參數(shù)資料
型號(hào): LTC4090EDJC#TRPBR
廠商: LINEAR TECHNOLOGY CORP
元件分類: 電源管理
英文描述: 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO22
封裝: 6 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-229WXXX, DFN-22
文件頁數(shù): 16/28頁
文件大?。?/td> 392K
代理商: LTC4090EDJC#TRPBR
LTC4090
23
4090f
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modied but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N02N1002J, used
in the following examples, has a nominal value of 10k
and follows the Vishay “Curve 2” resistance-temperature
characteristic. The LTC4090 trip points were designed
to work with thermistors whose resistance-temperature
characteristics follow Vishay Dale’s “R-T Curve 2.” The
Vishay NTHS0603N02N1002J is an example of such a
thermistor. However, Vishay Dale has many thermistor
products that follow the “R-T Curve 2” characteristic in a
variety of sizes. Furthermore, any thermistor whose ratio
of RCOLD to RHOT is about 7.0 will also work (Vishay Dale
R-T Curve 2 shows a ratio of 2.815 / 0.409 = 6.89).
In the explanation below, the following notation is used.
R25C = Value of the Thermistor at 25°C
RNTC|COLD = Value of Thermistor at the Cold Trip Point
RNTC|HOT = Value of the Thermistor at the Hot Trip Point
rcold = Ratio of RNTC|COLD to R25C
rHOT= Ratio of RNTC|HOT to R25C
RNOM = Primary Thermistor Bias Resistor (see Figure 8)
R1 = Optional Temperature Range Adjustment resistor
(see Figure 9)
The trip points for the LTC4090’s temperature qualica-
tion are internally programmed at 0.29 VNTC for the hot
threshold and 0.74 VNTC for the cold threshold.
Therefore, the hot trip point is set when:
R
RR
VNTC
NTCHOT
NOM
NTCHOT
|
.
+
= 029
and the cold trip point is set when:
R
RR
VNTC
NTC COLD
NOM
NTC COLD
|
.
+
= 074
Solving these equations for RNTC|COLDandRNTC|HOTresults
in the following:
RNTC|HOT = 0.409 RNOM
and
RNTC|COLD = 2.815 RNOM
By setting RNOM equal to R25C, the above equations result
in rHOT = 0.409 and rCOLD = 2.815. Referencing these ratios
to the Vishay Resistance-Temperature Curve 2 chart gives
a hot trip point of about 50°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 50°C.
By using a bias resistor, RNOM, different in value from
R25C, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
r
R
r
R
NOM
HOT
C
NOM
COLD
C
=
0 409
2 815
25
.
.
where rHOT and rCOLD are the resistance ratios at the de-
sired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen, the other is determined by the default ratios
designed in the IC. Consider an example where a 40°C
hot trip point is desired.
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