SS6639 데이터 시트보기 (PDF) - Silicon Standard Corp.
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SS6639 Datasheet PDF : 12 Pages
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Minimum and maximum input voltage, output voltage
and output current must be established before an
inductor can be selected.
In discontinuous mode operation, at the end of the
switch ON time, peak current and energy in the
inductor build according to
IPK = VIN * 1− exp(− RON + RS * TON)
RON + RS
L
≅ VIN * (TON) * 1− x
L
2
≅ VIN * TON
L
(simple lossless equation), where
X = (RON + RS) ∗ TON
L
EL = 1 L * IPK 2
2
SS6639
Power required from the inductor per cycle must be
equal to, or greater than
PL = (VOUT + VD − VIN) * (IOUT) * ( 1 )
fSW
fsw
in order for the converter to regulate the output.
When the loading exceeds IOB, the PFM controller
operates in continuous mode. Inductor peak current
can be derived from
IPK = VOUT + VD − VSW − x
VIN − VSW
2
* IOUT + VIN − VSW * TON * 1 − x
2L
2
Valley current (Iv) is
IV = VOUT + VD − VSW − x * IOUT
VIN − VSW
2
* VIN − VDE * TON * 1 − x
2L
2
Table 1 Indicates resistance and height for each coil.
Power Inductor Type
Inductance ( µH ) Resistance ( Ω ) Rated Current (A) height (mm)
47
0.25
0.7
Sumida SMT Type CD54
4.5
100
0.50
0.5
Hold SMT Type PM54
47
0.25
0.7
4.5
100
0.50
0.5
Hold SMT Type PM75
33
0.11
1.2
5.0
Huan Feng PIN Type V0810
33
40m
2
10.0
CAPACITOR SELECTION
A poor choice for an output capacitor can result in poor
efficiency and high output ripple. Ordinary aluminum
electrolytic capacitors, while inexpensive, may have
unacceptably poor ESR and ESL. There are low ESR
aluminum capacitors for switch mode DC-DC
converters which work much better than
general-purpose components. Tantalum capacitors
provide still better performance but are more
expensive. OS-CON capacitors have extremely low
ESR in a small size. If the capacitance is reduced, the
output ripple will increase.
As most of the input supply is applied across the input
bypass capacitor, the capacitor voltage rating should
be at least 1.25 times greater than the maximum input
voltage.
DIODE SELECTION
Speed, forward drop, and leakage current are three
main considerations in selecting a rectifier diode. The
best performance is obtained with a Schottky rectifier
Rev.2.01 6/26/2003
www.SiliconStandard.com
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