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参数资料
| 型号： | MIC28500 2A EV |
| 厂商： | Micrel Inc |
| 文件页数： | 18/29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.
high frequency operation of the MIC28500 requires the
use of ferrite materials for all but the most cost sensitive
applications. Lower cost iron powder cores may be used
but the increase in core loss will reduce the efficiency of
MIC28500
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 10:
Δ V OUT(pp) = ? ?
?
(
Δ I L(PP)
C OUT × f SW × 8 ? ?
)
the power supply. This is especially noticeable at low
output power. The winding resistance decreases
efficiency at the higher output current levels. The
winding resistance must be minimized although this
?
?
2
? + Δ I L(PP) × ESR C
OUT
2
usually comes at the expense of a larger inductor. The
power dissipated in the inductor is equal to the sum of
the core and copper losses. At higher output loads, the
core losses are usually insignificant and can be ignored.
At lower output currents, the core losses can be a
significant contributor. Core loss information is usually
available from the magnetics vendor. Copper loss in the
inductor is calculated by Equation 7:
Eq. 10
where:
C OUT = output capacitance value
f SW = switching frequency
As described in the “Theory of Operation” subsection in
P INDUCTOR(Cu) = I L(RMS)2 × R WINDING
Eq. 7
Functional Description , the MIC28500 requires at least
20mV peak-to-peak ripple at the FB pin to make the g m
amplifier and the error comparator behave properly. Also,
I C OUT (RMS) =
Δ I L(PP)
The resistance of the copper wire, R WINDING , increases
with the temperature. The value of the winding
resistance used should be at the operating temperature:
P WINDING(Ht) = R WINDING(20°C) × (1 + 0.0042 × (T H – T 20°C ))
Eq. 8
where:
T H = temperature of wire under full load
T 20°C = ambient temperature
R WINDING(20°C) = room temperature winding resistance
(usually specified by the manufacturer)
Output Capacitor Selection
The type of the output capacitor is usually determined by
its equivalent series resistance (ESR). Voltage and RMS
current capability are two other important factors for
selecting the output capacitor. Recommended capacitor
the output voltage ripple should be in phase with the
inductor current. Therefore, the output voltage ripple
caused by the output capacitors value should be much
smaller than the ripple caused by the output capacitor
ESR. If low-ESR capacitors, such as ceramic capacitors,
are selected as the output capacitors, a ripple injection
method should be applied to provide the enough
feedback voltage ripple. Please refer to the “Ripple
Injection” subsection for more details.
The voltage rating of the capacitor should be 20%
greater for aluminum electrolytic or OS-CON. The output
capacitor RMS current is calculated in Equation 11:
Eq. 11
12
The power dissipated in the output capacitor is:
P DISS(C OUT ) = I C OUT (RMS) × ESR C OUT
types are ceramic, low-ESR aluminum electrolytic, OS-
CON and POSCAP. The output capacitor’s ESR is
usually the main cause of the output ripple. The output
2
Eq. 12
capacitor ESR also affects the control loop from a
stability point of view. The maximum value of ESR is
calculated:
Input Capacitor Selection
The input capacitor for the power stage input V IN should
be selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to
ESR C OUT ≤
Δ V OUT(pp)
Δ I L(PP)
Eq. 9
high inrush currents, caused by turning the input supply
on. A tantalum input capacitor’s voltage rating should be
at least two times the maximum input voltage to
maximize reliability. Aluminum electrolytic, OS-CON, and
where:
Δ V OUT(pp) = peak-to-peak output voltage ripple
Δ I L(PP) = peak-to-peak inductor current ripple
multilayer polymer film capacitors can handle the higher
inrush currents without voltage de-rating. The input
voltage ripple will primarily depend on the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
June 2011
18
M9999-060311-B
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