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参数资料
| 型号： | MIC2155YML TR |
| 厂商： | Micrel Inc |
| 文件页数： | 29/35页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM VM 32MLF |
| 特色产品： | MIC2155/6 Buck Control IC |
| 标准包装： | 1 |
| PWM 型： | 电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 550kHz |
| 占空比： | 80% |
| 电源电压： | 4.5 V ~ 14.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 32-VFQFN 裸露焊盘，32-MLF? |
| 包装： | 标准包装 |
| 产品目录页面： | 1091 (CN2011-ZH PDF) |
| 其它名称： | 576-3527-6 |
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V IN
G MOD = ? ?
? ?
Micrel, Inc.
The Modulator Gain is proportional to the input voltage
and inversely proportional to the internal ramp voltage
generated by the oscillator. The MIC2155/6 peak-peak
ramp voltage is 1V:
? ?
? V RAMP ?
The output voltage divider attenuates V OUT and feeds it
back to the error amplifier. The divider gain is:
MIC2155/2156
As the frequency approaches the zero frequency (Fz),
formed by Co and it’s ESR, the slope of the gain curve
changes from -40db/dec. to -20db/dec and the phase
increases. The zero causes a 90° phase boost. Ceramic
capacitors, with their smaller values of capacitance and
ESR, push the zero and its phase boost out to higher
frequencies, which allow the phase lag from the LC filter
to drop closer to -180°. The system will be close to being
unstable if the overall open loop gain crosses 0dB while
the phase is close to -180°. If the output capacitance
and/or ESR is high, the zero moves lower in frequency
and helps to boost the phase, leading to a more stable
system.
H =
R 4
R 1 + R 4
=
V REF
V OUT
The error amplifier is a type III which has two zeros, two
poles and a pole at the origin. This type of error amplifier
works well when Ceramic output capacitors make up the
The modulator, filter and voltage divider gains can be
multiplied together to show the open loop gain of these
majority of C OUT because it introduces an extra zero that
helps improve phase margin:
? 1 +
? ? 1 +
?
s ? s ? ? s ?
ω po ? ?
ω p 1 ? ? ? ?
ω p 2 ? ?
parts:
G VD ( s ) = G FILTER ( s ) × H × G MOD
Gea ( s ) =
? s ? ? s ?
? ω z 1 ? ? ω z 2 ?
? 1 + ? ? 1 + ?
This transfer function is plotted in Figure 25. At low
frequency, the transfer function gain equals the
modulator gain times the voltage divider gain. As the
frequency increases toward the LC filter resonant
frequency, the gain starts to peak. The increase in the
gain’s amplitude equals Q. Just above the resonant
frequency, the gain drops at a -40db/decade rate. The
phase quickly drops from 0° to almost 180° before the
phase boost of the zero brings it back up to -90°. Higher
values of Q will cause the phase to drop quickly. In a
Where:
ω z 1 =
ω z 2 =
ω po =
1
R 2 × C 2
1
( R 1 + R 3 ) ? C 3
1
R 1 × C 1
well damped, low Q system the phase will change more
slowly.
90
45
ω p 1 =
ω p 2 =
1
C1 × C2
C 1 + C 2
1
R 3 × C 3
? R 2
0
Gain
Figure 26 shows the bode plot of the error amplifier
-45
-90
-135
Phase
transfer function.
-180
10
100
1k 10k 100k
1M
FREQUENCY
Figure 25. G VD Transfer Function
November 2009
29
M9999-111209-B
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