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| 型号： | MIC4742YTSE |
| 厂商： | Micrel Inc |
| 文件页数： | 13/23页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 2A DL 16TSSOP |
| 标准包装： | 94 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 2 |
| 输出电压： | 可调至 0.6V |
| 输入电压： | 2.9 V ~ 5.5 V |
| 频率 - 开关： | 2MHz |
| 电流 - 输出： | 2A |
| 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： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 16-EPAD TSSOP |
| 产品目录页面： | 1093 (CN2011-ZH PDF) |
| 其它名称： | 576-3355-5 |
V OUT = V FB ?
+ 1 ?
Micrel, Inc.
Component Selection
Input Capacitor
A 10μF ceramic is recommended on each VIN pin for
bypassing. X5R or X7R dielectrics are recommended for
the input capacitor. Y5V dielectrics lose most of their
capacitance over temperature and are therefore, not
recommended. Also, tantalum and electrolytic capacitors
alone are not recommended due to their reduced RMS
current handling, reliability, and ESR increases.
An additional 0.1μF is recommended close to the VIN
and PGND pins for high frequency filtering. Smaller case
size capacitors are recommended due to their lower
ESR and ESL. Please refer to layout recommendation
for proper layout of the input capacitor.
Output Capacitor
The MIC4742 is designed for a 4.7μF output capacitor.
X5R or X7R dielectrics are recommended for the output
capacitor. Y5V dielectrics lose most of their capacitance
over temperature and are therefore not recommended.
In addition to a 4.7μF, a small 0.1μF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to there
lower equivalent series ESR and ESL.
The MIC4742 utilizes type III voltage mode internal
compensation and utilizes an internal zero to
compensate for the double pole roll off of the LC filter.
For this reason, larger output capacitors can create
instabilities. In cases where a 4.7μF output capacitor is
not sufficient, other values of capacitance can be used
but the original LC filter pole frequency determined by
C OUT = 4.7μF and L = 1μH (which is approximately
73.4KHz) must remain fixed. Increasing C OUT forces L to
decrease and vice versa.
Inductor Selection
The MIC4742 is designed for use with a 1μH inductor.
MIC4742
Diode Selection
Since the MIC4742 is non-synchronous, a free-wheeling
diode is required for proper operation. A Schottky diode
is recommended due to the low forward voltage drop
and their fast reverse recovery time. The diode should
be rated to be able to handle the average output current.
Also, the reverse voltage rating of the diode should
exceed the maximum input voltage. The lower the
forward voltage drop of the diode the better the
efficiency. Please refer to the layout recommendation to
minimize switching noise.
Feedback Resistors
The feedback resistor set the output voltage by dividing
down the output and sending it to the feedback pin. The
feedback voltage is 0.6V. Calculating the set output
voltage is as follows;
? R1 ?
? R2 ?
Where R1 is the resistor from V OUT to FB and R2 is the
resistor from FB to GND. The recommended feedback
resistor values for common output voltages are available
in the bill of materials on page 17. Although the range of
resistance for the FB resistors is very wide, R1 is
recommended to be 10K. This minimizes the effect the
parasitic capacitance of the FB node.
Feedforward Capacitor (C FF )
A capacitor across the resistor from the output to the
feedback pin (R1) is recommended for most designs.
This capacitor can give a boost to phase margin and
increase the bandwidth for transient response. Also,
large values of feedforward capacitance can slow down
the turn-on characteristics, reducing inrush current. For
maximum phase boost, C FF can be calculated as follows;
Proper selection should ensure the inductor can handle
the maximum average and peak currents required by the
load. Maximum current ratings of the inductor are
C FF =
1
2 π × 200kHz × R1
generally given in two methods; permissible DC current
and saturation current. Permissible DC current can be
rated either for a 40°C temperature rise or a 10% to 20%
loss in inductance. Ensure the inductor selected can
handle the maximum operating current. When saturation
current is specified, make sure that there is enough
margin that the peak current will not saturate the
inductor.
Large values of feedforward capacitance may introduce
negative FB pin voltage during load shorting, which will
cause latch-off. In that case, a Schottky diode from FB
pin to the ground is recommended.
Bias Filter
A small 10 ? resistor is recommended from the input
supply to the bias pin along with a small 0.1μF ceramic
capacitor from bias to ground. This will bypass the high
frequency noise generated by the violent switching of
high currents from reaching the internal reference and
control circuitry. Tantalum and electrolytic capacitors are
not recommended for the bias, these types of capacitors
lose their ability to filter at high frequencies.
March 2009
13
M9999-030209-D
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