参数资料
型号: LT1961IMS8E#PBF
厂商: Linear Technology
文件页数: 10/16页
文件大小: 0K
描述: IC REG BOOST ADJ 1.5A 8MSOP
标准包装: 50
类型: 升压(升压)
输出类型: 可调式
输出数: 1
输出电压: 1.2 V ~ 35 V
输入电压: 3 V ~ 25 V
PWM 型: 电流模式
频率 - 开关: 1.25MHz
电流 - 输出: 1.5A
同步整流器:
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 8-TSSOP,8-MSOP(0.118",3.00mm 宽)裸露焊盘
包装: 管件
供应商设备封装: 8-MSOP-EP
LT1961
APPLICATIO N S I N FOR M ATIO N
SYNCHRONIZATION
The SYNC pin, is used to synchronize the internal oscilla-
tor to an external signal. The SYNC input must pass from
a logic level low, through the maximum synchronization
threshold with a duty cycle between 20% and 80%. The
input can be driven directly from a logic level output. The
synchronizing range is equal to initial operating frequency
up to 2MHz. This means that minimum practical sync
frequency is equal to the worst-case high self-oscillating
frequency (1.5MHz), not the typical operating frequency
of 1.25MHz. Caution should be used when synchronizing
above 1.7MHz because at higher sync frequencies the
amplitude of the internal slope compensation used to
prevent subharmonic switching is reduced. Higher induc-
tor values will tend to eliminate this problem. See Fre-
quency Compensation section for a discussion of an
entirely different cause of subharmonic switching before
assuming that the cause is insufficient slope compensa-
tion. Application Note 19 has more details on the theory
of slope compensation.
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering
layout, care must be taken to achieve optimal electrical,
thermal and noise performance. For maximum efficiency,
switch rise and fall times are typically in the nanosecond
range. To prevent noise both radiated and conducted, the
L1
C3
high speed switching current path, shown in Figure 5,
must be kept as short as possible. This is implemented in
the suggested layout of Figure 6. Shortening this path will
also reduce the parasitic trace inductance of approxi-
mately 25nH/inch. At switch off, this parasitic inductance
produces a flyback spike across the LT1961 switch. When
operating at higher currents and output voltages, with
poor layout, this spike can generate voltages across the
LT1961 that may exceed its absolute maximum rating. A
ground plane should always be used under the switcher
circuitry to prevent interplane coupling and overall noise.
The V C and FB components should be kept as far away as
possible from the switch node. The LT1961 pinout has
been designed to aid in this. The ground for these compo-
nents should be separated from the switch current path.
Failure to do so will result in poor stability or subharmonic
like oscillation.
Board layout also has a significant effect on thermal
resistance. The exposed pad is the copper plate that runs
under the LT1961 die. This is the best thermal path for heat
out of the package. Soldering the pad onto the board will
reduce die temperature and increase the power capability
of the LT1961. Provide as much copper area as possible
around this pad. Adding multiple solder filled feedthroughs
under and around the pad to the ground plane will also
help. Similar treatment to the catch diode and inductor
terminations will reduce any additional heating effects.
D1
V OUT
SW
LT1961
HIGH
V IN
FREQUENCY
SWITCHING
C1 LOAD
PATH
GND
1961 F05
Figure 5. High Speed Switching Path
1961fa
10
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