Datasheet
R
E
+
L
DCR
C
R
THE1
+
R
THE
(T1)
R
THE
(25)
R
THE2
+
R
THE
(T2)
R
THE
(25)
R
T
+
K
DIV
(T)
1 * K
DIV
(T)
R K
DIV
(T) +
K
DIV
(25)
1 ) 0.0039 (t * 25)
R1
R
+
ǒ
R
NTC1
* R
NTC2
Ǔ
R
E1
R
E2
* R
NTC1
R
E2
ǒ
1 * R
NTC2
Ǔ
) R
NTC2
R
E1
ǒ
1 * R
NTC1
Ǔ
R
NTC1
R
E1
ǒ
1 * R
NTC2
Ǔ
* R
NTC2
R
E2
ǒ
1 * R
NTC1
Ǔ
*
ǒ
R
NTC1
* R
NTC2
Ǔ
R2
R
+
ǒ
1 * R
NTC1
Ǔ
ƪ
1
1 * R1
R
*
R
NTC1
R
E1
* R1
R
ƫ
*1
RNTC
R
+
ƪ
ǒ
1 * R1
R
Ǔ
*1
*
ǒ
R2
R
Ǔ
*1
ƫ
*1
TPS40090
TPS40091
SLUS578B –OCTOBER 2003– REVISED MAY 2006
www.ti.com
The following algorithm and expressions help to determine components of the network.
1. Calculate the equivalent impedance of the network at 25°C that matches the inductor parameters in
Equation 18. Use of COG type capacitors for this application is recommended. For example, for L = 0.4 μH,
DCR = 1.22 mΩ, C = 10 nF; R
E
= 33.3 kΩ. It is recommended to keep R
E
< 50 kΩ as higher values may
produce false triggering of the current sense fault protection.
(18)
2. It is necessary to set the network attenuation value K
DIV
(25) at 25°C. For example, K
DIV
(25) = 0.85. The
attenuation values K
DIV
(25) > 0.9 produces higher values for NTC resistors that are harder to get from
suppliers. Attenuation values lower 0.7 substantially reduce the network output signal.
3. Based on calculated R
E
and K
DIV
(25) values, calculate and pick the closest standard value for the resistor R
= R
E
/K
DIV
(25). For the given example R = 33 kΩ/ 0.85 = 38.8 kΩ. The closest standard value from 1% line is
R = 39.2 kΩ.
4. Pick two temperature values at which curve fitting is made. For example T1 = 50°C and T2 = 90°C.
5. Find the relative values of R
THE
required on each of these temperatures.
(19)
(20)
For the given example R
THE1
= 0.606, R
THE2
=0.372.
6. From the NTC resistor datasheet get the relative resistance for resistors with desired curve. For the given
example and curve 17 for NTHS NTC resistors from Vishay R
NTC1
= 0.3507 and R
NTC2
= 0.08652.
7. Calculate relative values for network resistors including the NTC resistor.
(21)
(22)
(23)
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