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LM3554 Fiches technique(PDF) 36 Page - Texas Instruments

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No de pièce LM3554
Description  Synchronous Boost Converter with 1.2A Dual High Side LED Drivers and I2C Compatible
PDF  43 Pages
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Fabricant  TI1 [Texas Instruments]
Site Internet  http://www.ti.com
Logo TI1 - Texas Instruments

LM3554 Fiches technique(HTML) 36 Page - Texas Instruments

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+ E
R
V
TRIP x
3
C
°
x 298
E
C
°
273
-
LN
C x
°
298
C
T
=
° )
(
»
¼
º
«
¬
ª
(
V
V
TRIP
BIAS
x
-
)
R
C
°
25
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
70
75
80
85
90
95
100 105 110
TEMPERATURE (°C)
VBIAS = 2.5V,
@ +25°C, B = 4500,
R3 = 9 k:
RTHERMISTOR = 100 k:
R3 is then:
.
:
k
=
071
9
V
1
(
)
:
V
- 1
V
5
.
2
x
k
047
.
6
=
047
.
6
:
k
(T
R
)
x
:
=
k
100
e
-
+
298
1
273
93
1
E
»
¼
º
(
)
TRIP
- V
BIAS
V
)
TRIP
(
T
R
TRIP
V
=
3
R
e
x
( )
C
25
R
=
T
R
°
298
1
273
+
C
°
T
1
E
-
¹
·
©
§
LM3554
SNVS549A – JUNE 2009 – REVISED APRIL 2011
www.ti.com
Table 11. Recommended Inductors
Manufacturer
L
Part Number
Dimensions (L×W×H)
ISAT
TOKO
2.2µH
FDSE0312-2R2M
3mm×3mm×1.2mm
2A
TDK
2.2µH
VLS252012T-2R2M1R3
2mm×2.5mm×1.2mm
1.5A
Coilcraft
2.2µH
LPS4018-222ML
3.9mmx3.9mmx1.7mm
2.3A
NTC THERMISTOR SELECTION
NTC thermistors have a temperature to resistance relationship of:
(10)
where
β is given in the thermistor datasheet and R25C is the thermistors value at +25°C. R3 in Figure 28 is
chosen so that it is equal to:
(11)
where R(T)TRIP is the thermistors value at the temperature trip point, VBIAS is shown in Figure 28, and VTRIP =
1.05V (typical). Choosing R3 here gives a more linear response around the temperature trip voltage. For
example, with VBIAS = 2.5V, a thermistor whose nominal value at +25°C is 100kΩ and a β = 4500K, the trip point
is chosen to be +93°C. The value of R(T) at 93°C is:
(12)
Figure 27 shows the linearity of the thermistor resistive divider of the previous example.
Figure 27. Thermistor Resistive Divider Response vs Temperature
Another useful equation for the thermistor resistive divider is developed by combining the equations for R3, and
R(T) and solving for temperature. This gives the following relationship.
(13)
Using a spreadsheet such as Excel, different curves for the temperature trip point T(°C) can be created vs R3,
Beta, or VBIAS in order to help better choose the thermal components for practical values of thermistors, series
resistors (R3), or reference voltages VBIAS.
36
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Product Folder Links: LM3554



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