參數(shù)資料
型號(hào)： S-8520
廠商： Seiko Instruments Inc.
英文描述： PWM Control Step-Down Switching Regulator-Controller(PWM 控制，步降開關(guān)穩(wěn)壓器-控制器)
中文描述： PWM控制降壓型開關(guān)穩(wěn)壓控制器（脈寬調(diào)制控制，步降開關(guān)穩(wěn)壓器-控制器）
文件頁(yè)數(shù)： 11/30頁(yè)
文件大?。?/td> 650K
代理商： S-8520
PWM Control & PWM/PFM Control Step-Down Switching Regulator-Controllers
S-8520/8521 Series
Rev.6.1
10
Seiko Instruments Inc.
2. Inductor
The inductance value greatly affects the maximum output current Iout and the efficiency
h
.
As the L-value is reduced gradually, the peak current Ipk increases, to finally reach the maximum output current Iout when the L-
value has fallen to a certain point. If the L-value is made even smaller, Iout will begin decreasing because the current drive capacity
of the switching transistor becomes insufficient.
Conversely, as the L-value is augmented, the loss due to Ipk in the switching transistor will decrease until the efficiency is maximized
at a certain L-value. If the L-value is made even larger, the loss due to the series resistance of the coil will increase to the detriment
of the efficiency.
If the L-value is increased in an S-8520/21 Series product, the output voltage may turn unstable in some cases, depending on the
conditions of the input voltage, output voltage, and the load current. Perform thorough evaluations under the conditions of actual
service and decide on an optimum L-value.
In many applications, selecting a value of 47
m
H will allow a S-8520/21 Series product to yield its best characteristics in a well
balanced manner.
When choosing an inductor, pay attention to its allowable current, since a current applied in excess of the allowable value will
cause the inductor to produce magnetic saturation, leading to a marked decline in efficiency.
Therefore, select an inductor in which the peak current Ipk will not surpass its allowable current at any moment. The peak current
Ipk is represented by the following equation in continuous operation mode:
I
PK
= IOUT +
Where fosc is the oscillation frequency, L the inductance value of the coil, and VF the forward voltage of the diode.
3. Diode
The diode to be externally coupled to the IC should be a type that meets the following conditions:
Its forward voltage is low (Schottky barrier diode recommended).
Its switching speed is high (50 ns max.).
Its reverse direction voltage is higher than VIN.
Its current rating is higher than Ipk.
4. Capacitors (Cin, Cout)
The capacitor inserted on the input side (Cin) serves to lower the power impedance and to average the input current for better
efficiency. Select the Cin-value according to the impedance of the power supplied. As a rough rule of thumb, you should use a value
of 47
m
F to 100
m
F, although the actual value will depend on the impedance of the power in use and the load current value.
For the output side capacitor (Cout), select one of large capacitance with low ESR (Equivalent Series Resistance) for smoothing the
ripple voltage. However, notice that a capacitor with extremely low ESR (say, below 0.3
W
), such as a ceramic capacitor, could make
the output voltage unstable, depending on the input voltage and load current conditions. Instead, a tantalum electrolytic capacitor is
recommended. A capacitance value from 47
m
F to 100
m
F can serve as a rough yardstick for this selection.
5. External Switching Transistor
The S-8520/21 Series can be operated with an external switching transistor of the enhancement (Pch) MOS FET type or bipolar
(PNP) type.
5.1 Enhancement MOS FET type
The EXT terminal of the S-8520/21 Series is capable of directly driving a Pch power MOS FET with a gate capacity of some 1000 pF.
When a Pch power MOS FET is chosen, because it has a higher switching speed than a PNP type bipolar transistor and because
power losses due to the presence of a base current are avoided, efficiency will be 2% to 3% higher than when other types of transistor
are employed.
The important parameters to be kept in mind in selecting a Pch power MOS FET include the threshold voltage, breakdown voltage
between gate and source, breakdown voltage between drain and source, total gate capacity, on-resistance, and the current rating.
The EXT terminal swings from voltage VIN over to voltage Vss. If the input voltage is low, a MOS FET with a low threshold voltage
has to be used so that the MOS FET will come on as required. If, conversely, the input voltage is high, select a MOS FET whose
gate-source breakdown voltage is higher than the input voltage by at least several volts.
(VOUT + VF)
x (VIN - VOUT)
2
x fosc x L x (VIN + VF)
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S-8520A21MC-AVG-T2 功能描述:DC/DC 開關(guān)控制器 2.1V Step-Down PWM RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
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