参数资料
| 型号： | SC4508AMLTRT |
| 厂商： | Semtech |
| 文件页数： | 9/21页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST INV PWM 12-MLPQ |
| 标准包装： | 1 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.5MHz |
| 占空比： | 97% |
| 电源电压： | 2.7 V ~ 15 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 是 |
| 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： | 12-VQFN 裸露焊盘 |
| 包装： | 标准包装 |
| 产品目录页面： | 1358 (CN2011-ZH PDF) |
| 其它名称： | SC4508AMLDKR |
SC4508A
POWER MANAGEMENT
? V o
δ I o
α V o
Application Information (Cont.)
When determining the ESR value, both the steady state
ripple-voltage and the dynamic load transient need to be
considered. To keep the steady state output ripple-voltage
< ? V o , the ESR should satisfy:
R esr 1 <
To limit the dynamic output voltage overshoot/
undershoot within α (say 3%) of the steady state output
voltage) from no load to full load, the ESR value should
satisfy:
R esr 2 <
I o
Then, the required ESR value of the output capacitors
should be:
R esr = min{R esr1 ,R esr2 }
The voltage rating of aluminum capacitors should be at
least 1.5 V o . The RMS current ripple rating should also
be greater than:
δ I o
2 3
Usually it is necessary to have several capacitors of the
same type in parallel to satisfy the ESR requirement.
the following remarks are made to clarify some practical
issues.
Remark 1: High frequency ceramic capacitors may not
carry most of the ripple current. It also depends on the
capacitor value. Only when the capacitor value is set
properly, the effect of ceramic capacitor low ESR starts
to be significant.
For example, if a 10 μ F, 4m ? ceramic capacitor is
connected in parallel with 2x1500 μ F, 90m ? electrolytic
capacitors, the ripple current in the ceramic capacitor is
only about 42% of the current in the electrolytic
capacitors at the ripple frequency. If a 100 μ F, 2m ?
ceramic capacitor is used, the ripple current in the
ceramic capacitor will be about 4.2 times of that in the
electrolytic capacitors. When two 100 μ F, 2m ? ceramic
capacitors are used, the current ratio increases to 8.3.
In this case most of the ripple current flows in the
ceramic decoupling capacitor. The ESR of the ceramic
capacitors will then determine the output ripple-voltage.
Remark 2: The total equivalent capacitance of the filter
bank is not simply the sum of all the paralleled capacitors.
The total equivalent ESR is not simply the parallel
combination of all the individual ESRs either. Instead they
should be calculated using the following formulae.
( R 1 a + R 1 b ) 2 ω 2 C 1 a C 1 b + ( C 1 a + C 1 b ) 2
( R 1 a C 1 a + R 1 b C 1 b ) ω 2 C 1 a C 1 b + ( C 1 a + C 1 b )
C o >
R 1 a R 1 b ( R 1 a + R 1 b ) ω 2 C 1 a C 1 b + ( R 1 b C 1 b + R 1 a C 1 a )
( R 1 a + R 1 b ) 2 ω 2 C 1 a C 1 b + ( C 1 a + C 1 b ) 2
The voltage ripple cause by the capacitor charge/
discharge should be an order of magnitude smaller than
the voltage ripple caused by the ESR. To guarantee this,
the capacitance should satisfy:
10
2 π f s R esr
Buck-boost converter has higher ripple current than buck
C eq ( ω ) : =
R eq ( ω ) : =
2 2
2 2
2 2 2 2
2 2
I RMS _ CAP ≈ I O
in output. The RMS value is the most important factor to
consider. It has to be less than the output capacitor ripple
current rating.
The buck-boost output capacitor RMS current is:
V O + V d
V IN
? v ESR = R esr ? I d
where R 1a and C 1a are the ESR and capacitance of
electrolytic capacitors, and R 1b and C 1b are the ESR and
capacitance of the ceramic capacitors respectively.
(Figure 2)
where, V d and I d are rectifier forward voltage and current.
In many applications, several low ESR ceramic capacitors
C1a
R1a
C1b
R1b
Ceq
Req
are added in parallel with the aluminum capacitors in
order to further reduce ESR and improve high frequency
decoupling. Because the values of capacitance and ESR
are usually different in ceramic and aluminum capacitors,
Figure 2. Equivalent RC branch.
? 2007 Semtech Corp.
9
www.semtech.com
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