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参数资料
| 型号： | MIC3172BM TR |
| 厂商： | Micrel Inc |
| 文件页数： | 17/20页 |
| 文件大小： | 0K |
| 描述： | IC REG MULTI CONFIG 1.25A 8SOIC |
| 标准包装： | 2,500 |
| 类型： | 降压（降压），升压（升压），反相，Cuk |
| 输出数： | 1 |
| 输入电压： | 3 V ~ 40 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 100kHz |
| 电流 - 输出： | 1.25A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-SOIC |
| 其它名称： | MIC3172BMTR MIC3172BMTR-ND |
Micrel
Finally, recalculate the transformer turns ratio to insure
that it is less than the value earlier found in equation (9).
MIC2172/3172
requiring a transformer (forward), the MIC2172/3172 is a
good choice.
a ≤
Then:
a ≤
L PRI
L SEC
1.8 × 10 ? 5
2.54 × 10 ? 5
a ≤ 0.84 Use 0.8 (same as 1:1.25).
(12)
A 12V to 5V step-down converter using transformer
isolation (forward) is shown in figure 14. Unlike the
isolated flyback converter which stores energy in the
primary inductance during the controller’s on-time and
releases it to the load during the off-time, the forward
converter transfers energy to the output during the on-
time, using the off-time to reset the transformer core. In
the application shown, the transformer core is reset by
the tertiary winding discharging T1’s peak magnetizing
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know
the primary peak current which must be withstood
without saturating the transformer core.
current through D2.
For most forward converters the duty cycle is limited to
50%, allowing the transformer flux to reset with only two
times the input voltage appearing across the power
So:
I PEAK(pri) =
V IN(min) T ON
L PRI
switch. Although during normal operation this circuit’s
duty cycle is well below 50%, the MIC2172 (and
MIC3172) has a maximum duty cycle capability of 90%.
If 90% was required during operation (start-up and high
I PEAK(pri) =
4.0 × 5.5 × 10 ? 6
18 μ 8
(13)
load currents), a complete reset of the transformer
during the off-time would require the voltage across the
power switch to be ten times the input voltage. This
V IN(max) + ( V OUT a )
6.0 + ( 5.0 × 0.8 )
V BR ≥
I PEAK(pri) = 1.22A
Now find the minimum reverse voltage requirement for
the output rectifier. This rectifier must have an average
current rating greater than the maximum output current
of 0.25A.
V BR ≥ (14)
F BR a
Where:
V BR = output rectifier maximum peak reverse
voltage rating
a = transformer turns ratio (0.8)
F BR = reverse voltage safety derating factor (0.8)
Then:
0.8 × 0.8
V BR ≥ 15.625V
A 1N5817 will safely handle voltage and current
requirements in this example.
Forward Converters
Micrel’s MIC2172/3172 can be used in several circuit
configurations to generate an output voltage which is
less than the input voltage (buck or step-down topology).
Figure 13 shows the MIC3172 in a voltage step-down
application. Because of the internal architecture of these
devices, more external components are required to
implement a step-down regulator than with other devices
offered by Micrel (refer to the LM257x or LM457x family
of buck switchers). However, for step-down conversion
would limit the input voltage to 6V or less for forward
converter applications.
To prevent core saturation, the application given here
uses a duty cycle limiter consisting of Q1, C4 and R3.
Whenever the MIC3172 exceeds a duty cycle of 50%,
T1’s reset winding current turns Q1 on. This action
reduces the duty cycle of the MIC3172 until T1 is able to
reset during each cycle.
Fluorescent Lamp Supply
An extremely useful application of the MIC3172 is
generating an ac voltage for fluorescent lamps used as
liquid crystal display back lighting in portable computers.
Figure 15 shows a complete power supply for lighting a
fluorescent lamp. Transistors Q1 and Q2 together with
capacitor C2 form a Royer oscillator. The Royer
oscillator generates a sine wave whose frequency is
determined by the series L/C circuit comprised of T1 and
C2. Assuming that the MIC3172 and L1 are absent, and
the transistors’ emitters are grounded, circuit operation is
described in “Oscillator Operation.”
Oscillator Operation
Resistor R2 provides initial base current that turns
transistor Q1 on and impresses the input voltage across
one half of T1’s primary winding (Pri 1). T1’s feedback
winding provides additional base drive (positive
feedback) to Q1 forcing it well into saturation for a period
determined by the Pri 1/C2 time constant. Once the
voltage across C2 has reached its maximum circuit
value, Q1’s collector current will no longer increase.
Since T1 is in series with Q1, this drop in primary current
causes the flux in T1 to change and because of the
April 2006
17
M9999-041806
(408) 955-1690
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相关代理商/技术参数 |
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| MIC3172BN | 功能描述:IC REG BUCK BOOST INV CUK 8DIP RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 标准包装:20 系列:SIMPLE SWITCHER® 类型:降压（降压） 输出类型:固定 输出数:1 输出电压:12V 输入电压:4 V ~ 60 V PWM 型:电压模式 频率 - 开关:52kHz 电流 - 输出:1A 同步整流器:无 工作温度:-40°C ~ 125°C 安装类型:通孔 封装/外壳:16-DIP（0.300"，7.62mm） 包装:管件 供应商设备封装:16-DIP 其它名称:*LM2575HVN-12LM2575HVN-12 |
| MIC3172YM | 功能描述:直流/直流开关调节器 1.25A 100kHz Switching Regulator (Lead Free) RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| MIC3172YM TR | 功能描述:直流/直流开关调节器 1.25A 100kHz Switching Regulator (Lead Free) RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| MIC3172YM | 制造商:Micrel Inc 功能描述:IC POWER SUPPLY |
| MIC3172YM | 制造商:Micrel Inc 功能描述:Power Supply IC |
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