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參數(shù)資料
| 型號： | LTC3829IFE#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, PDSO38 |
| 封裝： | 4.4 MM, LEAD FREE, PLASTIC, TSSOP-38 |
| 文件頁數(shù)： | 11/40頁 |
| 文件大?。?/td> | 478K |
| 代理商： | LTC3829IFE#PBF |
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LTC3829
3829f
applicaTions inForMaTion
The NTC resistor has a negative temperature coefficient,
meaning its value decreases as temperature rises. The
VITEMP voltage, therefore, decreases as temperature in-
creases and in turn, the VSENSEMAX(ADJ) will increase to
compensate the DCR temperature coefficient. The NTC
resistor, however, is nonlinear and the user can linear-
ize its value by building a resistor network with regular
resistors. Consult the NTC manufacture data sheets for
detailed information.
Another use for the ITEMP pins, in addition to NTC com-
pensated DCR sensing, is adjusting VSENSE(MAX) to values
between the nominal values of 30mV, 50mV and 75mV for
a more precise current limit. This is done by applying a
voltage less than 0.5V to the ITEMP pin. VSENSE(MAX) will
bevariedperthepreviousequationandthesamedutycycle
limitationswillapply.Thecurrentlimitcanbeadjustedusing
this method either with a sense resistor or DCR sensing.
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
previousselection,exceptcalculatethedividercomponents
using the room temperature value of the DCR. For a single
output rail operating from one phase:
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
equations:
R
V
A
V
I
ITEMP
C
ITEMP
C
ITEMP
C
MAX
100
10
0 5
1 3
= . – .
(
)
/
–
. /
DCR MAX R
R R
C
VSENS
2
1 2
100
25
0 4 100
+
(
)
°
(
)
EE 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, which is 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
listed below:
NTC RO = 100k
RS = 20k
RP = 50k
But, the final values should be calculated using the above
equations and checked at 25°C and 100°C.
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