參數(shù)資料
型號(hào)： LT3645HMSE#TRPBF
廠商： Linear Technology
文件頁(yè)數(shù)： 12/24頁(yè)
文件大?。?/td> 300K
描述： IC REG DL BUCK/LINEAR 12-MSOP
標(biāo)準(zhǔn)包裝： 2,500
拓?fù)洌?/td> 降壓（降壓）（1），線性（LDO）（1）
功能： 車載
輸出數(shù)： 2
頻率 - 開(kāi)關(guān)： 750kHz
電壓/電流 - 輸出 1： 0.8 V ~ 16 V，500mA
電壓/電流 - 輸出 2： 0.8 V ~ 8 V，200mA
帶 LED 驅(qū)動(dòng)器： 無(wú)
帶監(jiān)控器： 無(wú)
帶序列發(fā)生器： 無(wú)
電源電壓： 3.6 V ~ 36 V
工作溫度： -40°C ~ 150°C
安裝類型： 表面貼裝
封裝/外殼： 12-TSSOP （0.118"，3.00mm 寬）裸露焊盤
供應(yīng)商設(shè)備封裝： 12-MSOP，裸露焊盤
包裝： 帶卷 (TR)
LT3645
12
3645f
APPLICATIONS INFORMATION
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it  lters the square wave generated
by the LT3645 to produce the DC output. In this role it
determines the output ripple so low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT3645s control loop.
Ceramic capacitors have very low equivalent series re-
sistance (ESR) and provide the best ripple performance.
A good value is:
  C
OUT
 = 26.4/(V
OUT
 " ?
where f is the switching frequency in MHz and C
OUT
 is in
糉. This choice will provide low output ripple and good
transient response. C
OUT
 = 10糉 is a good choice for
output voltages above 2.5V. For lower output voltages
use 22糉 or higher.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor R1 may be required to get the full bene t (see the
Compensation section). Using a small output capacitor
results in an increased loop crossover frequency.
Use X5R or X7R types and keep in mind that a ceramic
capacitor biased with V
OUT
 will have less than its nominal
capacitance. High performance electrolytic capacitors can
be used for the output capacitor. Low ESR is important, so
choose one that is intended for use in switching regulators.
The ESR should be speci ed by the supplier and should be
0.1?or less. Such a capacitor will be larger than a ceramic
capacitor and will have a larger capacitance, because the
capacitor must be large to achieve low ESR.
Table 2 lists several capacitor vendors.
Table 2. Capacitor Vendors
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
BOOST Pin Considerations
The external capacitor C2 and an internal Schottky diode
connected between the V
CC2
 and BOOST pins form a
charge pump circuit which is used to generate a boost
voltage that is higher than the input voltage (V
IN
). In most
application circuits where the duty cycle is less than 50%,
use C2 = 0.1糉. If the duty cycle is higher than 50% then
use C2 = 0.22糉.
The BOOST pin must be at least 2.2V above the SW pin
to fully saturate the NPN power switch (Q1). The forward
drop of the internal Schottky diode is 0.8V. This means
that V
CC2
 must be tied to a supply greater than 2.6V.
V
CC2
 may be tied to a supply between 2.2V and 2.6V if an
external Schottky diode (such as a BAS70) is connected
from V
CC2
 (anode) to BOOST (cathode).
If no voltage supply greater than 2.6V is available, then
an external boost Schottky diode can be tied from the
V
IN
 pin (anode) to the BOOST pin (cathode) as shown in
Figure 3. In this con guration, the BOOST capacitor will be
charged to approximately the V
IN
 voltage, and will change
if V
IN
 changes. In this con guration the maximum operat-
ing V
IN
 is 25V, because when V
IN
 = 25V, then when the
power switch Q1 turns on, V
SW
 ~ 25V, and since the boost
capacitor is charged to 25V, the BOOST pin will be at 50V.
This connection is not as ef cient as the others because
the BOOST pin current comes from a higher voltage.
The minimum operating voltage of an LT3645 application
is limited by the undervoltage lockout (~3.4V) and by
the maximum duty cycle as outlined above. For proper
startup, the minimum input voltage is also limited by the
Figure 3.
V
IN
BOOST
GND
SW
V
IN
LT3645
D2
3645 F03
V
OUT
C3
V
BOOST
  V
SW
  V
IN
MAX V
BOOST
  2V
IN
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