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參數(shù)資料
| 型號： | LT3645IUD#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.25 A SWITCHING REGULATOR, 825 kHz SWITCHING FREQ-MAX, PQCC16 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-220WEED-2, QFN-16 |
| 文件頁數(shù)： | 3/24頁 |
| 文件大?。?/td> | 296K |
| 代理商： | LT3645IUD#TRPBF |
LT3645
11
3645f
APPLICATIONS INFORMATION
Now the required on time has decreased below the mini-
mum on time of 100ns. Instead of the switch pulse width
becoming narrower to accommodate the lower duty cycle
requirement, the part skips a few pulses so that the aver-
age inductor current meets and does not exceed the load
current requirement.
The LT3645 is robust enough to survive prolonged opera-
tion under these conditions as long as the peak inductor
current does not exceed 1.2A. Inductor saturation due
to high current may further limit performance in this
operating region.
Inductor Selection and Maximum Output Current
Choose the inductor value according to:
L = 2.2 (VOUT + VD)/
where VD is the forward voltage drop of the catch diode
(~0.4V), f is the switching frequency in MHz and L is in
μH. With this value, there will be no subharmonic oscilla-
tion for applications with 50% or greater duty cycle. For
robust operation in fault conditions, the saturation current
should be above 1.5A. To keep efciency high, the series
resistance (DCR) should be less than 0.1Ω. Table 1 lists
several inductor vendors. If the buck load current is less
than 500mA, then a lower valued inductor can be used.
Catch Diode
Depending on load current, a 500mA to 1A Schottky diode
is recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
overvoltage lockout voltage (38.5V). The ON Semiconduc-
tor MBRA140T3 and Central Semiconductor CMMSH1-40
are good choices, as they are rated for 1A continuous
forward current and a maximum reverse voltage of 40V.
Input Filter Network
Bypass VIN with a 1μF or higher ceramic capacitor of X7R
or X5R type. Y5V types have poor performance over tem-
perature and applied voltage and should not be used. A 1μF
ceramic capacitor is adequate to bypass the LT3645 and
will easily handle the ripple current. However, if the input
power source has high impedance, or there is signicant
inductance due to long wires or cables, additional bulk
capacitance might be necessary. This can be provided
with a low performance (high ESR) electrolytic capacitor
in parallel with the ceramic device. Step-down regulators
draw current from the input supply in pulses with very
fast rise and fall times. The input capacitor is required to
reduce the resulting voltage ripple at the LT3645 input
and to force this very high frequency switching current
into a tight local loop, minimizing EMI. A 1μF capacitor
is capable of this task, but only if it is placed close to the
LT3645 and catch diode (see the PCB layout section). A
second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3645.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (underdamped) tank cir-
cuit. If the LT3645 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possibly
exceeding the LT3645’s voltage rating. This situation can
easily be avoided. For more details, see Linear Technology
Application Note 88.
Table 1. Inductor Vendors
Vendor
URL
Part Series
Inductance Range (μH)
Size (mm)
Sumida
www.sumida.com
CDRH4D28
CDRH5D28
CDRH8D28
1.2 to 4.7
2.5 to 10
2.5 to 33
4.5
× 4.5
5.5
× 5.5
8.3
× 8.3
Toko
www.toko.com
A916CY
D585LC
2 to 12
1.1 to 39
6.3
× 6.2
8.1
× 8.0
Würth Elektronik
www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 to 10
2.2 to 22
1 to 27
4.8
× 4.8
5.2
× 5.8
7.3
× 7.3
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