Theory of Operation / Application Information
9
Component Selection
Inductor Selection
Although the inductor does not influence the operating frequency, the inductor value has a direct
effect on ripple current. The inductor ripple current ∆I
L
decreases with higher inductance and
increases with higher VIN or V
OUT
.
Accepting larger values of ∆I
L
allows the use of low inductances, but results in higher output voltage
ripple, greater core losses, and lower output current capability. A typical ∆I
L
value is 20% to 40% of
output current.
Another important parameter for the inductor is the current rating. Exceeding an inductor's maximum
current rating may cause the inductor to saturate and overheat. Once the inductor value has been
selected, the peak inductor current can be calculated as the following:
It should be ensured that the current rating of the selected inductor is 1.5 times of the I
PEAK
.
Input Capacitor Selection
Because the buck converter has a pulsating input current, a low ESR input capacitor is required. This
results in the best input voltage filtering and minimizing the interference with other circuits caused by
high input voltage spikes. Also the input capacitor must be sufficiently large to stabilize the input
voltage during heavy load transients. Ceramic capacitors show a good performance because of the
low ESR value, and they are less sensitive to voltage transients and spikes. Place the input capacitor
as close as possible to the input pin of the device for best performance. The typical value is about
4.7µF. The X5R or X7R ceramic capacitors have the best temperature and voltage characteristics,
which is good for the input capacitor.
Output Capacitor Selection
The output capacitor is the most critical component of a switching regulator, it is used for output
filtering and keeping the loop stable. The selection of C
OUT
is driven by the required ESR to minimize
voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple (∆V
OUT
) is determined by:
The output ripple is highest at maximum input voltage since ∆I
L
increases with input voltage.
Once the ESR requirements for C
OUT
have been met, the RMS current rating generally far exceeds
the I
RIPPLE
(P-P) requirement, except for an all ceramic solution. In most applications, a 22µF ceramic
capacitor is usually enough for these conditions.
At light load currents, the device operates in PSM mode, and the output voltage ripple is independent
of the output capacitor value. The output voltage ripple is set by the internal comparator thresholds.
The typical output voltage ripple is 1% of the output voltage V
OUT
.
