IR3725MPBF International Rectifier, IR3725MPBF Datasheet - Page 11

• Manufacturer Part Number: IR3725MPBF
• Description: Input Power Monitor IC with Digital Interface
• Manufacturer: International Rectifier

Specifications of IR3725MPBF

• Package: 12 Lead DFN
• Bias Supply Voltage: +3.3V +/-5%
• Junction Temperature: 0oC to 125oC
• Pbf: Yes

THERMAL COMPENSATION FOR INDUCTOR DCR CURRENT
SENSING
The positive temperature coefficient of the inductor
DCR can be compensated if R
proportional to the DCR. DCR of a copper coil, as a
function of temperature, is approximated by
T
TCR
copper, usually assumed to be 0.39 %/°C near room
temperature. Note that equation 1 is linearly
increasing with temperature and has an offset of
DCR(T
If R
thermistor then temperature effects of DCR can be
minimized. Consider a circuit of two resistors and a
thermistor as shown in the R
Figure 3 R
If Rth is an NTC thermistor then the resistance of the
network will decrease as temperature increases.
Unfortunately, most thermistors exhibit far more
variation with temperature than copper wire. One
equation used to model thermistors is
Page 11 of 19
Rp
R
is some reference temperature, usually 25 °C, and
T
DCR
Cu
incorporates a negative temperature coefficient
Rs
R
is the resistive temperature coefficient of
) at the reference temperature.
(
T
)
T

Network
DCR
Rth
(
T
R
)

1 (

(
T
T

network below.
T
T
varies inversely
R
)

TCR
Cu
)
www.irf.com
.
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where R
temperature T, R
the reference temperature T
constant provided by the thermistor manufacturer.
Kelvin degrees are used in the exponential term of
equation 2. If R
curvature of the equivalent network resistance can be
reduced from the curvature of the thermistor alone.
Although the exponential equation 2 can never
compensate linear equation 1 at all temperatures, a
spreadsheet can be constructed to minimize error
over the temperature interval of interest. The
resistance R
temperature is
using R
Equation 4 may be written as a function of
temperature using equations 1 and 3 as follows:
With Rs and Rp as additional free variables, use a
spreadsheet to solve equation 4 for the desired full
scale current while minimizing the I
over temperature.
R
th
th
I
th
FS
(
(T) from equation 2.
T
(T) is the thermistor resistance at some
R

)
T
T

T


T
of the network shown as a function of
R


S
th

R
is large and R
(
th
T
V
(T
R
T
0
IG

)
s
T
0

) is the thermistor resistance at


e





R


1

R




p
T
1
CS
DCR


0
T
1
1
, and β is the material
1
R
0





P
th




1
R

is small, the

T
T
CS


FS
2
(T) variation

.
IR3725
Data Sheet
2011_11_16
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