Datasheet

LTC3855
19
3855f
R
V
µA
ITEMP C
ITEMP C
100
100
10
=
V V
I DCR MAX R
R R
ITEMP C
MAX
100
0 5 1 3
2
1 2
= − •
• •
+
•
. .
( )
(
1100 25 0 4
100
° − ° •C C
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 B
T T
O
O
= • •
+
−
+












exp
1
273
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
V V
DCR MAX
MAX
SENSEMAX ADJ SENSE
=
−
°
( )
( )
∆
2
at 25 CC T C
L MAX
• + − °
( )
•






1 25
0 4
100
( )
.
where
V V
V V
A
SENSEMAX ADJ SENSE MAX
ITEMP
( ) ( )
.
.
= •
−
−
1 8
1 3
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
GND FLOAT INTV
CC
V
SENSE(MAX)
TYP 30mV 50mV 75mV
A 5mV 5mV 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
applicaTions inForMaTion