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Inductor DCR Current Sense
V
DCR
+
ǒ
V
IN
* V
OUT
Ǔ
DCR
DCR ) w L
(23)
V
C
+
ǒ
V
IN
* V
OUT
Ǔ
1
w C
ǒ
R
CS
)
1
w C
Ǔ
(24)
V
C
+
1
w C
ǒ
R
CS
)
1
w C
Ǔ
+
DCR
DCR ) w L
;
L
DCR
+ R
CS
C; t
DCRL
+ t
RC
(25)
V
OUT
V
C
Switch
Node
UDG−03142
L
DCR
C
V
DCR
R
CS
CS
CSRT
C
L
DCR
CSRTCS
R
0.1%
R
0.1%
R
0.1%
R
0.1%
UDG−05079
R
CS
V
DCR
Switch
Node
V
OUT
V
C
Inductor DCR Current Sensing with 5-V Outputs
TPS40130
SLUS602B – JUNE 2004 – REVISED SEPTEMBER 2005
Inductor DCR current sensing is a known lossless technique to retrieve current proportional signal. Referring to
Figure 11 .
At any given frequency the DCR voltage can be calculated using Equation 23 and Equation 24 .
Voltage across the capacitor is equal to voltage drop across the inductor DCR, V
C
=V
DCR
when time constant of
the inductor and the time constant of the RC network are equal, see Equation 25 . Setting the value of the
capacitor to 0.1 µF or 0.01 µF provides for reasonable resistor values.
The output signal generated by the network shown in Figure 11 is temperature dependent due to positive
thermal coefficient of copper specific resistance K
T
=1+0.0039 ×(T-25). The temperature variation of the inductor
coil can easily exceed 100°C in a practical application leading to approximately 40% variation in the output
signal and, in turn, respectively moving the overcurrent threshold and the load line.
Figure 11. Inductor Current Sense Configuration Figure 12. Inductor Current Sense Configuration
for 1.2-V Output for 5-V Output
Due to the current sense operational amplifier input common mode voltage range, it is necessary to divide the
current sense information before connecting to the TPS40130 current sense pins (CS1, CSRT1, CS2 and
CSRT2). Figure 12 shows how this is achieved. R
CS
and C are selected as normal using the method described
in Equation 24 and Equation 25 . The divider resistors (R) are chosen to be much smaller than R
CS.
They are
typically 100 times smaller. However, if these resitors are too small, they may dissipate too much power. In that
case, choose a smaller capacitance value, and select a new R
CS
. The resistors used in the divider should be
precision resistors each having the same value.
20
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