參數(shù)資料
型號(hào)： LTC3553EUD#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 電源管理
英文描述： 1-CHANNEL POWER SUPPLY SUPPORT CKT, PQCC20
封裝： 3 X 3 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, QFN-20
文件頁(yè)數(shù)： 11/36頁(yè)
文件大小： 518K
代理商： LTC3553EUD#PBF
LTC3553
19
3553fa
By using a bias resistor, RNOM, different in value from
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
R
NOM =
r
HOT
0.538
R25
R
NOM =
r
COLD
3.17
R25
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 independently set, the other is determined by the de-
fault ratios designed in the IC.
Consider an example where a 60°C hot trip point is desired.
From the Vishay curve 1 R-T characteristics, rHOT is 0.2488
at 60°C. Using the above equation, RNOM should be set
to 46.4k. With this value of RNOM, the cold trip point is
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in tem-
perature gain of the thermistor as absolute temperature
increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 2. The following formulas can
be used to compute the values of RNOM and R1:
R
NOM =
r
COLD –r HOT
2.714
R25
R1 = 0.536 RNOM –rHOT R25
For example, to set the trip points to 0°C and 45°C with
a Vishay curve 1 thermistor choose:
R
NOM =
3.266 – 0.4368
2.714
100k =104.2k
the nearest 1% value is 105k:
R1 = 0.536 105k – 0.4368 100k = 12.6k
The nearest 1% value is 12.7k. The nal solution is shown
in Figure 2 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
OPERATION
Figure 2. NTC Thermistor Circuit With Additional Bias Resistor
–
+
–
+
RNOM
105k
RNTC
100k
R1
12.7k
NTC
VBUS
NTC_ENABLE
3553 F02
TOO_COLD
TOO_HOT
0.76 VBUS
(NTC RISING)
0.35 VBUS
(NTC FALLING)
0.017 VBUS
(NTC FALLING)
–
+
20
15
The trip points for the LTC3553’s temperature qualica-
tion are internally programmed at 0.35 VBUS for the hot
threshold and 0.76 VBUS for the cold threshold.
Therefore, the hot trip point is set when:
R
NTC|HOT
R
NOM + RNTC|HOT
V
BUS = 0.35 VBUS
and the cold trip point is set when:
R
NTC|COLD
R
NOM + RNTC|COLD
V
BUS = 0.76 V BUS
Solving these equations for RNTC|COLD and RNTC|HOT
results in the following:
RNTC|HOT = 0.538 RNOM
and
RNTC|COLD = 3.17 RNOM
By setting RNOM equal to R25, the above equations result
in rHOT = 0.538 and rCOLD = 3.17. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
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