参数资料
型号: LT3574IMS#TRPBF
厂商: Linear Technology
文件页数: 16/24页
文件大小: 0K
描述: IC REG FLYBK ISO ADJ .65A 16MSOP
标准包装: 2,500
类型: 回扫,隔离
输出类型: 可调式
输出数: 1
输出电压: 可调至 60V
输入电压: 3 V ~ 40 V
PWM 型: 电流模式,混合
频率 - 开关: 40kHz ~ 1MHz
电流 - 输出: 650mA
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: 16-TFSOP(0.118",3.00mm 宽)
包装: 带卷 (TR)
供应商设备封装: 16-MSOP
相关产品: 732-2669-2-ND - TRANS FLYBACK LT3748 20UH SMD
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732-2667-6-ND - TRANS FLYBACK LT3748 500UH SMD
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LT3574
APPLICATIONS INFORMATION
BIAS winding turns ratio is selected to program the BIAS
voltage to 3~5V. The BIAS voltage shall not exceed the
input voltage.
The turns ratio is then selected as primary: secondary:
BIAS = 3:1:1.
2. Select the transformer primary inductance for target
switching frequency.
The LT3574 requires a minimum amount of time to sample
the output voltage during the off-time. This off-time,
t OFF(MIN) , shall be greater than 350ns over all operating
conditions. The converter also has a minimum current limit,
I MIN , of 175mA to help create this off-time. This defines
the minimum required inductance as defined as:
In this design example, the minimum primary inductance is
used to achieve a nominal switching frequency of 350kHz
at full load. The PA2627NL from Pulse Engineering is
chosen as the flyback transformer.
Given the turns ratio and primary inductance, a custom-
ized transformer can be designed by magnetic component
manufacturer or a multi-winding transformer such as a
Coiltronics Versa-Pac may be used.
3. Select the output diodes and output capacitor.
The output diode voltage stress V D is the summation of
the output voltage and reflection of input voltage to the
secondary side. The average diode current is the load
current.
L MIN =
N( V OUT + V F )
I MIN
? t OFF ( MIN )
V D = V OUT +
V IN
N
f SW =
=
t ON + t OFF I PK I PK
+ V
V IN
N PS OUT F )
+
LI 2 PK
D V MAX =
The transformer primary inductance also affects the
switching frequency which is related to the output ripple. If
above the minimum inductance, the transformer’s primary
inductance may be selected for a target switching frequency
range in order to minimize the output ripple.
The following equation estimates the switching frequency.
1 1
( V
L L
Table 7. Switching Frequency at Different Primary Inductance
at I PK
f SW AT V IN(MIN) f SW AT V IN(MAX)
L (μH) (kHz) (kHz)
30 317 373
60 159 187
The output capacitor should be chosen to minimize the
output voltage ripple while considering the increase in
size and cost of a larger capacitor. The following equation
calculates the output voltage ripple.
2 CV OUT
4. Select the snubber circuit to clamp the switch voltage
spike.
A flyback converter generates a voltage spike during switch
turn-off due to the leakage inductance of the transformer.
In order to clamp the voltage spike below the maximum
rating of the switch, a snubber circuit is used. There are
many types of snubber circuits, for example R-C, R-C-D and
Zener clamps. Among them, RCD is widely used. Figure 9
shows the RCD snubber in a flyback converter.
120
79
93
A typical switch node waveform is shown in Figure 10.
Note: The switching frequency is calculated at maximum output.
3574f
  
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