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