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参数资料
| 型号： | MIC28500 2A EV |
| 厂商： | Micrel Inc |
| 文件页数： | 19/29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.
MIC28500
Δ V IN = I L(pk) × C ESR
Eq. 13
Δ V FB(pp) ≈ ESR × Δ I L (pp)
Eq. 17
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determined at the maximum output current. Assuming
the peak-to-peak inductor current ripple is low:
3) Virtually no ripple at the FB pin voltage due to the very
low ESR of the output capacitors.
I CIN(RMS) ≈ I OUT(max) × D × (1 ? D)
The power dissipated in the input capacitor is:
P DISS(CIN) = I CIN(RMS)2 × C ESR
Ripple Injection
Eq. 14
Eq. 15
Figure 7a. Enough Ripple at FB
The V FB ripple required for proper operation of the
MIC28500 g m amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For a low
output voltage, such as a 1V, the output voltage ripple is
only 10mV to 20mV, and the feedback voltage ripple is
less than 20mV. If the feedback voltage ripple is so small
that the g m amplifier and error comparator can’t sense it,
then the MIC28500 will lose control and the output
voltage is not regulated. In order to have some amount
of V FB ripple, a ripple injection method is applied for low
output voltage ripple applications.
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1) Enough ripple at the feedback voltage due to the large
ESR of the output capacitors.
As shown in Figure 7a, the converter is stable without
any ripple injection. The feedback voltage ripple is:
Figure 7b. Inadequate Ripple at FB
Δ V FB(pp) =
R 2
R 1 + R 2
× ESR C OUT × Δ I L (pp)
Eq. 16
where Δ I L(pp) is the peak-to-peak value of the inductor
current ripple.
2) Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors.
The output voltage ripple is fed into the FB pin through a
feedforward capacitor C ff in this situation, as shown in
Figure 7b. The typical C ff value is between 1nF and
22nF.
With the feedforward capacitor, the feedback voltage
ripple is very close to the output voltage ripple:
Figure 7c. Invisible Ripple at FB
In this situation, the output voltage ripple is less than
20mV. Therefore, additional ripple is injected into the FB
pin from the switching node SW via a resistor R inj and a
capacitor C inj , as shown in Figure 7c.
June 2011
19
M9999-060311-B
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