參數(shù)資料
型號： LTC3856IFE#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PDSO38
封裝： 4.4 MM, LEAD FREE, PLASTIC, TSSOP-38
文件頁數(shù)： 11/40頁
文件大小： 2222K
代理商： LTC3856IFE#PBF
LTC3856
3856f
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 RS and RP 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
10A 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
V
A
V
ITEMP
C
ITEMP
C
ITEMP
C
100
10
0 5
1
=
.
.33
2
1
2
100
25
0 4
100
+
° °
I
DCR
R
R R
C
V
MAX
(
)
.
SSENSE MAX
(
)
Calculate the values for RP and RS. A simple method is to
graph the following RS versus RP equations with RS on
the y-axis and RP on the x-axis.
RS = RITEMP25C – RNTC25C||RP
RS = RITEMP100C – RNTC100C||RP
Next, find the value of RP that satisfies both equations,
which will be the point where the curves intersect. Once
RP is known, solve for RS.
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
O
=
+
+
exp
1
273
1
273
Where
R = resistance at temperature T, in degrees C
RO = resistance at temperature TO, typically 25°C
B = B-constant of the thermistor.
Figure5showsatypicalresistancecurvefora100ktherm-
istor and the ITEMP pin network over temperature.
Starting values for the NTC compensation network are:
NTC RO = 100k
RS = 20k
RP = 50k
But, the final values should be calculated using the previ-
ous equations and checked at 25°C and 100°C.
Figure 5. Resistance vs Temperature for the
ITEMP Pin Network and the 100k NTC
INDUCTOR TEMPERATURE (°C)
–40
RESIST
ANCE
(k)
1000
100
10
0
40
–20
20
60
80
120
0
10000
3856 F05
RITMP:
RS = 20k
RP = 43.2k
100k NTC
THERMISTOR RESISTANCE:
RO = 100k
TO = 25°C
B = 4334 for 25°C/100°C
相關PDF資料
PDF描述
LTC3856EFE#PBF SWITCHING CONTROLLER, PDSO38
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LTC3856EFE#TRPBF SWITCHING CONTROLLER, PDSO38
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