参数资料
型号: LT3512MPMS#TRPBF
厂商: Linear Technology
文件页数: 13/26页
文件大小: 0K
描述: IC REG FLYBK ISO ADJ .42A 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
输入电压: 4.5 V ~ 100 V
PWM 型: 电流模式,混合
频率 - 开关: 40kHz ~ 650kHz
电流 - 输出: 420mA
同步整流器:
工作温度: -55°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 16-TFSOP(0.118",3.00mm),12 引线
包装: 带卷 (TR)
供应商设备封装: 16-MSOP
LT3512
APPLICATIONS INFORMATION
DZ Power Loss = ? L ? ? I PK(VIN(MIN))2 ? f SW ?
? 1 + V
ZENER – N PS ? ( V OUT + V F ) ?
?
I PK(VIN(MIN)) =
f SW =
=
t ON + t OFF L PRI ? I PK(VIN(MIN)) L PRI ? I PK(VIN(MIN))
N PS ? ( V OUT + V F )
V IN(MIN)
currentishighestatthispointalongwiththeenergystored
in the leakage inductance. A 0.5W Zener will satisfy most
applications when the highest V ZENER is chosen. Choosing
a low value for V ZENER will cause excessive power loss as
shown in the following equations:
1
2
? N PS ? ( V OUT + V F ) ?
?
L ? = Leakage Inductance
V OUT ? I OUT ? 2
η ? V IN(MIN) ? D VIN(MIN)
1 1
+
?
Table 2 and 3 show some recommended diodes and Zener
diodes.
Table 2. Recommended Zener Diodes
V ZENER POWER
PART (V) (W) CASE VENDOR
and transformer leakage inductance cause the delay. The
leakage inductance also causes a very fast voltage spike
on the primary side of the transformer. The amplitude of
the leakage spike is largest when power switch current is
highest. Introduction of an internal fixed delay between
switch turn-off and the start of sampling provides im-
munity to the phenomena discussed above. The LT3512
sets internal blanking to 150ns. In certain cases leakage
inductance spikes last longer than the internal blanking,
but will not significantly affect output regulation.
Secondary Leakage Inductance
In addition to primary leakage inductance, secondary leak-
age inductance exhibits an important effect on application
design. Secondary leakage inductance forms an inductive
divider on the transformer secondary. The inductive divider
effectively reduces the size of the primary-referred flyback
pulse. The smaller flyback pulse results in a higher regulated
output voltage. The inductive divider effect of secondary
leakage inductance is load independent. R FB /R REF ratio
adjustments can accommodate this effect to the extent
secondary leakage inductance is a constant percentage
of mutual inductance (over manufacturing variations).
MMSZ5266BT1G
MMSZ5270BT1G
CMHZ5266B
CMHZ5267B
BZX84J-68
BZX100A
68
91
68
75
68
100
0.5
0.5
0.5
0.5
0.5
0.5
SOD-123 On Semi
SOD-123
SOD-123 Central
SOD-123 Semiconductor
SOD323F NXP
SOD323F
Winding Resistance Effects
Resistance in either the primary or secondary will reduce
overall efficiency (P OUT /P IN ). Good output voltage regula-
tion will be maintained independent of winding resistance
due to the boundary mode operation of the LT3512.
Table 3. Recommended Diodes
V REVERSE
PART I (A) (V) VENDOR
DFLS1200 1.0 200 Diodes Inc.
DFLS1150 1.0 150
Leakage Inductance Blanking
When the power switch turns off, the flyback pulse ap-
pears. However, a finite time passes before the trans-
former primary-side voltage waveform approximately
represents the output voltage. Rise time on the SW node
Bifilar Winding
A bifilar, or similar winding technique, is a good way to
minimize troublesome leakage inductances. However, re-
member that this will also increase primary-to-secondary
capacitance and limit the primary-to-secondary breakdown
voltage, so bifilar winding is not always practical. The
Linear Technology applications group is available and
extremely qualified to assist in the selection and/or design
of the transformer.
3512fb
13
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