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SIP12201 Datasheet PDF : 12 Pages
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SiP12201
Vishay Semiconductors
APPLICATION NOTES
Inductor Selection:
An inductor is one of the energy storage component in
a converter. Choosing an inductor means specifying its
size, structure, material, inductance, saturation level,
DC-resistance (DCR), and core loss. Fortunately,
there are many inductor vendors that offer wide selec-
tions with ample specifications and test data, such as
Vishay Dale.
The following are some key parameters that users
should focus on. In PWM mode, inductance has a
direct impact on the ripple current. The peak-to-peak
inductor ripple current can be calculated as
VOUT (VIN − V ) OUT
IP − P =
VIN Lf
where f = switching frequency.
Higher inductance means lower ripple current, lower
rms current, lower voltage ripple on both input and out-
put, and higher efficiency, unless the resistive loss of
the inductor dominates the overall conduction loss.
However, higher inductance also means a bigger
inductor size and a slower response to transients. In
PSM mode, inductance affects inductor peak current,
and consequently impacts the load capability and
switching frequency. For fixed line and load conditions,
higher inductance results in a lower peak current for
each pulse, a lower load capability, and a higher
switching frequency.
The saturation level is another important parameter in
choosing inductors. Note that the saturation levels
specified in data sheets are maximum currents. For a
dc-to-dc converter operating in PWM mode, it is the
maximum peak inductor current that is relevant, and
which can be calculated using these equations:
IP − P
I PK = I OUT +
2
This peak current varies with inductance tolerance and
other errors, and the rated saturation level varies over
temperature. So a sufficient design margin is required
when choosing current ratings.
A high-frequency core material, such as ferrite, should
be chosen, the core loss could lead to serious effi-
ciency penalties. The DCR should be kept as low as
possible to reduce conduction losses.
Input Capacitor Selection:
To minimize current pulse induced ripple caused by
the step-down controller and interference of large volt-
age spikes from other circuits, a low-ESR input capac-
itor is required to filter the input voltage. The input
capacitor should be rated for the maximum RMS input
current:
I = I RMS LOAD(m ax) VOUT ⎜⎛1− VOUT ⎟⎞
VIN ⎝ VIN ⎠
It is common practice to rate for the worst-case RMS
ripple that occurs when the duty cycle is at 50%:
I = I RMS
LOAD(max)
2
Output Capacitor Selection:
The selection of the output capacitor is primarily deter-
mined by the ESR required to minimize voltage ripple
and current ripple. The desired output ripple ∆VOUT
can be calculated by:
( ) ∆VOUT =
Im ax- Im in
⎜⎛ ESR + 1 ⎟⎞
⎝
8fCOUT ⎠
Current ripple can be calculated by:
( ) Imax- Imin = T VOUT (VIN - VOUT)
L VIN
Where: ∆VOUT = Desired Output Ripple Voltage
f = Switching frequency
Imax = Maximum Inductor Current
Imin = Minimum Inductor Current
T = Switching Period
Multiple capacitors placed in parallel may be needed to
meet the ESR requirements. However if the ESR is too
low it can cause instability problems.
MOSFET Selection:
The key selection criteria for the MOSFETs include
maximum specifications for on-resistance, drain-
source voltage, gate source, current, and total gate
charge Qg. While the voltage ratings are fairly straight-
forward, it is important to carefully balance on-resis-
tance and gate charge. In typical MOSFETs, the lower
the on-resistance, the higher the gate charge. The
power loss of a MOSFET consists of conduction, gate
charge, and crossover losses. For lower-current appli-
cations, gate charge losses become a significant fac-
tor, so low gate charge MOSFETs, such as Vishay
Siliconix's LITTLE FOOT family of PWM-optimized
devices, are desirable.
Document Number: 73541
S-52332–Rev. B, 07-Nov-05
www.vishay.com
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