S
ITEMP25C
NTC25C
R
= R
– R
S
ITEMP100C
NTC100C
Next, find the value of R
which will be the point where the curves intersect. Once
R
is known, solve for R
P
S
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
(
)
100°C – 25°C
• 0.4
100
V
SENSE(MAX)
• DCR (Max):
= 8.33k
. A simple method is to graph
S
equations with R
on the y-axis
S
||R
P
||R
P
that satisfies both equations
P
.
For more information
from the following equation:
R = R
• exp B •
O
where:
R = Resistance at temperature T, which is in degrees C.
R
= Resistance at temperature T
O
B = B-constant of the thermistor.
Figure 5 shows a typical resistance curve for a 100k thermis-
tor and the TCOMP/ITEMP pin network over temperature.
Starting values for the NTC compensation network are:
• NTC R
= 100k
O
• R
= 3.92k
S
• R
= 24.3k
P
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, check the results by plotting I
temperature using the following equations:
I
=
DC(MAX)
V
SENSEMAX(ADJ)
DCR(MAX) at 25°C • 1+ T
where:
10000
1000
100
RITMP
R
S
R
10
P
100k NTC
1
–40
–20
Figure 5. Resistance Versus Temperature for
I
Pin Network and the 100k NTC
TEMP
www.linear.com/LTC3875
LTC3875
1
1
–
T + 273
T
+ 273
O
, typically 25°C.
O
versus inductor
MAX
∆V
SENSE
–
2
(
– 25°C
L(MAX)
THERMISTOR RESISTANCE
R
= 100k, T
= 25°C
O
O
B = 4334 for 25°C/100°C
= 20k
= 43.2k
0
20
40
60
80
100
120
INDUCTOR TEMPERATURE (°C)
3875 F05
)
0.4
•
100
3875fb
21
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