CS5212GD14 ONSEMI [ON Semiconductor], CS5212GD14 Datasheet - Page 8

• Manufacturer Part Number: CS5212GD14
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ONSEMI [ON Semiconductor]

Inductor Component Selection
in the converter because it will directly effect the choice of
other components and dictate both the steady−state and
transient performance of the converter. When selecting an
inductor the designer must consider factors such as DC
current, peak current, output voltage ripple, core material,
magnetic saturation, temperature, physical size, and cost
(usually the primary concern).
and physically small as possible to provide the best transient
response and minimum cost. If a large inductance value is
used, the converter will not respond quickly to rapid changes
in the load current. On the other hand, too low an inductance
value will result in very large ripple currents in the power
components (MOSFETs, capacitors, etc) resulting in
increased dissipation and lower converter efficiency. Also,
increased ripple currents will force the designer to use
higher rated MOSFETs, oversize the thermal solution, and
use more, higher rated input and output capacitors − the
converter cost will be adversely effected.
size the inductor to produce a specified maximum ripple
current in the inductor. Lower ripple currents will result in
less core and MOSFET losses and higher converter
efficiency. The following equation may be used to calculate
the minimum inductor value to produce a given maximum
ripple current (α ⋅ I
this equation is a minimum because values less than this will
produce more ripple current than desired. Conversely,
higher inductor values will result in less than the maximum
ripple current.
output current (α = 0.15 for ±15%, α = 0.25 for ±25%, etc)
and f
value is used, the inductor current will swing ± α/2% about
Iout. Therefore, the inductor must be designed or selected
such that it will not saturate with a peak current of (1 + α/2)
⋅ I
from the RMS current level. The RMS of the AC component
of the inductor is given by the following relationship:
where I
from:
APPLICATIONS AND COMPONENT SELECTION
Lo MIN + (Vin * Vout) @ Vout (a @ I O,MAX @ Vin @ f SW )
O,MAX
The output inductor may be the most critical component
In general, the output inductance value should be as low
One method of calculating an output inductor value is to
α is the ripple current as a percentage of the maximum
Power dissipation in the inductor can now be calculated
The total I
The power dissipation for the inductor can be determined
sw
PP
is the switching frequency. If the minimum inductor
.
= α ⋅ I
RMS
I RMS +
O,MAX
of the current will be calculated from:
O,MAX
P + I RMS 2
I AC +
.
). The inductor value calculated by
I OUT 2 ) I AC 2
I PP
12
R L
APPLICATIONS INFORMATION
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8
Input Capacitor Selection and Considerations
caused by conduction of current of the top pass transistor
charging the PWM inductor.
going from 0 to peak current in the inductor. The duty factor
will be a function of the ratio of the input to output voltage
and of the efficiency.
now be calculated:
possible duty factor closest to 50% will give the worst case
dissipation in the capacitors. The power dissipation of the
input capacitors can be calculated by multiplying the square
of the RMS current by the ESR of the capacitor.
Output Capacitor
and provides low impedance for load current changes. The
effect of the capacitance for handling the power supply
induced ripple will be discussed here. Effects of load
transient behavior can be considered separately.
ripple current induced by the switches through the inductor.
This ripple current was calculated as I
discussion of the inductor. This ripple component will
induce heating in the capacitor by a factor of the RMS
current squared multiplied by the ESR of the output
capacitor section. It will also create output ripple voltage.
current times the ESR of the capacitor, plus the ripple current
integrating in the capacitor, and the rate of change in current
times the total series inductance of the capacitor and
connections.
output capacitor is the major contributor to the output ripple
voltage. This fact can be used as a criterion to select the
output capacitor.
calculated from:
where:
MOSFET & Heatsink Selection
drive MOSFET selection. To adequately size the heat sink,
The input capacitor is used to reduce the current surges
The input current is pulsing at the switching frequency
The RMS value of the ripple into the input capacitors can
The input RMS is maximum at 50% DF, so selection of the
The output capacitor filters output inductor ripple current
The principle consideration for the output capacitor is the
The ripple voltage will be a vector summation of the ripple
The inductor ripple current acting against the ESR of the
The power dissipation in the output capacitor can be
I
C
network.
Power dissipation, package size, and thermal solution
AC
ESR
= AC RMS of the inductor
= Effective series resistance of the output capacitor
I IN(RMS) + I OUT DF * DF 2
V PP + I PP
P + I AC 2
DF +
V O
V I
C ESR
C ESR
Eff
1
AC
in the above