LT3492 Просмотр технического описания (PDF) - Linear Technology
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LT3492 Datasheet PDF : 20 Pages
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LT3492
APPLICATIONS INFORMATION
Table 1. Ceramic Capacitor Manufacturers
VENDOR
TYPE
Taiyo Yuden
Ceramic
AVX
Ceramic
Murata
Ceramic
Kemet
Ceramic
TDK
Ceramic
SERIES
X5R, X7R
X5R, X7R
X5R, X7R
X5R, X7R
X5R, X7R
Inductor Selection
Inductor value is selected based on switching frequency
and desired transient response. The data sheet applica-
tions show appropriate selections for a 1.3MHz switching
frequency. Proportionately higher values may be used if a
lower switching frequency is selected.
Several inductors that work well with the LT3492 are listed
in Table 2. However, there are many other manufacturers
and devices that can be used. Consult each manufacturer
for more detailed information and their entire range of
parts. Ferrite core inductors should be used to obtain the
best efficiency. Choose an inductor that can handle the
necessary peak current without saturating, and ensure that
the inductor has a low DCR (copper-wire resistance) to
minimize I2R power losses. An inductor with a magnetic
shield should be used to prevent noise radiation and cross
coupling among the three channels.
Diode Selection
The Schottky diode conducts current during the interval
when the switch is turned off. Select a diode VR rated
for the maximum SW voltage. It is not necessary that
the forward current rating of the diode equal the switch
current limit. The average current, IF, through the diode
is a function of the switch duty cycle. Select a diode with
forward current rating of:
IF = IL • (1 – D)
where IL is the inductor current.
If using the PWM feature for dimming, it is important to
consider diode leakage, which increases with the tem-
perature from the output during the PWM low interval.
Therefore, choose the Schottky diode with sufficient low
leakage current at hot temperature. Table 3 shows several
Schottky diodes that work well with the LT3492.
10
Table 2. Surface Mount Inductors
VALUE DCR
PART NUMBER (μH) (Ω MAX)
Sumida
CDRH4D28
15
0.149
CDRH5D28
22
0.122
33
0.189
CooperET
SD20
15
0.1655
22
0.2053
SD25
33
0.2149
Taiyo Yuden
NP04SZB
15
0.180
22
0.210
TDK
VLF5014A
15
0.32
22
0.46
Würth Electronics
7447789133
33
0.24
Coilcraft
M556132
22
0.19
IRMS (A)
0.76
0.9
0.75
1.25
1.12
1.11
0.95
0.77
0.97
0.51
1.22
1.45
SIZE
W × L × H (mm3)
5.0 × 5.0 × 3.0
6.0 × 6.0 × 3.0
5.0 × 5.0 × 2.0
5.0 × 5.0 × 2.5
4.0 × 4.0 × 1.8
4.5 × 4.7 × 1.4
7.3 × 7.3 × 3.2
6.1 × 6.1 × 3.2
Table 3. Schottky Diodes
PART NUMBER
ZETEX
VR (V)
ZLLS350
40
ZLLS400
40
DIODES
B1100
100
ROHM
RB160M-60
60
IF (A)
0.38
0.52
1.0
1.0
PACKAGE
SOD523
SOD323
SMA
PMDU/SOD-123
Undervoltage Lockout
The LT3492 has an undervoltage lockout circuit that
shuts down all the three converters when the input volt-
age drops below 2.1V. This prevents the converter from
switching in an erratic mode when powered from a low
supply voltage.
Programming the LED Current
An important consideration when using a switch with a
fixed current limit is whether the regulator will be able to
supply the load at the extremes of input and output voltage
range. Several equations are provided to help determine
3492f
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