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| 型号： | LTC3642EMS8E#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 50MA 8MSOP |
| 标准包装： | 50 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 45 V |
| 输入电压： | 4.5 V ~ 45 V |
| PWM 型： | Burst Mode? |
| 电流 - 输出： | 50mA |
| 同步整流器： | 是 |
| 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 |
| 产品目录页面： | 1335 (CN2011-ZH PDF) |
LTC3642
APPLICATIONS INFORMATION
L = á
? @ 100μH
? á 1–
L MIN =
? 22μH
I RMS = 50mA ?
– 1 ? 18mA RMS
?
C OUT =
2 ? 50mV –
160 ?
R1 = ? OUT – 1 ? ? R2 = 750k
Thegatechargecurrentresultsfromswitchingthegate
capacitance of the internal power MOSFET switches.
Each time the gate is switched from high to low to
high again, a packet of charge, dQ, moves from V IN to
ground. The resulting dQ/dt is the current out of V IN
that is typically larger than the DC bias current.
2. I 2 R losses are calculated from the resistances of the
internal switches, R SW , and external inductor R L . When
switching, the average output current flowing through
the inductor is “chopped” between the high side PMOS
switch and the low side NMOS switch. Thus, the series
resistance looking back into the switch pin is a function
of the top and bottom switch R DS(ON) values and the
duty cycle (DC = V OUT /V IN ) as follows:
R SW = (R DS(ON)TOP )DC + (R DS(ON)BOT )(1 – DC)
The R DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain the I 2 R losses, simply add
R SW to R L and multiply the result by the square of the
average output current:
I 2 R Loss = I O2 (R SW + R L )
Other losses, including C IN and C OUT ESR dissipative
losses and inductor core losses, generally account for
less than 2% of the total power loss.
Thermal Considerations
The LTC3642 does not dissipate much heat due to its high
efficiency and low peak current level. Even in worst-case
conditions (high ambient temperature, maximum peak
current and high duty cycle), the junction temperature will
exceed ambient temperature by only a few degrees.
Design Example
As a design example, consider using the LTC3642 in an
application with the following specifications: V IN = 24V,
V OUT = 3.3V, I OUT = 50mA, f = 250kHz. Furthermore, as-
sume for this example that switching should start when
V IN is greater than 12V and should stop when V IN is less
than 8V.
First, calculate the inductor value that gives the required
switching frequency:
ê 3.3V ? ê 3.3V ?
ˉ
? 250kHz ? 115mA ? ? 24V ˉ
Next, verify that this value meets the L MIN requirement.
For this input voltage and peak current, the minimum
inductor value is:
24V ? 100ns
115mA
Therefore, the minimum inductor requirement is satisfied,
and the 100μH inductor value may be used.
Next, C IN and C OUT are selected. For this design, C IN should
be size for a current rating of at least:
3.3V 24V
24V 3.3V
Due to the low peak current of the LTC3642, decoupling
the V IN supply with a 1μF capacitor is adequate for most
applications.
C OUT will be selected based on the output voltage ripple
requirement. For a 1.5% (50mV) output voltage ripple at
no load, C OUT can be calculated from:
115mA ? 4 ? 10 –6
? 3.3V ?
?
?
A 7.8μF capacitor gives this typical output voltage ripple at
no load. Choose a 10μF capacitor as a standard value.
The output voltage can now be programmed by choosing
the values of R1 and R2. Choose R2 = 240k and calculate
R1 as:
? V ?
? 0.8V ?
3642fc
15
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