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参数资料
| 型号： | MIC2155YML TR |
| 厂商： | Micrel Inc |
| 文件页数： | 27/35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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f ' = 1
Micrel, Inc.
One method of reducing the ringing is to use a resistor
and capacitor to lower the Q of the resonant circuit. The
circuit in Figure 23 shows the resistor in between the
switch node and ground. Capacitor Cs is used to block
DC and minimize the power dissipation in the resistor.
This capacitor value should be between 5 and 10 times
the parasitic capacitance of the MOSFET C OSS . A
capacitor that is too small will have high impedance and
prevent the resistor from damping the ringing. A
capacitor that is too large causes unnecessary power
MIC2155/2156
Define f’ as:
f
f 2
Combining the equations for f 1 , f 2 and f’ to derive C P , the
parasitic capacitance:
2 × ( f ) 2 ? 1
dissipation in the resistor, which lowers efficiency.
The snubber components should be placed as close as
possible to the low-side MOSFET and/or external
C P =
C S
'
schottky diode since in contributes to most of the stray
capacitance. Placing the snubber too far from the FET or
using etch that is too long or thin will add inductance to
L P is solved by re-arranging the equation for f 1 :
( 2 π )
the snubber and diminishes its effectiveness.
A proper snubber design requires the parasitic
inductance and capacitance be known. A method of
L P =
2
1
× C P × ( f 1 ) 2
determining these values and calculating the damping
resistor value is outlined below.
Step 3: Calculate the damping resistor.
Q =
1. Measure the ringing frequency at the switch node
which is determined by parasitic L P and C P . Define this
frequency as f 1 .
2. Add a capacitor C S (normally at least 3 times as big as
the C OSS of the FET) from the switch node to ground and
measure the new ringing frequency. Define this new
(lower) frequency as f 2 . L P and C P can now be solved
using the values of f 1 , f 2 and C S .
Critical damping occurs at Q = 1:
1 L P
R S C S
Solving for R S
= 1
3. Add a resistor R S in series with C S to generate critical
damping.
Step 1: First measure the ringing frequency on the
R S =
L P
C S
switch node voltage when the high-side MOSFET turns
on. This ringing is characterized by the equation:
Figure 23 shows the snubber in the circuit and the
damped switch node waveform.
f 1 =
1
2 π L P × C P
L STRAY1
L STRAY2
Where:
C P and L P are the parasitic capacitance and
inductance
Step 2: Add a capacitor, C S , in parallel with the
synchronous MOSFET, Q2. The capacitor value should
be approximately 3 times the C OSS of Q2. Measure the
frequency of the switch node ringing, f 2 :
R DS
L STRAY3
C OSS2
L STRAY4
R S
C S
f 2 =
November 2009
1
2 π Lp × ( Cs + Cp )
27
Figure 23. Snubber Circuit
M9999-111209-B
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