LTC3856_15 Datasheet, PDF (19/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
NTC Compensated DCR Sensing
For DCR sensing applications where a more accurate
current limit is required, a network consisting of an NTC
thermistor placed from the ITEMP pin to ground will
provide correction of the current limit over temperature.
Figure 2b shows this network. Resistors R_S and R_P will
linearize the impedance the ITEMP pin sees. To implement
NTC compensated DCR sensing, design the DCR sense
filter network per the same procedure mentioned in the
previous selection, except calculate the divider compo-
nents using the room temperature value of the DCR. For
a typical application:
1. Set the ITEMP pin resistance to 50k at 25°C. With
10µA flowing out of the ITEMP pin, the voltage on the
ITEMP pin will be 0.5V at room temperature. Current
limit correction will occur for inductor temperatures
greater than 25°C.
2. Calculate the ITEMP pin resistance and the maximum
inductor temperature, which is typically 100°C. Use the
following equations:
R_ITEMP100C
=
V_ITEMP100C
10µA
V_ITEMP100C = 0.5V − 1.3 •
I_MAX
•
DCR_MAX
•
R2
R1+ R2
•
(100°C
−
25°C) •
0.4
100
V_SENSE(MAX)
Calculate the values for 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 •



T
1
+ 273
−
T_O
1^
+ 273_



Where
R = resistance at temperature T, 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 therm-
istor and the ITEMP pin network over temperature.
Starting values for the NTC compensation network are:
• NTC R_O = 100k
• R_S = 20k
• R_P = 50k
But, the final values should be calculated using the previ-
ous equations and checked at 25°C and 100°C.
10000
1000
THERMISTOR RESISTANCE:
R_O = 100k
T_O = 25°C
B = 4334 for 25°C/100°C
100
R_ITMP:
R_S = 20k
10
R_P = 43.2k
100k NTC
0
–40 –20 0 20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
3856 F05
Figure 5. Resistance vs Temperature for the
ITEMP Pin Network and the 100k NTC
3856f
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