LT3469 Ver la hoja de datos (PDF) - Linear Technology
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LT3469 Datasheet PDF : 8 Pages
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LT3469
U
OPERATIO
INPUT
+
gm
–
R1
VOUT
CBOOST R2
3469 F04
Figure 4. Boosting the Bandwidth of the gm Amplifier
with Capacitance On the Inverting Input
simply an amplified version of the difference between the
feedback voltage and the reference voltage of 1.23V. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output; if it decreases, less current is delivered. The
switching regulator provides the boosted supply voltage
for the gm amplifier.
Inductor Selection
A 47µH inductor is recommended for most LT3469 appli-
cations. Some suitable inductors with small size are listed
in Table 1. The efficiency comparison of different induc-
tors is shown in Figure 5.
Table 1. Recommended Inductors
PART NUMBER
CURRENT
DCR
RATING
(Ω)
(mA)
LQH32CN470
1.3
170
CMD4D11-470
2.8
180
LBC2518T470M
1.9
150
MANUFACTURER
Murata
814-237-1431
www.murata.com
Sumida
847-545-6700
www.Sumida.com
Taiyo Yuden
408-573-4150
www.t-yuden.com
Capacitor Selection
The small size of ceramic capacitors makes them ideal for
LT3469 applications. X5R and X7R types are recommended
because they retain their capacitance over wider voltage and
temperature ranges than other types such as Y5V or Z5U.
A 1µF input capacitor is sufficient for most LT3469 appli-
cations. A 0.22µF output capacitor is sufficient for stable
6
90 VIN = 12V
85 VOUT = 35V
80
75
70
65
60
55
50
45
40
0
MURATA LQH32CN470
SUMIDA CMD4011-470
TAIYO YUDEN LBC2518T470M
5 10 15 20 25 30
LOAD CURRENT (mA)
3469 F05
Figure 5. Efficiency Comparison of Different Inductors
transient response, however, more output capacitance can
help limit the voltage droop on VCC during transients.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
PHONE
URL
Taiyo Yuden
408-573-4150
www.t-yuden.com
AVX
843-448-9411
www.avxcorp.com
Murata
814-237-1431
www.murata.com
Kemet
408-986-0424
www.kemet.com
Inrush Current Considerations When Hot Plugging
When the supply voltage is applied to VIN, the voltage
difference between VIN and VCC generates inrush current
flowing from the input through the inductor, the SW pin,
and the integrated Schottky diode to charge the output
capacitor. Care should be taken not to exceed the LT3469
maximum SW pin current rating of 1A. Worst-case inrush
current occurs when the application circuit is hot plugged
into a live supply with a large output capacitance. The
typical application circuit will maintain a peak SW pin
current below 1A when it is hot plugged into a 5V supply.
To keep SW pin current below 1A during a hot plug into
a 12V supply, 4.7Ω must be added between the supply
and the LT3469 input capacitor. During normal operation,
the SW pin current remains significantly less than 1A.
Layout Hints
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
3469f
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