SMB218 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
Manufacturer
SMB218 Datasheet PDF : 37 Pages
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PLEASE NOTE: The SMB118 has entered End-of-Life.
SMB118/218
APPLICATIONS INFORMATION (CONTINUED)
COMPONENT SELECTION
Buck Outputs:
Inductor:
The starting point design of any and DC/DC converter
is the selection of the appropriate inductor for the
application. The optimal inductor value will set the
inductor current at 30% of the maximum expected load
current. The inductors current for Buck and Boost
converters are as follows:
Buck: Equation
3:
L
=
Vo(VIN − Vo)
Vin * 0.3 * I MAX *
f
Boost: Equation 4:
L
=
VIN (VO
VO * 0.3 *
− VIN
I MAX
)
*
f
Where Vo is the output voltage, VIN is the input
voltage, f is the frequency, and IMAX is the max load
current.
For example: For a 1.2V output and a 3.6V input with a
500mA max load, and a 1MHz switching frequency the
optimal inductor value is:
L
=
1.2(3.6 − 1.2)
3.6 * 0.3 * 0.5 *1E6
=
5.3uH
Choosing the nearest standard inductor value we select
a 5.6uH inductor. It is important that the inductor has a
saturation current level greater than 1.2 times the max
load current.
Other parameters of interest when selecting an inductor
are the DCR (DC winding resistance). This has a direct
impact on the efficiency of the converter. In general, the
smaller the size of the inductor is the larger the
resistance. As the DCR goes up the power loss
increases according to the I2R relation. As a result
choosing a correct inductor is often a trade off between
size and efficiency.
Input Capacitor
Each converter should have a high value low
impedance input (or bulk) capacitor to act as a current
reservoir for the converter stage. This capacitor should
be either a X5R or X7R MLCC (multi-layer-ceramic
capacitor). The value of this capacitor is normally
chosen to reflect the ratio of the input and output
voltage with respect to the output capacitor. Typical
values range from 2.2uF to 10uF.
For Buck converters, the input capacitor supplies
square wave current to the inductor and thus it is critical
to place this capacitor as close to the PFET as possible
in order to minimize trace inductance that would
otherwise limit the rate of change of the current. While
the placement of this inductor for Boost channels is not
as critical as with the Buck channels, each Boost must
still have its own reservoir capacitor.
Output capacitor
Each converter should have a high value low
impedance output capacitor to act as a current
reservoir for current transients and to. This capacitor
should be either a X5R or X7R MLCC.
For a Buck converter, the value of this capacitance is
determined by the maximum expected transient
current. Since the converter has a finite response time,
during a load transient the current is provided by the
output capacitor. Since the voltage across the capacitor
drops proportionally to the capacitance, a higher output
capacitor reduces the voltage drop until the feedback
loop can react to increase the voltage to equilibrium.
For the Boost converters, the output is disconnected
from the inductor while the diode is reverse biased.
This means that the entire load current is being taken
from the output capacitance for this portion of the duty
cycle. For this reason it is necessary to choose the
output capacitor such that the cycle-to-cycle voltage
droop is minimized to be within system limits.
The voltage drop can be calculated according to:
Equation 5: V = I *T
C
Where I is the load or transient current, T is the time the
output capacitor is supporting the output and C is the
output capacitance. Typical values range from 10uF to
44uF.
Other important capacitor parameters include the
Equivalent Series Resistance (E.S.R) of the capacitor.
The ESR in conjunction with the ripple current
determines the ripple voltage on the output, for typical
values of MLCC the ESR ranges from 2-10mΩ. In
addition, carful attention must be paid to the voltage
rating of the capacitor the voltage rating of a capacitor
must never be exceeded. In addition, the DC bias
voltage rating can reduce the measured capacitance by
as much as 50% when the voltage is at half of the max
rating, make sure to look at the DC bias de-rating
curves when selecting a capacitor.
Summit Microelectronics, Inc
2107 3.0 10/15/2008
28
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