SMB113A/B/SMB117/A
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
COMPONENT SELECTION
possible in order to minimize trace inductance that
would otherwise limit the rate of change of the current.
Buck Outputs:
Inductor:
Output capacitor
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 a Buck converter is
as follows:
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.
Vo(VIN −Vo)
Vin * 0.3* IMAX * f
Buck: Equation 1: L =
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:
The voltage drop can be calculated according to:
I *T
C
V =
Equation 2:
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.
1.2(3.6 −1.2)
3.6*0.3*0.5 *1E6
L =
= 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.
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.
Input Capacitor
MOSFETS
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.
When selecting the appropriate FET to use attention
must be paid to the gate to source rating, input
capacitance, and maximum power dissipation.
Most FETs are specified by an on resistance (RDSON)
for a given gate to source voltage (VGS). It is essential
to ensure that the FETs used will always have a VGS
voltage grater then the minimum value shown on the
datasheet. It is worth noting that the specified VGS
voltage must not be confused with the threshold
voltage of the FET.
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
Summit Microelectronics, Inc
2111 2.4 6/24/2008
25
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
