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参数资料
| 型号： | MIC4685WR |
| 厂商： | Micrel Inc |
| 文件页数： | 11/17页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 3A SPAK-7 |
| 标准包装： | 48 |
| 系列： | SuperSwitcher™ |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调至 1.235V |
| 输入电压： | 4 V ~ 30 V |
| PWM 型： | 电压模式 |
| 频率 - 开关： | 200kHz |
| 电流 - 输出： | 3A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | SPak（7 引线 + 接片） |
| 包装： | 散装 |
| 供应商设备封装： | S-PAK-7 |
| 产品目录页面： | 1093 (CN2011-ZH PDF) |
| 其它名称： | 576-1224 |
P D =
? P OUT
Micrel, Inc.
Application Information
Adjustable Regulators
Adjustable regulators require a 1.235V feedback signal.
Recommended voltage-divider resistor values for
common output voltages are detailed in Table 1.
MIC4685
The ef ? ciency is used to determine how much of the
output power (P OUT ) is dissipated in the regulator circuit
(P D ).
P OUT
η
For other voltages, the resistor values can be
determined using the following formulas:
P D =
7.5W
0.84
? 7 . 5 W
V OUT = V REF ?
+ 1 ?
? R1
? R2
?
?
P D = 1.43W
A worst-case rule of thumb is to assume that 80% of the
R1 = R2 ? ? OUT ? 1 ? ?
? V ?
? V REF ?
V REF = 1.235V
Thermal Considerations
The MIC4685 is capable of high current due to the
thermally optimized SPAK package.
One limitation of the maximum output current on any
MIC4685 design is the junction-to-ambient thermal
resistance ( θ JA ) of the design (package and ground
plane).
Examining θ JA in more detail:
θ JA = ( θ JC + θ CA )
where:
θ JC = junction-to-case thermal resistance
θ CA = case-to-ambient thermal resistance
θ JC is a relatively constant 2.2°C/W for a 7-pin SPAK.
θ CA is dependent upon layout and is primarily governed
by the connection of pins 4, and Tab to the ground
plane. The purpose of the ground plane is to function as
a heat sink.
Checking the Maximum Junction Temperature:
For this example, with an output power (P OUT ) of 7.5W,
(5V output at 1.5A with V IN = 12V) and 60°C maximum
ambient temperature, what is the junction temperature?
Referring to the “Typical Characteristics: 5V Output
Ef ? ciency” graph, read the ef ? ciency ( η ) for 1.5A output
current at V IN = 12V or perform you own measurement.
η = 84%
total output power dissipation is in the MIC4685 (P D(IC) )
and 20% is in the diode-inductor-capacitor circuit.
P D(IC) = 0.8 P D
P D(IC) = 0.8 × 1.43W
P D(IC) = 1.14W
Calculate the worst-case junction temperature:
T J = P D(IC) θ JC + (T C – T A ) + T A(max)
where:
T J = MIC4685 junction temperature
P D(IC) = MIC4685 power dissipation
θ JC = junction-to-case thermal resistance.
The θ JC for the MIC4685’s 7-pin SPAK is approximately
2.2°C/W.
T C = “pin” temperature measurement taken at
the Tab.
T A = ambient temperature
T A(max) = maximum ambient operating temp-
erature for the speci ? c design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
T J = 1.14 × 2.2°C + (46°C – 25°C) + 60°C
T J = 83.5°C
This value is within the allowable maximum operating
junction temperature of 125°C as listed in “Operating
Ratings.” Typical thermal shutdown is 160°C and is
listed in “Electrical Characteristics.” Also refer to the
“Typical Safe Operating Area (SOA)” graphs in this
document.
January 2010
11
M9999-012610
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