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| 型號： | S-8520E47MC-BKG-T2 |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.05 A SWITCHING CONTROLLER, 375 kHz SWITCHING FREQ-MAX, PDSO5 |
| 封裝： | SOT-23, 5 PIN |
| 文件頁數(shù)： | 9/43頁 |
| 文件大小： | 499K |
| 代理商： | S-8520E47MC-BKG-T2 |
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PWM CONTROL, PWM/PFM SWITCHING CONTROL STEP-DOWN SWITCHING REGULATOR
Rev.7.4_20
S-8520/8521 Series
Seiko Instruments Inc.
17
2. Inductor
The inductance value greatly affects the maximum output current (IOUT) and the efficiency (
η).
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 the peak current (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-8520/8521 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 A, B, C and D types 47
H, E and F types 22 H will allow a
S-8520/8521 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:
)
V
(
L
f
2
)
V
(
)
V
(
I
F
IN
osc
OUT
IN
F
OUT
PK
+
×
×
+
=
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
F to 100 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
), 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
F to 100 F can serve as a rough yardstick for this selection.
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