LTC3855_15 Datasheet, PDF (19/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

LTC3855
Applications Information
R_ITEMP100C
=
V_ITEMP100C
10µA
V_ITEMP100C = 0.5V − 1.3•
I_MAX •DCR(MAX)•R2 _• (100°C − 25°C)• 0.4
R1+ R2
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, 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 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 above
equations and checked at 25°C and 100°C.
After determining the components for the temperature
compensation network, check the results by plotting
I_MAX versus inductor temperature using the following
equations:
I_MAX =
V_{SENSEMAX(ADJ)} − ∆V_SENSE
2
( ) ^DCR(MAX)
at
25°C
•


1+
T_L(MAX) − 25°C
•
0.4
100


where
V_{SENSEMAX(ADJ)}
=
V_SENSE(MAX)
•
1.8V
− V_ITEMP
1.3
−
A
V_ITEMP = 10µA • (R_S + R_P|| R_NTC)
Use typical values for V_SENSE(MAX). Subtracting constant
A will provide a minimum value for V_SENSE(MAX). These
values are summarized in Table 2.
Table 2.
I_LIM
V_SENSE(MAX) TYP
A
GND
30mV
5mV
FLOAT
50mV
5mV
INTV_CC
75mV
7mV
The resulting current limit should be greater than or
equal to I_MAX for inductor temperatures between 25°C
and 100°C.
Typical values for the NTC compensation network are:
• NTC R_O = 100k, B-constant = 3000 to 4000
• R_S ≈ 20k
• R_P ≈ 50k
Generating the I_MAX versus inductor temperature curve plot
first using the above values as a starting point and then
adjusting the R_S and R_P values as necessary is another
approach. Figure 6 shows a typical curve of I_MAX versus
inductor temperature. For PolyPhase applications, tie the
ITEMP pins together and calculate for an ITEMP pin cur-
rent of 10µA • #phases.
The same thermistor network can be used to correct for
temperatures less than 25°C. But make sure V_ITEMP is
3855f
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