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参数资料
| 型号： | MIC28500 2A EV |
| 厂商： | Micrel Inc |
| 文件页数： | 20/29页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR MIC28500 2A |
| 标准包装： | 1 |
| 系列： | Hyper Speed Control™，SuperSwitcher II™ |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，非隔离 |
| 输出电压： | 12V |
| 电流 - 输出： | 2A |
| 输入电压： | 30 ~ 75V |
| 稳压器拓扑结构： | 降压 |
| 频率 - 开关： | 250kHz |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | MIC28500 |
| 其它名称： | 576-4019 |
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Micrel, Inc.
The injected ripple is:
MIC28500
Setting Output Voltage
The MIC28500 requires two resistors to set the output
Δ V FB(pp) = V IN × K div × D × (1 - D) ×
R 1 //R 2
K div =
R inj + R1//R2
1
f SW × τ
Eq. 18
Eq. 19
voltage as shown in Figure 8.
where
V IN = Power stage input voltage
D = duty cycle
f SW = switching frequency
Figure 8. Voltage-Divider Configuration
τ = (R1//R2//R inj ) × C ff
The output voltage is determined by Equation 23:
V O = V FB × ? 1 +
?
In Equations 18 and 19, it is assumed that the time
constant associated with C ff must be much greater than
the switching period:
?
?
R 1 ?
R 2 ?
Eq. 23
1
f SW × τ
=
T
τ
<< 1
Eq. 20
where, V FB = 0.8V. A typical value of R1 can be between
3k ? and 10k ? . If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small, it will decrease the efficiency of the power supply,
If the voltage divider resistors R1 and R2 are in the k ?
range, a C ff of 1nF to 22nF can easily satisfy the large
time constant requirements. Also, a 100nF injection
especially at light loads. Once R1 is selected, R2 can be
calculated using:
capacitor C inj is used in order to be considered as short
for a wide range of the frequencies.
The process of sizing the ripple injection resistor and
R 2 =
V FB × R 1
V OUT ? V FB
Eq. 24
capacitors is:
Step 1. Select C ff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of C ff is 1nF to
22nF if R1 and R2 are in k ? range.
Step 2. Select R inj according to the expected feedback
voltage ripple using Equation 21:
The inverting input voltage V INJ is clamped to 1.2V. As
the injected ripple increases, the swing of V INJ will be
clamped. The clamped V INJ reduces the line regulation
because it is reflected back as a DC error on the FB
terminal.
Thermal Measurements
K div =
Δ V FB(pp)
V IN
×
f SW × τ
D × (1 ? D)
Eq. 21
Measuring the IC’s case temperature is recommended to
ensure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
Then the value of R inj is obtained as:
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
R inj = (R 1 //R 2 ) × (
1
K div
? 1)
Eq. 22
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared thermometer.
If a thermal couple wire is used, it must be constructed
Step 3. Select C inj as 100nF, which could be considered
as short for a wide range of the frequencies.
of 36 gauge wire or higher then (smaller wire size) to
minimize the wire heat-sinking effect. In addition, the
thermal couple tip must be covered in either thermal
June 2011
20
M9999-060311-B
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