LTC3875_15 Datasheet, PDF (20/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

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
V_{SENSE(MAX )}
– 25°C)
100
•
0.4




= 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.
from the following equation:
R
=
R_O
•

exp_
B
•


T
1
+ 273
–
T_O
1
+ 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_{SENSEM AX( ADJ)}
–
∆V_SENSE
2
( ) ^DCR(MAX)
at
25°C
•


1+
T_L(MAX) – 25°C
•
0.4
100


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
where: ₁₀₀₀₀
1000
THERMISTOR RESISTANCE
R_O = 100k, T_O = 25°C
B = 4334 for 25°C/100°C
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
100
RITMP
R_S = 20kΩ
10 RP = 43.2kΩ
100k NTC
1
–40 –20 0 20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
3875 F05
Figure 5. Resistance Versus Temperature for
I_TEMP Pin Network and the 100k NTC
3875fa
20
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